Publications by year
In Press
Price N, Green S, Troscianko J, Tregenza T, Stevens M (In Press). Background matching and disruptive coloration as habitat-specific strategies for camouflage. Scientific Reports
Green S, Duarte RC, Kellett E, Alagaratnam N, Stevens M (In Press). Color change and behavioral choice facilitate chameleon prawn camouflage against different seaweed backgrounds. Communications Biology
Galloway J, Green SD, Stevens M, Kelley L (In Press). Finding a signal hidden among noise: how can predators overcome camouflage strategies?. Philosophical Transactions of the Royal Society B: Biological Sciences
Paul S, Stevens M (In Press). Horse vision and obstacle visibility in horseracing. Applied Animal Behaviour Science
Carter EE, Tregenza T, Stevens M (In Press). Ship noise inhibits colour change, camouflage, and anti-predator behaviour in shore crabs. Current Biology
Nokelainen O, Maynes R, Mynott S, Price N, Stevens M (In Press). Supplementary data for: Improved camouflage through ontogenetic colour change confers reduced detection risk in shore crabs.
Dell’Aglio DD, Troscianko J, Stevens M, McMillan WO, Jiggins CD (In Press). The conspicuousness of the toxic<i>Heliconius</i>butterflies across time and habitat.
Abstract:
The conspicuousness of the toxicHeliconiusbutterflies across time and habitat
AbstractForests are a mosaic of light spectra, and colour signal efficiency might change in different light environments. Local adaptation inHeliconiusbutterflies is linked to microhabitat use and the colourful wing colour patterns may be adapted for signalling in different light environments. These toxic butterflies exhibit conspicuous colours as a warning to predators that they should be avoided, but also find and choose potential mates based on colour signals. The two selection pressures of predation and mate preference are therefore acting together. Colour conspicuousness should show habitat-specific contrast for the butterflies, which would facilitate detection and species identification. On the other hand, selection for signal stability would be stronger in the avian visual system. In this study we analysed the contrast of twoHeliconiusmimicry rings in their natural habitats under varying degrees of forest fragmentation and light conditions. We used digital image analyses and mapped the bird and butterfly vision colour space in order to examine whether warning colours have greater contrast and if they transmit a consistent signal across time of the day and habitat in a tropical forest. We tested conspicuousness using opponent colour channels against a natural green background. For avian vision, colours are generally very stable through time and habitat. For butterfly vision, there is some evidence that species are more contrasting in their own habitats, where conspicuousness is higher for red and yellow bands in the border and for white in the forest. Light environment affectsHeliconiusbutterflies’ warning signal transmission to a higher degree through their own vision, but to a lesser degree through avian predator vision. This work provides insight into the use of colour signals in sexual and natural selection in the light of ecological adaptation.
Abstract.
Mynott S, Daniels C, Widdicombe S, Stevens M (In Press). Using camouflage for conservation: colour change in juvenile European lobster.
Abstract:
Using camouflage for conservation: colour change in juvenile European lobster
AbstractChanges in coloration enable animals to refine their camouflage to match different visual environments. Such plasticity provides ecological benefits and could potentially be exploited to support conservation or stock enhancement efforts. One application could be ensuring that hatchery-reared animals, reared to stock wild populations, are appropriately matched to their environment on release. European lobster (Homarus gammarus) hatcheries aim to restock or enhance local lobster populations by rearing juveniles through their most vulnerable stages, then releasing them into the wild. However, little consideration has been given to their camouflage and the implications of matching individuals to their release site. This study assesses to what extent juvenile lobsters can change coloration to match their background and whether hatchery practices could be altered to enhance lobster camouflage. We test this by switching individuals between black or white backgrounds in the laboratory and monitoring their coloration over time. Our work demonstrates the capacity of juvenile lobsters to change lightness in response to their surroundings. We show that juvenile lobsters are capable of small changes in luminance (perceived lightness) to better match their background over 2-3 weeks. These changes potentially correspond to improved camouflage, based on a model of predator (European pollack,Pollachius pollachius) vision. However, over a longer period (5 weeks), lobsters maintained on either background converged on the same darker coloration, suggesting that lobsters also experience changes in appearance associated with ontogeny. By refining the approaches used here, there is potential for hatcheries to rear lobsters on backgrounds that better match their release site. However, such manipulations should be considered in the context of ontogenetic changes and release timing (which varies between stocking programmes). This study highlights the potential to use colour change in stocking and aquaculture, as well as gaps that could be addressed through further research in this area.
Abstract.
Hughes A, Briolat E, Arenas L, Liggins E, Stevens M (In Press). Varying benefits of generalist and specialist camouflage in two versus four background environments. Behavioral Ecology
2022
Rising K, Hardege J, Tregenza T, Stevens M (2022). Anthropogenic noise may impair the mating behaviour of the Shore Crab Carcinus Maenas.
PLoS One,
17(10).
Abstract:
Anthropogenic noise may impair the mating behaviour of the Shore Crab Carcinus Maenas.
Anthropogenic noise is a recent addition to the list of human-made threats to the environment, with potential and established negative impacts on a wide range of animals. Despite their economic and ecological significance, few studies have considered the impact of anthropogenic noise on crustaceans, though past studies have shown that it can cause significant effects to crustacean physiology, anatomy, and behaviour. Mating behaviour in crustaceans could potentially be severely affected by anthropogenic noise, given that noise has been demonstrated to impact some crustacean's ability to detect and respond to chemical, visual, and acoustic cues, all of which are vital in courtship rituals. To explore if noise has an impact on crustacean mating, we tested the responses of male green shore crabs (Carcinus maenas) from the southwest UK coast by exposing them to ship noise recordings while simultaneously presenting them with a dummy-female soaked in the female-sex pheromone uridine diphosphate (UDP) in an experimental tank setup (recording treatment: n = 15, control treatment: n = 15). We found a significant, negative effect of noise on the occurrence of mating behaviour compared to no noise conditions, though no significant effect of noise on the time it took for a crab to respond to the pheromone. Such effects suggest reproductive impairment due to anthropogenic noise, which could potentially contribute to decreased crustacean populations and subsequent ecological and economic repercussions. Given the findings of our preliminary study, more research should be undertaken that includes larger sample sizes, double blind setups, and controlled laboratory trials in order to more fully extrapolate the potential impact of noise on mating in the natural environment.
Abstract.
Author URL.
2021
Niu Y, Stevens M, Sun H (2021). Commercial harvesting has driven the evolution of camouflage in an alpine plant. Current Biology, 31, 446-449.
Duarte RC, Dias GM, Flores AAV, Stevens M (2021). Different ontogenetic trajectories of body colour, pattern and crypsis in two sympatric intertidal crab species.
Biological Journal of the Linnean Society,
132(1), 17-31.
Abstract:
Different ontogenetic trajectories of body colour, pattern and crypsis in two sympatric intertidal crab species
Animals frequently exhibit great variation in appearance, especially in heterogeneous habitats where individuals can be concealed differentially against backgrounds. Although background matching is a common anti-predator strategy, gaps exist in our understanding of within- and among-species variation. Specifically, the drivers of changes in appearance associated with habitat use and occurring through ontogeny are poorly understood. Using image analysis, we tested how individual appearance and camouflage in two intertidal crab species, the mud crab Panopeus americanus and the mottled crab Pachygrapsus transversus, relate to ontogeny and habitat use. We predicted that both species would change appearance with ontogeny, but that resident mud crabs would exhibit higher background similarity than generalist mottled crabs. Both species showed ontogenetic changes; the mud crabs became darker, whereas mottled crabs became more green. Small mud crabs were highly variable in colour and pattern, probably stemming from the use of camouflage in heterogeneous habitats during the most vulnerable life stage. Being habitat specialists, mud crabs were better concealed against all backgrounds than mottled crabs. Mottled crabs are motile and generalist, occupying macroalgae-covered rocks when adults, which explains why they are greener and why matches to specific habitats are less valuable. Differential habitat use in crabs can be associated with different coloration and camouflage strategies to avoid predation.
Abstract.
Briolat ES, Arenas LM, Hughes AE, Liggins E, Stevens M (2021). Generalist camouflage can be more successful than microhabitat specialisation in natural environments.
BMC Ecol Evol,
21(1).
Abstract:
Generalist camouflage can be more successful than microhabitat specialisation in natural environments.
BACKGROUND: Crypsis by background-matching is a critical form of anti-predator defence for animals exposed to visual predators, but achieving effective camouflage in patchy and variable natural environments is not straightforward. To cope with heterogeneous backgrounds, animals could either specialise on particular microhabitat patches, appearing cryptic in some areas but mismatching others, or adopt a compromise strategy, providing partial matching across different patch types. Existing studies have tested the effectiveness of compromise strategies in only a limited set of circumstances, primarily with small targets varying in pattern, and usually in screen-based tasks. Here, we measured the detection risk associated with different background-matching strategies for relatively large targets, with human observers searching for them in natural scenes, and focusing on colour. Model prey were designed to either 'specialise' on the colour of common microhabitat patches, or 'generalise' by matching the average colour of the whole visual scenes. RESULTS: in both the field and an equivalent online computer-based search task, targets adopting the generalist strategy were more successful in evading detection than those matching microhabitat patches. This advantage occurred because, across all possible locations in these experiments, targets were typically viewed against a patchwork of different microhabitat areas; the putatively generalist targets were thus more similar on average to their various immediate surroundings than were the specialists. CONCLUSIONS: Demonstrating close agreement between the results of field and online search experiments provides useful validation of online citizen science methods commonly used to test principles of camouflage, at least for human observers. In finding a survival benefit to matching the average colour of the visual scenes in our chosen environment, our results highlight the importance of relative scales in determining optimal camouflage strategies, and suggest how compromise coloration can succeed in nature.
Abstract.
Author URL.
Caves EM, Dixit T, Colebrook-Robjent JFR, Hamusikili L, Stevens M, Thorogood R, Spottiswoode CN (2021). Hosts elevate either within-clutch consistency or between-clutch distinctiveness of egg phenotypes in defence against brood parasites.
Proc Biol Sci,
288(1953).
Abstract:
Hosts elevate either within-clutch consistency or between-clutch distinctiveness of egg phenotypes in defence against brood parasites.
In host-parasite arms races, hosts can evolve signatures of identity to enhance the detection of parasite mimics. In theory, signatures are most effective when within-individual variation is low ('consistency'), and between-individual variation is high ('distinctiveness'). However, empirical support for positive covariation in signature consistency and distinctiveness across species is mixed. Here, we attempt to resolve this puzzle by partitioning distinctiveness according to how it is achieved: (i) greater variation within each trait, contributing to elevated 'absolute distinctiveness' or (ii) combining phenotypic traits in unpredictable combinations ('combinatorial distinctiveness'). We tested how consistency covaries with each type of distinctiveness by measuring variation in egg colour and pattern in two African bird families (Cisticolidae and Ploceidae) that experience mimetic brood parasitism. Contrary to predictions, parasitized species, but not unparasitized species, exhibited a negative relationship between consistency and combinatorial distinctiveness. Moreover, regardless of parasitism status, consistency was negatively correlated with absolute distinctiveness across species. Together, these results suggest that (i) selection from parasites acts on how traits combine rather than absolute variation in traits, (ii) consistency and distinctiveness are alternative rather than complementary elements of signatures and (iii) mechanistic constraints may explain the negative relationship between consistency and absolute distinctiveness across species.
Abstract.
Author URL.
Stevens M (2021).
Life in Colour How Animals See the World., Random House.
Abstract:
Life in Colour How Animals See the World
Abstract.
Troscianko J, Nokelainen O, Skelhorn J, Stevens M (2021). Variable crab camouflage patterns defeat search image formation.
Commun Biol,
4(1).
Abstract:
Variable crab camouflage patterns defeat search image formation.
Understanding what maintains the broad spectrum of variation in animal phenotypes and how this influences survival is a key question in biology. Frequency dependent selection - where predators temporarily focus on one morph at the expense of others by forming a "search image" - can help explain this phenomenon. However, past work has never tested real prey colour patterns, and rarely considered the role of different types of camouflage. Using a novel citizen science computer experiment that presented crab "prey" to humans against natural backgrounds in specific sequences, we were able to test a range of key hypotheses concerning the interactions between predator learning, camouflage and morph. As predicted, switching between morphs did hinder detection, and this effect was most pronounced when crabs had "disruptive" markings that were more effective at destroying the body outline. To our knowledge, this is the first evidence for variability in natural colour patterns hindering search image formation in predators, and as such presents a mechanism that facilitates phenotypic diversity in nature.
Abstract.
Author URL.
2020
Lee V (2020). Avian cognition in a changing world.
Abstract:
Avian cognition in a changing world
Humans are altering the natural environment at an unprecedented rate, with profound consequences for non-human animals. However, species differ in how they respond to these ecological changes. Understanding the responses of wildlife to environmental change is vital to conserve biodiversity and mitigate anthropogenic impacts. Behaviour can often act as a rapid adaptation to ecological change, and is influenced by an organism’s ability to acquire and process information from their environment. Despite the importance of cognition in shaping behaviour, little is known about the role of cognition in allowing some species to thrive in human-dominated habitats. In this thesis, I examine how the cognitive abilities of wild jackdaws allow these birds to cope with the challenges of a rapidly changing world. Specifically, I focus on the need to navigate a dynamic social environment, and the need to learn about anthropogenic threats. Firstly, I investigate how jackdaws track their social environment by recognising conspecifics and their relationships. In Chapter 3, I demonstrate that jackdaws individually recognise the contact calls of their breeding partner, but I find no evidence of vocal discrimination beyond the pair bond. In Chapter 4, I use infidelity simulations to investigate whether jackdaws track changes to prevailing social relationships, although I find no evidence that jackdaws respond to relationship information in this experimental context. Secondly, I investigate how jackdaws’ cognitive abilities shape their behaviour during encounters with people, allowing birds to avoid danger whilst exploiting anthropogenic resources. I test the commonly-held preconception that jackdaws identify people carrying shotguns as dangerous (Chapter 5), but find no evidence that jackdaws use objects being carried by people to inform their escape decisions in this case. I also demonstrate that jackdaws learn socially about dangerous people (Chapter 6). Throughout these experiments, jackdaws differed considerably in their behaviour, which may influence individual success in anthropogenic habitats. In Chapter 7, I find that individual jackdaws differ consistently in their responses to human disturbance, but that these differences do not appear to impact reproductive success. Together, my findings highlight the importance of fundamental behavioural and cognitive research in predicting animals’ responses to environmental change.
Abstract.
Spaniol RL, Mendonça MDS, Hartz SM, Iserhard CA, Stevens M (2020). Discolouring the Amazon Rainforest: how deforestation is affecting butterfly coloration. Biodiversity and Conservation, 29(9-10), 2821-2838.
Stephenson JF, Stevens M, Troscianko J, Jokela J (2020). The Size, Symmetry, and Color Saturation of a Male Guppy's Ornaments Forecast His Resistance to Parasites.
AMERICAN NATURALIST,
196(5), 597-608.
Author URL.
2019
Green SD, Duarte RC, Kellett E, Alagaratnam N, Stevens M (2019). Colour change and behavioural choice facilitate chameleon prawn camouflage against different seaweed backgrounds.
Commun Biol,
2(1).
Abstract:
Colour change and behavioural choice facilitate chameleon prawn camouflage against different seaweed backgrounds.
Camouflage is driven by matching the visual environment, yet natural habitats are rarely uniform and comprise many backgrounds. Therefore, species often exhibit adaptive traits to maintain crypsis, including colour change and behavioural choice of substrates. However, previous work largely considered these solutions in isolation, whereas many species may use a combination of behaviour and appearance to facilitate concealment. Here we show that green and red chameleon prawns (Hippolyte varians) closely resemble their associated seaweed substrates to the vision of predatory fish, and that they can change colour to effectively match new backgrounds. Prawns also select colour-matching substrates when offered a choice. However, colour change occurs over weeks, consistent with seasonal changes in algal cover, whereas behavioural choice of matching substrates occurs in the short-term, facilitating matches within heterogeneous environments. We demonstrate how colour change and behaviour combine to facilitate camouflage against different substrates in environments varying spatially and temporally.
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Author URL.
Carter E (2019). Consequences of ship noise for camouflage, anti-predation, and movement in crabs.
Abstract:
Consequences of ship noise for camouflage, anti-predation, and movement in crabs.
The marine environment is becoming increasingly polluted, with unprecedented levels of anthropogenic noise changing the marine soundscape. Mounting evidence shows that exposure to this noise can cause numerous adverse effects across taxa. However, invertebrates, juveniles, and behaviours not dependent on acoustics have received relatively little attention. Furthermore, research into how individuals may cope with these pressures is lacking. I address these knowledge gaps through a series of laboratory-based playback experiments focussed on juvenile shore crabs (Carcinus maenas), using three noise treatments: ship noise, ambient underwater sounds (control), and ambient underwater sounds played at the same amplitude as the ship treatment (loud control).
In chapter 2, I examined the effects of ship noise on brightness change; a strategy employed by juvenile shore crabs to increase their level of camouflage and reduce predation risk. Individuals were repeatedly exposed to one of the aforementioned noise treatments for 8 weeks. Photographs of individuals, taken regularly throughout, were analysed using a predator vision model to determine the level of brightness change and camouflage in an ecologically relevant context. Ship noise reduced the overall brightness change and camouflage, though it did not affect the change in brightness per moult. The level of growth per moult was reduced by ship noise however, and the timing of moulting events was delayed. In chapter 3 I investigated the effects of noise on antipredator behaviour (using the response to a simulated predator) and locomotion, including the frequency of pausing and directionality of movement. By comparing the effects between individuals with varying levels of previous noise exposure, I also tested for signs of acclimatisation. Ship noise reduced the likelihood of individuals responding to a predator and increased their latency of response. Locomotion was not disrupted, but individuals moved away from ship noise, positioning themselves in quieter areas. These findings were consistent for all individuals, regardless of their previous level of noise exposure.
The negative consequences of anthropogenic noise in the marine environment are clearly not constrained to species or behaviours reliant on acoustics, as juvenile shore crabs exposed to ship noise suffered decreased levels of camouflage and reduced growth. Individuals also displayed maladaptive behavioural responses to a simulated predator when exposed to ship noise. There is no evidence that acclimatisation occurred, but individuals did attempt to physically avoid noisy areas. Loud natural sounds did not affect any behaviours studied, suggesting the type of noise is important in determining how individuals may be affected. Overall, this thesis shows that juvenile shore crabs suffer multiple negative effects from noise pollution, including the disruption of critical behaviours that are pervasive in the marine environment, with potential implications for survival.
Abstract.
Narasimha S, Nagornov KO, Menin L, Mucciolo A, Rohwedder A, Humbel BM, Stevens M, Thum AS, Tsybin YO, Vijendravarma RK, et al (2019). Drosophila melanogaster cloak their eggs with pheromones, which prevents cannibalism. PLoS Biology, 17
Stoddard MC, Hogan BG, Stevens M, Spottiswoode CN (2019). Higher-level pattern features provide additional information to birds when recognizing and rejecting parasitic eggs.
Philosophical Transactions of the Royal Society B: Biological Sciences,
374(1769).
Abstract:
Higher-level pattern features provide additional information to birds when recognizing and rejecting parasitic eggs
Despite a recent explosion of research on pattern recognition, in both neuroscience and computer vision, we lack a basic understanding of how most animals perceive and respond to patterns in the wild. Avian brood parasites and their hosts provide an ideal study system for investigating the mechanisms of pattern recognition. The cuckoo finch, Anomalospiza imberbis, and its host the tawny-flanked prinia, Prinia subflava, lay highly polymorphic eggs with a great deal of variation in colour and patterning, with the cuckoo finch capable of close egg mimicry. Behavioural experiments in Zambia have previously shown that prinias use colour and multiple ‘low-level’ (occurring in early stages of visual processing) pattern attributes, derived from spatial frequency analysis, when rejecting foreign eggs. Here, we explore the extent to which host birds might also use ‘higher-level’ pattern attributes, derived from a feature detection algorithm, to make rejection decisions. Using a SIFT-based pattern recognition algorithm, NATUREPATTERNMATCH, we show that hosts are more likely to reject a foreign egg if its higher-level pattern features—which capture information about the shape and orientation of markings—differ from those of the host eggs. A revised statistical model explains about 37% variance in egg rejection behaviour, and differences in colour, low-level and higher-level pattern features all predict rejection, accounting for 42, 44 and 14% of the explained variance, respectively. Thus, higher-level pattern features provide a small but measurable improvement to the original model and may be especially useful when colour and low-level pattern features provide hosts with little information. Understanding the relative importance of low- and higher-level pattern features is a valuable goal for future work on animal coloration, especially in the contexts of mimicry, camouflage and individual recognition. This article is part of the theme issue ‘The coevolutionary biology of brood parasitism: from mechanism to pattern’.
Abstract.
Hughes A, Liggins E, Stevens M (2019). Imperfect camouflage: how to hide in a variable world?. Proceedings of the Royal Society B: Biological Sciences, 286, 20190646-20190646.
Nokelainen O, Maynes R, Mynott S, Price N, Stevens M (2019). Improved camouflage through ontogenetic colour change confers reduced detection risk in shore crabs.
Functional Ecology,
33(4), 654-669.
Abstract:
Improved camouflage through ontogenetic colour change confers reduced detection risk in shore crabs
Animals from many taxa, from snakes and crabs to caterpillars and lobsters, change appearance with age, but the reasons why this occurs are rarely tested. We show the importance that ontogenetic changes in coloration have on the camouflage of the green shore crabs (Carcinus maenas), known for their remarkable phenotypic variation and plasticity in colour and pattern. In controlled conditions, we reared juvenile crabs of two shades, pale or dark, on two background types simulating different habitats for 10 weeks. In contrast to expectations for reversible colour change, crabs did not tune their background match to specific microhabitats, but instead, and regardless of treatment, all developed a uniform dark green phenotype. This parallels changes in shore crab appearance with age observed in the field. Next, we undertook a citizen science experiment at the Natural History Museum London, where human subjects (“predators”) searched for crabs representing natural colour variation from different habitats, simulating predator vision. In concert, crabs were not hardest to find against their original habitat, but instead, the dark green phenotype was hardest to detect against all backgrounds. The evolution of camouflage can be better understood by acknowledging that the optimal phenotype to hide from predators may change over the life history of many animals, including the utilization of a generalist camouflage strategy. A plain language summary is available for this article.
Abstract.
Briolat E, Zagrobelny M, Olsen C, Blount J, Stevens M (2019). No evidence of quantitative signal honesty across species of aposematic burnet moths (Lepidoptera: Zygaenidae). Journal of Evolutionary Biology, 32, 31-48.
Carter EE, Tregenza T, Stevens M (2019). Ship Noise Inhibits Colour Change, Camouflage, and Anti-Predator Behaviour in Shore Crabs.
Mynott S (2019). The impact of climate change on intertidal species, camouflage and predation.
Abstract:
The impact of climate change on intertidal species, camouflage and predation
To understand the impact of climate change on ecosystems we need to know not only how individual species will be affected, but also the relationships between them. Predator-prey relationships determine the structure and function of ecosystems worldwide, governing the abundance of populations, the distribution of different species within habitats and, ultimately, the composition of communities. Many predator-prey relationships are shifting as a result of environmental change, with climate change causing both mismatches in the abundance and distribution of species and changes in predator and prey behaviour. However, few studies have addressed how climate change might impact the interactions between species, particularly the development of anti-predator defences, which enable prey to limit their predation risk. One of the most widespread defences in nature is camouflage, with many species capable of changing colour to match their background to avoid being seen and eaten. The impact of climate change on this process is largely unknown, save for studies on species that exhibit seasonal changes in coloration. Using behavioural assays with predatory rock gobies (Gobius paganellus) and chameleon prawn prey (Hippolyte varians), I first demonstrate how background matching affects survival, shedding light on the fitness benefits of camouflage. Building on this fundamental understanding, this project explores how defensive coloration may be affected by anthropogenic climate change. Through a series of laboratory studies I test what impact ocean warming and ocean acidification have on the development of camouflage in intertidal crustaceans (chameleon prawns and common shore crabs, Carcinus maenas). Camouflage is modelled according to the visual systems of relevant predators, allowing us to understand what implications their coloration has for detectability, predation risk, and associated trophic links. Finally, this project investigates how camouflage can be applied to conservation and aquaculture. By rearing juvenile European lobster (Homarus gammarus) on different backgrounds, I show that they are capable of colour change for camouflage, as well as colour change throughout ontogeny. This capacity could be harnessed to help improve survival on release into the wild. As such, this thesis explores the fundamental science of camouflage, anthropogenic impacts on this process and its applications for conservation.
Abstract.
Stevens M, Ruxton GD (2019). The key role of behaviour in animal camouflage.
Biological Reviews,
94(1), 116-134.
Abstract:
The key role of behaviour in animal camouflage
Animal camouflage represents one of the most important ways of preventing (or facilitating) predation. It attracted the attention of the earliest evolutionary biologists, and today remains a focus of investigation in areas ranging from evolutionary ecology, animal decision-making, optimal strategies, visual psychology, computer science, to materials science. Most work focuses on the role of animal morphology per se, and its interactions with the background in affecting detection and recognition. However, the behaviour of organisms is likely to be crucial in affecting camouflage too, through background choice, body orientation and positioning; and strategies of camouflage that require movement. A wealth of potential mechanisms may affect such behaviours, from imprinting and self-assessment to genetics, and operate at several levels (species, morph, and individual). Over many years there have been numerous studies investigating the role of behaviour in camouflage, but to date, no effort to synthesise these studies and ideas into a coherent framework. Here, we review key work on behaviour and camouflage, highlight the mechanisms involved and implications of behaviour, discuss the importance of this in a changing world, and offer suggestions for addressing the many important gaps in our understanding of this subject.
Abstract.
Chan IZW, Stevens M, Todd PA (2019). pat-geom: a software package for the analysis of animal patterns.
Methods in Ecology and Evolution,
10(4), 591-600.
Abstract:
pat-geom: a software package for the analysis of animal patterns
Colour patterns often influence how animals interact with one another, but the ability of researchers to quantify pattern per se is hampered by a lack of easily accessible and user-friendly measurement software packages. We address this issue by releasing pat-geom, a free software package for use within ImageJ that allows users to measure seven properties of a pattern: (a) the shape of its markings, (b) the directionality in the shape of its markings, (c) the size of its markings, (d) the contrast of the pattern, (e) the distribution of its markings, (f) the directionality in the distribution of its markings, and (g) the randomness of the pattern. We provide examples of how pat-geom may be used, such as to visualise the “average pattern” of a population of animals, or to compare the patterns on two animals. Using data from two case studies, we also demonstrate pat-geom's ability to identify the specific aspects of an organism's pattern that match its background and to design artificial prey items that accurately resemble their model organism for use in predation experiments. pat-geom collates the tools to measure these seven diverse properties of animal colour patterns into one convenient, easy-to-use package. It can be employed in a wide range of studies on topics such as aposematism, camouflage and mimicry, and also has the potential to be applied to other research fields such as landscape ecology, botany and cellular biology.
Abstract.
2018
Walton OC, Stevens M (2018). Avian vision models and field experiments determine the survival value of peppered moth camouflage. Communications Biology, 1, 118-118.
Troscianko J, Skelhorn J, Stevens M (2018). Camouflage strategies interfere differently with observer search images.
Proceedings of the Royal Society B: Biological Sciences,
285(1886).
Abstract:
Camouflage strategies interfere differently with observer search images
Numerous animals rely on camouflage for defence. Substantial past work has identified the presence of multiple strategies for concealment, and tested the mechanisms underpinning how they work. These include background matching, D-RUP coloration to destroy target edges, and distractive markings that may divert attention from key target features. Despite considerable progress, work has focused on how camouflage types prevent initial detection by naive observers. However, predators will often encounter multiple targets over time, providing the opportunity to learn or focus attention through search images. At present, we know almost nothing about how camouflage types facilitate or hinder predator performance over repeated encounters. Here, we use experiments with human subjects searching for targets on touch screens with different camouflage strategies, and control the experience that subjects have with target types. We show that different camouflage strategies affect how subjects improve in detecting targets with repeated encounters, and how performance in detection of one camouflage type depends on experience of other strategies. In particular, disruptive coloration is effective at preventing improvements in camouflage breaking during search image formation, and experience with one camouflage type (distraction) can decrease the ability of subjects to switch to and from search images for new camouflage types (disruption). Our study is, to our knowledge, the first to show how the success of camouflage strategies depends on how they prevent initial and successive detection, and on predator experience of other strategies. This has implications for the evolution of prey phenotypes, how we assess the efficacy of defences, and predator-prey dynamics.
Abstract.
Troscianko J, Skelhorn J, Stevens M (2018). Camouflage strategies interfere differently with observer search images. Proceedings of the Royal Society B: Biological Sciences, 285, 20181386-20181386.
Badás EP, Martínez J, Rivero-de Aguilar J, Ponce C, Stevens M, Merino S (2018). Colour change in a structural ornament is related to individual quality, parasites and mating patterns in the blue tit.
Science of Nature,
105(1-2).
Abstract:
Colour change in a structural ornament is related to individual quality, parasites and mating patterns in the blue tit
Carry-over effects refer to processes that occur in one season and influence fitness in the following. In birds, two costly activities, namely reproduction and moult, are restricted to a small time window, and sometimes overlap. Thus, colour in newly moulted feathers is likely to be affected by the costs of reproduction. Using models of bird vision we investigated male colour change in a free-living population of blue tits (Cyanistes caeruleus) in three sampling occasions: spring 1, winter and spring 2. We related crown, tail, breast and cheek feather colouration after the moult (winter) to the intensity of infections by blood parasites during reproduction (spring 1). In the following spring (spring 2), we explored mating patterns with respect to changes in feather colour (springs 1 vs. 2). Males that were less intensely infected by the malaria parasite Plasmodium while breeding showed purer white cheek feathers in winter, which may indicate higher feather quality. Increased brightness in the white cheek was associated with better body condition during reproduction. In the following season, males with brighter cheeks paired with females that had noticeably brighter cheek patches compared to the male’s previous mate. These results suggest that the conditions experienced during reproduction are likely to affect moult and thus feather colouration, at least in the white patch. High quality individuals may allocate resources efficiently during reproduction increasing future reproductive success through variation in mating patterns. Carry-over effects from reproduction might extend not only to the non-breeding phase, but also to the following breeding season.
Abstract.
Nokelainen O, Stevens M, Caro T (2018). Colour polymorphism in the coconut crab (Birgus latro). Evolutionary Ecology, 32, 75-75.
Ligon RA, Diaz CD, Morano JL, Troscianko J, Stevens M, Moskeland A, Laman TG, Scholes E (2018). Evolution of correlated complexity in the radically different courtship signals of birds-of-paradise. PLoS Biology, 16, e2006962-e2006962.
Ligon RA, Diaz CD, Morano JL, Troscianko J, Stevens M, Moskeland A, Laman TG, Scholes E (2018). Evolution of correlated complexity in the radically different courtship signals of birds-of-paradise.
Gómez J, Ramo C, Troscianko J, Stevens M, Castro M, Pérez-Hurtado A, Liñán-Cembrano G, Amat JA (2018). Individual egg camouflage is influenced by microhabitat selection and use of nest materials in ground-nesting birds. Behavioral Ecology and Sociobiology, 72, 142-142.
Paul SC, Stevens M, Burton J, Pell JK, Birkett MA, Blount JD (2018). Invasive egg predators and food availability interactively affect maternal investment in egg chemical defense.
Frontiers in Ecology and Evolution,
6(JAN).
Abstract:
Invasive egg predators and food availability interactively affect maternal investment in egg chemical defense
Invasive species commonly predate the offspring of native species and eggs are the life stage most vulnerable to this predation. In many species with no maternal care, females can alter the phenotype of eggs to protect them, for instance through chemical defense. In ladybirds egg alkaloids deter predators, including invasive predatory species of ladybirds, but conversely may attract cannibals who benefit from the consumption of eggs with higher alkaloid levels. Invasive predators tend to be more abundant where resources are also abundant, but in high resource environments the maternal fitness benefits of sibling cannibalism are low. Consequently this presents a conflict for female ladybirds between the different factors that influence egg alkaloid level, as protecting her eggs from predators might come with the cost of inadvertently encouraging within-clutch cannibalism under circumstances where it is not beneficial. We investigated how the ladybird Adalia bipunctata addresses this trade-off experimentally, by measuring the quantity of alkaloids in eggs laid by ladybirds in environments that differed in levels of resource availability and perceived predation risk from an invasive predator Harmonia axyridis. Females did lay eggs with higher egg alkaloid levels under poor resource conditions, but only when predator cues were absent. The resulting negative correlation between egg number and egg alkaloid level under poor resource conditions indicates a trade-off between these two attributes of maternal investment, mediated by female response to offspring predation risk. This implies that selection pressures on mothers to adaptively adjust the risk of siblicide may outweigh the need to protect offspring from interspecific predation. Our results demonstrate that maternal effects are an important aspect of species' responses to invasive predators, and highlight the value of studying maternal effects in the context of the multifaceted environments in which they occur.
Abstract.
Gómez J, Ramo C, Stevens M, Liñán-Cembrano G, Rendón MA, Troscianko J, Amat JA (2018). Latitudinal variation in biophysical characteristics of avian eggshells to cope with differential effects of solar radiation. Ecology and Evolution, 8, 8019-8029.
Paul SC, Stevens M, Pell JK, Birkett MA, Blount JD (2018). Parental phenotype not predator cues influence egg warning coloration and defence levels.
Animal Behaviour,
140, 177-186.
Abstract:
Parental phenotype not predator cues influence egg warning coloration and defence levels
In species that advertise their toxicity to predators through visual signals, there is considerable variation among individuals in both signal appearance and levels of defence. Parental effects, a type of nongenetic inheritance, may play a key role in creating and maintaining this within-species diversity in aposematic signals; however, a comprehensive test of this notion is lacking. Using the ladybird Adalia bipunctata, we assessed how egg coloration and defence level (concentration of the toxic alkaloid (-)-adaline) is influenced both by simulated predation risk in the egg-laying environment and by parental phenotype (coloration and toxin level). We found that egg toxin level and colour were predicted by parental phenotype but were not altered in response to cues of egg predators. Egg luminance (lightness) was positively correlated with paternal elytral luminance, while maternal toxin level positively predicted egg toxin level. In response to egg predator cues, ladybird mothers altered the timing of laying and total egg number, but not egg toxin level or colour. It appears therefore that in A. bipunctata variation between individuals of the same morph in the colour and toxin level of the eggs they lay, that is, egg aposematic phenotype, is more strongly influenced by individual variation in parental aposematic traits than by environmental cues of egg predation risk. Furthermore, these results provide the first indication that, in a warningly coloured species, male coloration may play a dual role as predator deterrent and indicator of paternal quality, influencing maternal investment in offspring.
Abstract.
Niu Y, Sun H, Stevens M (2018). Plant camouflage: ecology, evolution, and implications. Trends in Ecology and Evolution, 33, 608-618.
Smithers SP, Rooney R, Wilson A, Stevens M (2018). Rock pool fish use a combination of colour change and substrate choice to improve camouflage.
Animal Behaviour,
144, 53-65.
Abstract:
Rock pool fish use a combination of colour change and substrate choice to improve camouflage
Camouflage can be achieved by both morphological (e.g. colour, brightness and pattern change) and behavioural (e.g. substrate preference) means. Much of the research on behavioural background matching has been conducted on species with fixed coloration and body patterns, while less is known about the role background choice plays in species capable of rapid (within minutes or seconds) colour change. One candidate species is the rock goby, Gobius paganellus, a common rock pool fish capable of rapid changes in colour and brightness when placed on different backgrounds. However, their ability to match different backgrounds is not unbounded, with some colours and brightness being easier to match than others, thus raising the possibility that gobies may use behavioural background matching to make up for their limited ability to match certain backgrounds. We used digital image analysis and a model of predator vision to investigate the ability of rock gobies to match chromatic (beige and greenish-grey) and achromatic (varying brightness) backgrounds. We then conducted choice experiments to determine whether gobies exhibited a behavioural preference for the backgrounds they were best at matching. Gobies rapidly changed their colour and brightness when placed on the different backgrounds. However, the level of camouflage differed between backgrounds: fish were better at matching beige than greenish-grey, and darker than lighter backgrounds. When given the choice, gobies displayed a behavioural preference for the backgrounds they were best at matching. Our findings therefore show that rock gobies, and probably other animals, use a combination of morphological and behavioural means to achieve camouflage and in doing so mitigate limitations in either approach alone.
Abstract.
Briolat E, Zagrobelny M, Olsen, CE, Blount J, Stevens M (2018). Sex differences but no evidence of quantitative honesty in the warning signals of six-spot burnet moths (Zygaena filipendulae L.). Evolution, 77, 1460-1474.
Duarte RC, Stevens M, Flores AAV (2018). The adaptive value of camouflage and colour change in a polymorphic prawn. Scientific Reports, 8, 16028-16028.
Dell'Aglio DD, Troscianko J, McMillan WO, Stevens M, Jiggins CD (2018). The appearance of mimetic Heliconius butterflies to predators and conspecifics.
Evolution,
72(10), 2156-2166.
Abstract:
The appearance of mimetic Heliconius butterflies to predators and conspecifics
Adaptive coloration is under conflicting selection pressures: choosing potential mates and warning signaling against visually guided predators. Different elements of the color signal may therefore be tuned by evolution for different functions. We investigated how mimicry in four pairs of Heliconius comimics is potentially seen both from the perspective of butterflies and birds. Visual sensitivities of eight candidate avian predators were predicted through genetic analysis of their opsin genes. Using digital image color analysis, combined with bird and butterfly visual system models, we explored how predators and conspecifics may visualize mimetic patterns. Ultraviolet vision (UVS) birds are able to discriminate between the yellow and white colors of comimics better than violet vision (VS) birds. For Heliconius vision, males and females differ in their ability to discriminate comimics. Female vision and red filtering pigments have a significant effect on the perception of the yellow forewing band and the red ventral forewing pattern. A behavioral experiment showed that UV cues are used in mating behavior; removal of such cues was associated with an increased tendency to approach comimics as compared to conspecifics. We have therefore shown that visual signals can act to both reduce the cost of confusion in courtship and maintain the advantages of mimicry.
Abstract.
2017
Duarte RC, Flores AAV, Stevens M (2017). Camouflage through colour change: mechanisms, adaptive value, and ecological significance. Philosophical Transactions B: Biological Sciences, 372, 20160342-20160342.
Niu Y, Chen Z, Stevens M, Sun H (2017). Divergence in cryptic leaf colour provides local camouflage in an alpine plant. Proceedings of the Royal Society B: Biological Sciences, 284, 20171654-20171654.
Arenas LM, Stevens M (2017). Diversity in warning coloration is easily recognised by avian predators. Journal of Evolutionary Biology, 30, 1288-1302.
Caves EM, Stevens M, Spottiswoode CN (2017). Does coevolution with a shared parasite drive hosts to partition their defences among species?.
Proceedings of the Royal Society B: Biological Sciences,
284(1854), 20170272-20170272.
Abstract:
Does coevolution with a shared parasite drive hosts to partition their defences among species?
© 2017 the Authors. When mimicry imposes costs on models, selection may drive the model’s phenotype to evolve away from its mimic. For example, brood parasitism often drives hosts to diversify in egg appearance among females within a species, making mimetic parasitic eggs easier to detect. However, when a single parasite species exploits multiple host species, parasitism could also drive host egg evolution away from other co-occurring hosts, to escape susceptibility to their respective mimics. This hypothesis predicts that sympatric hosts of the same parasite should partition egg phenotypic space (defined by egg colour, luminance and pattern) among species to avoid one another. We show that eggs of warbler species parasitized by the cuckoo finch Anomalospiza imberbis in Zambia partition phenotypic space much more distinctly than do eggs of sympatric but unparasitized warblers. Correspondingly, cuckoo finch host-races better match their own specialist host than other local host species. In the weaver family, parasitized by the diederik cuckoo Chrysococcyx caprius, by contrast, parasitized species were more closely related and overlapped extensively in phenotypic space; correspondingly, cuckoos did not match their own host better than others. These results suggest that coevolutionary arms races between hosts and parasites may be shaped by the wider community context in which they unfold.
Abstract.
Geltsch N, Moskat C, Elek Z, Ban M, Stevens M (2017). Egg spotting pattern in the common cuckoo and its great reed warbler host: a century perspective. Biological Journal of the Linnean Society, 121, 50-50.
Badás EP, Martínez J, Rivero-de Aguilar J, Stevens M, van der Velde M, Komdeur J, Merino S (2017). Eggshell pigmentation in the blue tit: male quality matters. Behavioral Ecology and Sociobiology, 71, 57-57.
Stevens M, Troscianko J, Wilson-Aggarwal JK, Spottiswoode CN (2017). Improvement of individual camouflage through background choice in ground-nesting birds. Nature Ecology and Evolution, 1, 1325-1325.
Koski T-M, Lindstedt C, Klemola T, Troscianko J, Mäntylä E, Tyystjärvi E, Stevens M, Helander M, Laaksonen T (2017). Insect herbivory may cause changes in the visual properties of leaves and affect the camouflage of herbivores to avian predators. Behavioral Ecology and Sociobiology, 31, 97-97.
Troscianko J, Skelhorn J, Stevens M (2017). Quantifying camouflage: how to predict detectability from appearance. BMC Evolutionary Biology, 17, 7-7.
Troscianko J, Wilson-Aggarwal J, Griffiths D, Spottiswoode CN, Stevens M (2017). Relative advantages of dichromatic and trichromatic color vision in camouflage breaking. Behavioral Ecology, 28, 556-556.
Smithers SP, Wilson A, Stevens M (2017). Rock pool gobies change their body pattern in response to background features. Biological Journal of the Linnean Society, 121, 109-121.
Pérez-Rodríguez L, Jovani R, Stevens M (2017). Shape matters: animal colour patterns as signals of individual quality. Proceedings of the Royal Society B: Biological Sciences, 284, 20162446-20162446.
Cuthill IC, Allen WL, Arbuckle K, Caspers B, Chaplin G, Hauber ME, Hill GE, Jablonski NG, Jiggins CD, Kelber A, et al (2017). The biology of color.
Science,
357(6350), eaan0221-eaan0221.
Abstract:
The biology of color
© 2017, American Association for the Advancement of Science. All rights reserved. Coloration mediates the relationship between an organism and its environment in important ways, including social signaling, antipredator defenses, parasitic exploitation, thermoregulation, and protection from ultraviolet light, microbes, and abrasion. Methodological breakthroughs are accelerating knowledge of the processes underlying both the production of animal coloration and its perception, experiments are advancing understanding of mechanism and function, an d measurements of color collected noninvasively and at a global scale are opening windows to evolutionary dynamics more generally. Here we provide a roadmap of these advances and identify hitherto unrecognized challenges for this multi- and interdisciplinary field.
Abstract.
Nokelainen O, Hubbard N, Lown AE, Wood LE, Stevens M (2017). Through predators’ eyes - phenotype-environment associations in shore crab coloration at different spatial scales. Biological Journal of the Linnean Society, 122, 738-751.
2016
Dell'aglio DD, Stevens M, Jiggins CD (2016). Avoidance of an aposematically coloured butterfly by wild birds in a tropical forest.
Ecological Entomology,
41(5), 627-632.
Abstract:
Avoidance of an aposematically coloured butterfly by wild birds in a tropical forest
1. Birds are considered to be the primary selective agents for warning colouration in butterflies, and select for aposematic mimicry by learning to avoid brightly coloured prey after unpleasant experiences. It has long been thought that bright colouration plays an important role in promoting the avoidance of distasteful prey by birds. 2. The hypothesis that warning colouration facilitates memorability and promotes predator avoidance was tested by means of a field experiment using distasteful model butterflies. Artificial butterflies with a Heliconius colour pattern unknown to local birds were generated using bird vision models, either coloured or achromatic, and hung in tree branches in a tropical forest. Two sequential trials were conducted at each site to test avoidance by naïve and experienced predators. 3. There was a significant reduction in predation in the second trial. Also, coloured models were attacked less than achromatic models. Specifically, coloured butterflies were attacked significantly less in the second trial, but there was no significant decrease in predation on achromatic models. 4. The present results imply an important role for colour in enhancing aversion of aposematic butterflies. It has also been demonstrated that previous experience of distasteful prey can lead to enhanced avoidance in subsequent trials, supporting mimicry theory.
Abstract.
Medina I, Troscianko J, Stevens M, Langmore NE (2016). Brood Parasitism is Linked to Egg Pattern Diversity within and among Species of Australian Passerines.
Am Nat,
187(3), 351-362.
Abstract:
Brood Parasitism is Linked to Egg Pattern Diversity within and among Species of Australian Passerines.
Bird eggs show striking diversity in color and pattern. One explanation for this is that interactions between avian brood parasites and their hosts drive egg phenotype evolution. Brood parasites lay their eggs in the nests of other species, their hosts. Many hosts defend their nests against parasitism by rejecting foreign eggs, which selects for parasite eggs that mimic those of the host. In theory, this may in turn select for changes in host egg phenotypes over time to facilitate discrimination of parasite eggs. Here, we test for the first time whether parasitism by brood parasites has led to increased divergence in egg phenotype among host species. Using Australian host and nonhost species and objective measures of egg color and pattern, we show that (i) hosts of brood parasites have higher within-species variation in egg pattern than nonhosts, supporting previous findings in other systems, and (ii) host species have diverged more in their egg patterns than nonhost species after controlling for divergence time. Overall, our results suggest that brood parasitism has played a significant role in the evolution of egg diversity and that these effects are evident, not only within species, but also among species.
Abstract.
Author URL.
Troscianko J, Wilson-Aggarwal J, Stevens M, Spottiswoode CN (2016). Camou age predicts survival in ground-nesting birds. Scientific Reports, 6
Nokelainen O, Stevens M (2016). Camouflage.
Current Biology,
26(14), R654-R656.
Abstract:
Camouflage
© 2016 Elsevier Ltd Nokelainen and Stevens introduce strategies of concealment among animals and plants.
Abstract.
Stevens M (2016).
Cheats and Deceits How Animals and Plants Exploit and Mislead., Oxford University Press.
Abstract:
Cheats and Deceits How Animals and Plants Exploit and Mislead
Abstract.
Stevens M (2016). Color Change, Phenotypic Plasticity, and Camouflage. Frontiers in Ecology and Evolution, 4
Wilson-Aggarwal, Troscianko, Stevens M, Spottiswoode CN (2016). Escape distance in ground-nesting birds differs with individual level of camouflage. The American Naturalist, 188, 231-231.
Marshall KLA, Philpott KE, Stevens M (2016). Microhabitat choice in island lizards enhances camou age against avian predators. Scientific Reports, 6
Troscianko, Wilson-Aggarwal, Spottiswoode CN, Stevens M (2016). Nest covering in plovers: how modifying the visual environment influences egg camouflage. Ecology and Evolution, 6, 7536-7545.
Chan IZW, Stevens M, Todd PA (2016). Quantifying shell pattern and colour polymorphism in the button snail Umbonium vestiarium (Mollusca: Gastropoda: Trochacea) and comparing morph frequencies between two populations using the Mantel test.
Raffles Bulletin of Zoology,
2016, 22-32.
Abstract:
Quantifying shell pattern and colour polymorphism in the button snail Umbonium vestiarium (Mollusca: Gastropoda: Trochacea) and comparing morph frequencies between two populations using the Mantel test
© National University of Singapore.The button snail, Umbonium vestiarium, is a small intertidal snail with a wide geographic range. It displays a high degree of phenotypic polymorphism in shell pattern and colour, but this is poorly researched. To examine morph frequencies, we collected 2845 empty shells and 207 live individuals from Tanah Merah and Changi beaches in Singapore. From these, two 2-dimensional matrices of morph types were constructed. The ‘Comprehensive Matrix’ classifies individuals based on 22 distinct band designs (which differ based on the presence, number and continuity of bands) and nine whorl designs (which differ based on marking colour and type: pink or brown uniform, dotted, wavy-lined, striped and with or without stripes on the lowest whorl). In total, 96 different morphs were observed. These morphs were then pooled into morph-groups to form a 5 × 5 ‘Condensed Matrix’ that represents a useful approach for quickly categorising live individuals and comparing morph frequencies among populations. Using the Mantel test on the Condensed Matrices, morph frequencies for the two populations studied here were statistically compared and found to be similar (p-values of 0.001 for empty shells and 0.01 for live snails). Together, these two matrices form a basis for future investigation into the mechanisms behind the maintenance of phenotypic polymorphism in Umbonium vestiarium.
Abstract.
Duarte RC, Stevens M, Flores AAV (2016). Shape, colour plasticity, and habitat use indicate morph-specific camouflage strategies in a marine shrimp. BMC Evolutionary Biology, 16, 218-218.
Caro T, Sherratt TN, Stevens M (2016). The ecology of multiple colour defences. Evolutionary Ecology, 30, 797-809.
Stevens M (2016). When to attack defended prey? a comment on Skelhorn et al. Behavioral Ecology, 27(4).
2015
Stevens M (2015). Anti-Predator Coloration and Behaviour: a Longstanding Topic with Many Outstanding Questions.
CURRENT ZOOLOGY,
61(4), 702-707.
Author URL.
Flores EE, Stevens M, Moore AJ, Rowland HM, Blount JD (2015). Body size but not warning signal luminance influences predation risk in recently metamorphosed poison frogs.
Ecology and evolution,
5(20), 4603-4616.
Abstract:
Body size but not warning signal luminance influences predation risk in recently metamorphosed poison frogs.
During early development, many aposematic species have bright and conspicuous warning appearance, but have yet to acquire chemical defenses, a phenotypic state which presumably makes them vulnerable to predation. Body size and signal luminance in particular are known to be sensitive to variation in early nutrition. However, the relative importance of these traits as determinants of predation risk in juveniles is not known. To address this question, we utilized computer-assisted design (CAD) and information on putative predator visual sensitivities to produce artificial models of postmetamorphic froglets that varied in terms of body size and signal luminance. We then deployed the artificial models in the field and measured rates of attack by birds and unknown predators. Our results indicate that body size was a significant predictor of artificial prey survival. Rates of attack by bird predators were significantly higher on smaller models. However, predation by birds did not differ between artificial models of varying signal luminance. This suggests that at the completion of metamorphosis, smaller froglets may be at a selective disadvantage, potentially because predators can discern they have relatively low levels of chemical defense compared to larger froglets. There is likely to be a premium on efficient foraging, giving rise to rapid growth and the acquisition of toxins from dietary sources in juvenile poison frogs.
Abstract.
Burriss R, Troscianko J, Lovell PG, Fulford AJC, Stevens M, Quigley R, Payne J, Saxton TK, Rowland HM (2015). Changes in Women’s Facial Skin Color over the Ovulatory Cycle are Not Detectable by the Human Visual System. PLoS ONE, 10, e0130093-e0130093.
Stevens M, Arenas LM, Lown AE (2015). Color in camouflage, mimicry, and warning signals. In (Ed) Handbook of Color Psychology, 357-376.
Marshall KLA, Philpot KE, Stevens M (2015). Conspicuous male coloration impairs survival against avian predators in Aegean wall lizards, Podarcis erhardii.
Ecology and evolution,
5(18), 4115-4131.
Abstract:
Conspicuous male coloration impairs survival against avian predators in Aegean wall lizards, Podarcis erhardii.
Animal coloration is strikingly diverse in nature. Within-species color variation can arise through local adaptation for camouflage, sexual dimorphism and conspicuous sexual signals, which often have conflicting effects on survival. Here, we tested whether color variation between two island populations of Aegean wall lizards (Podarcis erhardii) is due to sexual dimorphism and differential survival of individuals varying in appearance. On both islands, we measured attack rates by wild avian predators on clay models matching the coloration of real male and female P. erhardii from each island population, modeled to avian predator vision. Avian predator attack rates differed among model treatments, although only on one island. Male-colored models, which were more conspicuous against their experimental backgrounds to avian predators, were accordingly detected and attacked more frequently by birds than less conspicuous female-colored models. This suggests that female coloration has evolved primarily under selection for camouflage, whereas sexually competing males exhibit costly conspicuous coloration. Unexpectedly, there was no difference in avian attack frequency between local and non-local model types. This may have arisen if the models did not resemble lizard coloration with sufficient precision, or if real lizards behaviorally choose backgrounds that improve camouflage. Overall, these results show that sexually dimorphic coloration can affect the risk of predator attacks, indicating that color variation within a species can be caused by interactions between natural and sexual selection. However, more work is needed to determine how these findings depend on the island environment that each population inhabits.
Abstract.
Troscianko J, Stevens M, Skelhorn J (2015). Does Disruptive Camouflage Disrupt Search Image Formation?.
PERCEPTION,
44(10), 1239-1240.
Author URL.
Stevens M (2015). Evolutionary Ecology: Insect Mothers Control Their Egg Colours.
Curr Biol,
25(17), R755-R757.
Abstract:
Evolutionary Ecology: Insect Mothers Control Their Egg Colours.
Animal egg coloration has long provided a valuable testing ground for evolutionary ideas. A new study shows that female stink bugs can flexibly control the colour of their eggs depending on the prevailing conditions, including for protection from ultraviolet light.
Abstract.
Author URL.
Caves EM, Stevens M, Iversen ES, Spottiswoode CN (2015). Hosts of avian brood parasites have evolved egg signatures with elevated information content.
Proc Biol Sci,
282(1810).
Abstract:
Hosts of avian brood parasites have evolved egg signatures with elevated information content.
Hosts of brood-parasitic birds must distinguish their own eggs from parasitic mimics, or pay the cost of mistakenly raising a foreign chick. Egg discrimination is easier when different host females of the same species each lay visually distinctive eggs (egg 'signatures'), which helps to foil mimicry by parasites. Here, we ask whether brood parasitism is associated with lower levels of correlation between different egg traits in hosts, making individual host signatures more distinctive and informative. We used entropy as an index of the potential information content encoded by nine aspects of colour, pattern and luminance of eggs of different species in two African bird families (Cisticolidae parasitized by cuckoo finches Anomalospiza imberbis, and Ploceidae by diederik cuckoos Chrysococcyx caprius). Parasitized species showed consistently higher entropy in egg traits than did related, unparasitized species. Decomposing entropy into two variation components revealed that this was mainly driven by parasitized species having lower levels of correlation between different egg traits, rather than higher overall levels of variation in each individual egg trait. This suggests that irrespective of the constraints that might operate on individual egg traits, hosts can further improve their defensive 'signatures' by arranging suites of egg traits into unpredictable combinations.
Abstract.
Author URL.
Troscianko J, Stevens M (2015). Image Calibration and Analysis Toolbox – a free software suite for objectively measuring reflectance, colour and pattern. Methods in Ecology and Evolution, 6, 1320-1331.
Marshall KLA, Philpot KE, Damas-Moreira I, Stevens M (2015). Intraspecific Colour Variation among Lizards in Distinct Island Environments Enhances Local Camouflage.
PLoS One,
10(9).
Abstract:
Intraspecific Colour Variation among Lizards in Distinct Island Environments Enhances Local Camouflage.
Within-species colour variation is widespread among animals. Understanding how this arises can elucidate evolutionary mechanisms, such as those underlying reproductive isolation and speciation. Here, we investigated whether five island populations of Aegean wall lizards (Podarcis erhardii) have more effective camouflage against their own (local) island substrates than against other (non-local) island substrates to avian predators, and whether this was linked to island differences in substrate appearance. We also investigated whether degree of local substrate matching varied among island populations and between sexes. In most populations, both sexes were better matched against local backgrounds than against non-local backgrounds, particularly in terms of luminance (perceived lightness), which usually occurred when local and non-local backgrounds were different in appearance. This was found even between island populations that historically had a land connection and in populations that have been isolated relatively recently, suggesting that isolation in these distinct island environments has been sufficient to cause enhanced local background matching, sometimes on a rapid evolutionary time-scale. However, heightened local matching was poorer in populations inhabiting more variable and unstable environments with a prolonged history of volcanic activity. Overall, these results show that lizard coloration is tuned to provide camouflage in local environments, either due to genetic adaptation or changes during development. Yet, the occurrence and extent of selection for local matching may depend on specific conditions associated with local ecology and biogeographic history. These results emphasize how anti-predator adaptations to different environments can drive divergence within a species, which may contribute to reproductive isolation among populations and lead to ecological speciation.
Abstract.
Author URL.
Stevens M, Broderick AC, Godley BJ, Lown AE, Troscianko J, Weber N, Weber SB (2015). Phenotype-environment matching in sand fleas.
Biol Lett,
11(8).
Abstract:
Phenotype-environment matching in sand fleas.
Camouflage is perhaps the most widespread anti-predator strategy in nature, found in numerous animal groups. A long-standing prediction is that individuals should have camouflage tuned to the visual backgrounds where they live. However, while several studies have demonstrated phenotype-environment associations, few have directly shown that this confers an improvement in camouflage, particularly with respect to predator vision. Here, we show that an intertidal crustacean, the sand flea (Hippa testudinaria), has coloration tuned to the different substrates on which it occurs when viewed by potential avian predators. Individual sand fleas from a small, oceanic island (Ascension) matched the colour and luminance of their own beaches more closely than neighbouring beaches to a model of avian vision. Based on past work, this phenotype-environment matching is likely to be driven through ontogenetic changes rather than genetic adaptation. Our work provides some of the first direct evidence that animal coloration is tuned to provide camouflage to prospective predators against a range of visual backgrounds, in a population of animals occurring over a small geographical range.
Abstract.
Author URL.
Arenas LM, Walter D, Stevens M (2015). Signal honesty and predation risk among a closely related group of aposematic species. Scientific Reports, 5, 11021-11021.
Ruxton GD, Stevens M (2015). The evolutionary ecology of decorating behaviour.
Biol Lett,
11(6).
Abstract:
The evolutionary ecology of decorating behaviour.
Many animals decorate themselves through the accumulation of environmental material on their exterior. Decoration has been studied across a range of different taxa, but there are substantial limits to current understanding. Decoration in non-humans appears to function predominantly in defence against predators and parasites, although an adaptive function is often assumed rather than comprehensively demonstrated. It seems predominantly an aquatic phenomenon-presumably because buoyancy helps reduce energetic costs associated with carrying the decorative material. In terrestrial examples, decorating is relatively common in the larval stages of insects. Insects are small and thus able to generate the power to carry a greater mass of material relative to their own body weight. In adult forms, the need to be lightweight for flight probably rules out decoration. We emphasize that both benefits and costs to decoration are rarely quantified, and that costs should include those associated with collecting as well as carrying the material.
Abstract.
Author URL.
Hughes AE, Magor-Elliott RS, Stevens M (2015). The role of stripe orientation in target capture success. Frontiers in Zoology, 12, 17-17.
2014
Salazar PA, Stevens M, Jones RT, Ogilvie I, Jiggins CD (2014). A field test for frequency-dependent selection on mimetic colour patterns in Heliconius butterflies.
Stevens M, Lown AE, Wood LE (2014). Camouflage and individual variation in shore crabs (Carcinus maenas) from different habitats.
PLoS One,
9(12).
Abstract:
Camouflage and individual variation in shore crabs (Carcinus maenas) from different habitats.
Camouflage is widespread throughout the natural world and conceals animals from predators in a vast range of habitats. Because successful camouflage usually involves matching aspects of the background environment, species and populations should evolve appearances tuned to their local habitat, termed phenotype-environment associations. However, although this has been studied in various species, little work has objectively quantified the appearances of camouflaged animals from different habitats, or related this to factors such as ontogeny and individual variation. Here, we tested for phenotype-environment associations in the common shore crab (Carcinus maenas), a species highly variable in appearance and found in a wide range of habitats. We used field surveys and digital image analysis of the colors and patterns of crabs found in four locations around Cornwall in the UK to quantify how individuals vary with habitat (predominantly rockpool, mussel bed, and mudflat). We find that individuals from sites comprising different backgrounds show substantial differences in several aspects of color and pattern, and that this is also dependent on life stage (adult or juvenile). Furthermore, the level of individual variation is dependent on site and life stage, with juvenile crabs often more variable than adults, and individuals from more homogenous habitats less diverse. Ours is the most comprehensive study to date exploring phenotype-environment associations for camouflage and individual variation in a species, and we discuss the implications of our results in terms of the mechanisms and selection pressures that may drive this.
Abstract.
Author URL.
Kang C, Stevens M, Moon J-Y, Lee S-I, Jablonski PG (2014). Camouflage through behavior in moths: the role of background matching and disruptive coloration. Behavioral Ecology
Allen WL, Stevens M, Higham JP (2014). Character displacement of Cercopithecini primate visual signals.
Nat Commun,
5Abstract:
Character displacement of Cercopithecini primate visual signals.
Animal visual signals have the potential to act as an isolating barrier to prevent interbreeding of populations through a role in species recognition. Within communities of competing species, species recognition signals are predicted to undergo character displacement, becoming more visually distinctive from each other; however, this pattern has rarely been identified. Using computational face recognition algorithms to model primate face processing, we demonstrate that the face patterns of guenons (tribe: Cercopithecini) have evolved under selection to become more visually distinctive from those of other guenon species with whom they are sympatric. The relationship between the appearances of sympatric species suggests that distinguishing conspecifics from other guenon species has been a major driver of diversification in guenon face appearance. Visual signals that have undergone character displacement may have had an important role in the tribe's radiation, keeping populations that became geographically separated reproductively isolated on secondary contact.
Abstract.
Author URL.
Arenas LM, Troscianko J, Stevens M (2014). Color contrast and stability as key elements for effective warning signals. Frontiers in Ecology and Evolution, 2
Stevens M, Lown AE, Wood LE (2014). Colour change and camouflage in juvenile shore crabs Carcinus maenas. Frontiers in Ecology and Evolution, 2
Stevens M (2014). Confusion and illusion: understanding visual traits and behavior. A comment on Kelley and Kelley. Behavioral Ecology, 25(3), 464-465.
Stevens M, Ruxton GD (2014). Do animal eyespots really mimic eyes?.
Current Zoology,
60(1), 26-36.
Abstract:
Do animal eyespots really mimic eyes?
The diversity of anti-predator adaptations in the natural world has long been an active area of research in evolutionary and behavioural biology. A common visually-obvious feature found on prey are 'eyespots', being approximately circular markings often with concentric rings and conspicuous colours. These are found on a range of animals, especially adult and larval Lepidoptera and fish. One of the most widespread functions of eyespots seems to be to intimidate or startle predators: delaying, preventing or halting an attack. However, while the fact that they can influence predators in this way is uncontroversial, the mechanism(s) behind why they are effective is debated. Traditionally, they have been assumed to work by mimicking the eyes of the predator's own enemies, and much research in this field is conducted under the implicit or explicit assumption that this theory is correct. However, eyespots might work simply by being highly salient stimuli that promote sensory overload, biases, or neophobic reactions in predators. A range of recent studies has aimed to test these alternatives. Here, we critically evaluate this work and what it tells us about the mechanisms underlying eyespot function. We conclude that although eye mimicry is plausible, there remains a lack of evidence to support it and most observations are at least equally consistent with alternative mechanisms. Finally, we also discuss how the debate can be resolved. © 2014 Current Zoology.
Abstract.
Stevens M (2014). Evolution: predator versus parasite.
Curr Biol,
24(10), R388-R390.
Abstract:
Evolution: predator versus parasite.
Both predators and brood parasites can be major threats to the reproduction of many birds. A new study shows that some cuckoo chicks can help deter nest predators, potentially improving host reproductive success when predation risks are high.
Abstract.
Author URL.
Winters AE, Stevens M, Mitchell C, Blomberg SP, Blount JD (2014). Maternal effects and warning signal honesty in eggs and offspring of an aposematic ladybird beetle. Functional Ecology, 28(5), 1187-1196.
Hughes AE, Stevens M, Tolhurst DJ (2014). Methods and mechanisms of motion dazzle.
I-PERCEPTION,
5(5).
Author URL.
Hughes AE, Troscianko J, Stevens M (2014). Motion dazzle and the effects of target patterning on capture success. BMC Evolutionary Biology, 14
Hughes AE, Troscianko J, Stevens M (2014). Motion dazzle and the effects of target patterning on capture success.
BMC Evol Biol,
14Abstract:
Motion dazzle and the effects of target patterning on capture success.
BACKGROUND: Stripes and other high contrast patterns found on animals have been hypothesised to cause "motion dazzle", a type of defensive coloration that operates when in motion, causing predators to misjudge the speed and direction of object movement. Several recent studies have found some support for this idea, but little is currently understood about the mechanisms underlying this effect. Using humans as model 'predators' in a touch screen experiment we investigated further the effectiveness of striped targets in preventing capture, and considered how stripes compare to other types of patterning in order to understand what aspects of target patterning are important in making a target difficult to capture. RESULTS: We find that striped targets are among the most difficult to capture, but that other patterning types are also highly effective at preventing capture in this task. Several target types, including background sampled targets and targets with a 'spot' on were significantly easier to capture than striped targets. We also show differences in capture attempt rates between different target types, but we find no differences in learning rates between target types. CONCLUSIONS: We conclude that striped targets are effective in preventing capture, but are not uniquely difficult to catch, with luminance matched grey targets also showing a similar capture rate. We show that key factors in making capture easier are a lack of average background luminance matching and having trackable 'features' on the target body. We also find that striped patterns are attempted relatively quickly, despite being difficult to catch. We discuss these findings in relation to the motion dazzle hypothesis and how capture rates may be affected more generally by pattern type.
Abstract.
Author URL.
Stevens M, Lown AE, Denton AM (2014). Rockpool gobies change colour for camouflage. PLoS One, 9
Marshall KLA, Stevens M (2014). Wall lizards display conspicuous signals to conspecifics and reduce detection by avian predators. Behavioral Ecology
2013
Walker LK, Stevens M, Karadaş F, Kilner RM, Ewen JG (2013). A window on the past: male ornamental plumage reveals the quality of their early-life environment.
Proc Biol Sci,
280(1756).
Abstract:
A window on the past: male ornamental plumage reveals the quality of their early-life environment.
It is well established that the expression of many ornamental traits is dependent on the current condition of the bearer. However, conditions experienced in early life are also known to be important for an individual's subsequent fitness and therefore, directly or indirectly, for the fitness of their mate. Specifically, a recent hypothesis suggests that sexually selected traits might be sensitive to conditions experienced during early-life development and thereby function as honest indicators of developmental history. Whether this applies to colourful male plumage, however, is largely unknown. We tested this idea with a field experiment by manipulating neonatal nutrition in a sexually dichromatic passerine, the hihi (Notymystis cincta). We found that carotenoid supplementation increased nestling plasma carotenoid concentration, which was in turn correlated with increased yellow saturation in male breeding plumage after moulting. We also found that the post-moult luminance (lightness) of the white ear-tufts tended to be reduced in males that had received an all-round nutritional supplement as nestlings. Black breeding plumage was not affected by neonatal nutritional treatment. Although the mechanisms that generate colourful plumage are evidently diverse, our results show that at least some parts of this display are accurate indicators of environmental conditions during development.
Abstract.
Author URL.
(2013). A window on the past: male ornamental plumage reveals the quality of their early-life environment.
Proceedings. Biological sciences / the Royal Society,
280(1756).
Abstract:
A window on the past: male ornamental plumage reveals the quality of their early-life environment.
It is well established that the expression of many ornamental traits is dependent on the current condition of the bearer. However, conditions experienced in early life are also known to be important for an individual's subsequent fitness and therefore, directly or indirectly, for the fitness of their mate. Specifically, a recent hypothesis suggests that sexually selected traits might be sensitive to conditions experienced during early-life development and thereby function as honest indicators of developmental history. Whether this applies to colourful male plumage, however, is largely unknown. We tested this idea with a field experiment by manipulating neonatal nutrition in a sexually dichromatic passerine, the hihi (Notymystis cincta). We found that carotenoid supplementation increased nestling plasma carotenoid concentration, which was in turn correlated with increased yellow saturation in male breeding plumage after moulting. We also found that the post-moult luminance (lightness) of the white ear-tufts tended to be reduced in males that had received an all-round nutritional supplement as nestlings. Black breeding plumage was not affected by neonatal nutritional treatment. Although the mechanisms that generate colourful plumage are evidently diverse, our results show that at least some parts of this display are accurate indicators of environmental conditions during development.
Abstract.
Stevens M (2013). Bird brood parasitism.
Curr Biol,
23(20), R909-R913.
Abstract:
Bird brood parasitism.
For many animals, the effort to rear their young is considerable. In birds, this often includes building nests, incubating eggs, feeding the chicks, and protecting them from predators. Perhaps for this reason, about 1% of birds (around 100 species) save themselves the effort and cheat instead. They are obligate brood parasites, laying their eggs in the nests of other species and leaving the hosts or foster parents to rear the foreign chicks for them. Some birds also cheat on individuals of the same species (intraspecific brood parasitism). Intraspecific brood parasitism has been reported in around 200 species, but is likely to be higher, as it can often only be detected by genetic analyses.
Abstract.
Author URL.
Stevens M, Rong CP, Todd PA (2013). Colour change and camouflage in the horned ghost crab Ocypode ceratophthalmus.
Biological Journal of the Linnean Society,
109(2), 257-270.
Abstract:
Colour change and camouflage in the horned ghost crab Ocypode ceratophthalmus
Species that change colour present an ideal opportunity to study the control and tuning of camouflage with regards to the background. However, most research on colour-pattern change and camouflage has been undertaken with species that rapidly alter appearance (in seconds), despite the fact that most species change appearance over longer time periods (e.g. minutes, hours, or days). We investigated whether individuals of the horned ghost crab (Ocypode ceratophthalmus) from Singapore can change colour, when this occurs, and how it influences camouflage. Individuals showed a clear daily rhythm of colour change, becoming lighter during the day and darker at night, and this significantly improved their camouflage to the sand substrate upon which they live. Individuals did not change colour when put into dark conditions, but they did become brighter when placed on a white versus a black substrate. Our findings show that ghost crabs have a circadian rhythm of colour change mediating camouflage, which is fine-tuned by adaptation to the background brightness. These types of colour change can enable individuals to achieve effective camouflage under a range of environmental conditions, substrates, and time periods, and may be widespread in other species. © 2013 the Linnean Society of London.
Abstract.
Stevens M (2013). Concealing Coloration in Animals. Animal Behaviour, 86(6), 1333-1334.
Troscianko J, Lown AE, Hughes AE, Stevens M (2013). Defeating crypsis: detection and learning of camouflage strategies.
PLoS One,
8(9).
Abstract:
Defeating crypsis: detection and learning of camouflage strategies.
Camouflage is perhaps the most widespread defence against predators in nature and an active area of interdisciplinary research. Recent work has aimed to understand what camouflage types exist (e.g. background matching, disruptive, and distractive patterns) and their effectiveness. However, work has almost exclusively focused on the efficacy of these strategies in preventing initial detection, despite the fact that predators often encounter the same prey phenotype repeatedly, affording them opportunities to learn to find those prey more effectively. The overall value of a camouflage strategy may, therefore, reflect both its ability to prevent detection by predators and resist predator learning. We conducted four experiments with humans searching for hidden targets of different camouflage types (disruptive, distractive, and background matching of various contrast levels) over a series of touch screen trials. As with previous work, disruptive coloration was the most successful method of concealment overall, especially with relatively high contrast patterns, whereas potentially distractive markings were either neutral or costly. However, high contrast patterns incurred faster decreases in detection times over trials compared to other stimuli. In addition, potentially distractive markings were sometimes learnt more slowly than background matching markings, despite being found more readily overall. Finally, learning effects were highly dependent upon the experimental paradigm, including the number of prey types seen and whether subjects encountered targets simultaneously or sequentially. Our results show that the survival advantage of camouflage strategies reflects both their ability to avoid initial detection (sensory mechanisms) and predator learning (perceptual mechanisms).
Abstract.
Author URL.
Flores EE, Stevens M, Moore AJ, Blount JD (2013). Diet, development and the optimization of warning signals in post-metamorphic green and black poison frogs.
Functional Ecology,
27(3), 816-829.
Abstract:
Diet, development and the optimization of warning signals in post-metamorphic green and black poison frogs
Many prey species are chemically defended and have conspicuous appearance to deter predators (i.e. aposematism). Such warning signals work because predators pay attention to the colour and size of signals, which they associate with unprofitability. Paradoxically, in early life stages, aposematic species are often warningly coloured, but their chemical defences are lacking because they have yet to be acquired through the diet or synthesized endogenously. This state of being conspicuous yet poorly defended must place individuals at increased risk of predation, but how they minimize this risk during development is unclear. We reared larval green and black poison frogs (Dendrobates auratus) on a relatively low or a higher food supply and tested the hypothesis that individuals with more resources should grow larger while reducing their investment in warning signals at metamorphic completion. We also assayed markers of oxidative balance (malondialdehyde, superoxide dismutase and total antioxidant capacity) to ascertain whether there were resource-allocation trade-offs that differed with diet treatments. Low-food froglets were relatively small, and their body size and signal luminance (perceived brightness) were positively correlated. In contrast, in high-food froglets body size and warning signal luminance were negatively correlated, suggesting either a resource-allocation trade-off or alternatively a facultative reduction in luminance exhibited by larger froglets. The reduction in luminance in relatively large, high-food froglets did not appear to arise because of oxidative stress: signal luminance and markers of oxidative stress were positively correlated in high-food froglets, but were negatively correlated in low-food froglets suggesting a trade-off. Our results highlight developmental plasticity in body size and coloration as affected by resource (i.e. food) supply. Such plasticity seems likely to minimize predation risk during the vulnerable period early in life when individuals are warningly coloured and must make the transition from an undefended phenotype to a mature aposematic state. © 2013 the Authors. Functional Ecology © 2013 British Ecological Society.
Abstract.
Flores EE, Stevens M, Moore AJ, Blount JD (2013). Diet, development and the optimization of warning signals in post-metamorphic green and black poison frogs. Functional Ecology
Teasdale LC, Stevens M, Stuart-Fox D (2013). Discrete colour polymorphism in the tawny dragon lizard (Ctenophorus decresii) and differences in signal conspicuousness among morphs. Journal of Evolutionary Biology
Teasdale LC, Stevens M, Stuart-Fox D (2013). Discrete colour polymorphism in the tawny dragon lizard (Ctenophorus decresii) and differences in signal conspicuousness among morphs.
Journal of Evolutionary Biology,
26(5), 1035-1046.
Abstract:
Discrete colour polymorphism in the tawny dragon lizard (Ctenophorus decresii) and differences in signal conspicuousness among morphs
Intraspecific colour variation is common in nature and can vary from the coexistence of discrete colour variants in polymorphic species to continuous variation. Whether coloration is continuous or discrete is often ambiguous and many species exhibit a combination of the two. The nature of the variation (discrete or continuous) has implications for both the genetic basis of the colour variation and the evolutionary processes generating and maintaining it. Consequently, it is important to qualify the existence of discrete morphs, particularly in relation to the animal's visual system. In this study, we quantified male throat colour variation in Ctenophorus decresii tawny dragon lizard and tested for morphological and ecological correlates of the colour variants. We confirmed that discrete throat colour morphs can be defined based on colour and pattern analyses independent of the human visual system. We also found that the colour variants differed in their conspicuousness from the background, to the lizard's visual system, which has implications for signalling. However, the morphs did not differ in morphology or microhabitat use, which suggests that these characteristics are not involved in the evolutionary maintenance of the polymorphism. © 2013 the Authors. © 2013 European Society for Evolutionary Biology.
Abstract.
Stevens M (2013). Evolutionary ecology: knowing how to hide your eggs.
Curr Biol,
23(3), R106-R108.
Abstract:
Evolutionary ecology: knowing how to hide your eggs.
A new study of camouflage in quail shows that individual birds know the appearance of their own eggs and select backgrounds that maximise concealment.
Abstract.
Author URL.
Stevens M (2013). Exchanging messages between plants and animals. Trends in Ecology & Evolution, 28(7), 386-387.
Stevens M, Troscianko J, Spottiswoode CN (2013). Repeated targeting of the same hosts by a brood parasite compromises host egg rejection.
Nat Commun,
4Abstract:
Repeated targeting of the same hosts by a brood parasite compromises host egg rejection.
Cuckoo eggs famously mimic those of their foster parents to evade rejection from discriminating hosts. Here we test whether parasites benefit by repeatedly parasitizing the same host nest. This should make accurate rejection decisions harder, regardless of the mechanism that hosts use to identify foreign eggs. Here we find strong support for this prediction in the African tawny-flanked prinia (Prinia subflava), the most common host of the cuckoo finch (Anomalospiza imberbis). We show experimentally that hosts reject eggs that differ from an internal template, but crucially, as the proportion of foreign eggs increases, hosts are less likely to reject them and require greater differences in appearance to do so. Repeated parasitism by the same cuckoo finch female is common in host nests and likely to be an adaptation to increase the probability of host acceptance. Thus, repeated parasitism interacts with egg mimicry to exploit cognitive and sensory limitations in host defences.
Abstract.
Author URL.
Stevens M, Marshall KLA, Troscianko J, Finlay S, Burnand D, Chadwick SL (2013). Revealed by conspicuousness: Distractive markings reduce camouflage.
Behavioral Ecology,
24(1), 213-222.
Abstract:
Revealed by conspicuousness: Distractive markings reduce camouflage
Animal camouflage is a textbook example of natural selection. Despite substantial progress, one historical theory remains controversial: that conspicuous "distractive" markings draw predator attention away from the prey outline, preventing detection. Here, we present evidence from 4 experiments to resolve this controversy. In field experiments, we measured bird predation on artificial cryptic prey that were either unmarked or had distractive markings of various attributes (number, color, and location). Prey with 3 high-contrast distractive markings, and with markings located away from the body outline, suffered reduced survival compared with unmarked controls or prey with low-contrast markings. There was no effect of small single markings with different colors on the survival of targets. In 2 computer-based experiments with human subjects searching for hidden targets, distractive markings of various types (number, size, and location) reduced detection times compared with controls. This effect was greatest for targets that had large or 3 markings. In addition, small and centrally placed markings facilitated faster learning. Therefore, these 2 experimental approaches show that distractive markings are detrimental to camouflage, both facilitating initial detection and increasing the speed of predator learning. Our experiments also suggest that learning of camouflaged prey is dependent on the type of camouflage present. Contrary to current and historical discussion, conspicuous markings are more likely to impair camouflage than enhance it, presenting important implications for the optimization of prey coloration in general. © 2012 the Author.
Abstract.
Stevens M (2013). Sensing the World. In (Ed) Sensory Ecology, Behaviour, and Evolution, Oxford University Press, 21-39.
Stevens M (2013). Sensory Ecology, Behaviour, and Evolution. In Stevens M (Ed) Sensory Ecology, Behaviour, and Evolution, OUP Oxford.
Stevens M (2013). Sensory Ecology, Behaviour, and Evolution., OUP Oxford.
Stevens M (2013). Sensory Ecology, Information, and Decision-Making. In (Ed) Sensory Ecology, Behaviour, and Evolution, Oxford University Press, 2-18.
Higham JP, Pfefferle D, Heistermann M, Maestripieri D, Stevens M (2013). Signaling in multiple modalities in male rhesus macaques: sex skin coloration and barks in relation to androgen levels, social status, and mating behavior.
BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY,
67(9), 1457-1469.
Author URL.
Higham JP, Pfefferle D, Heistermann M, Maestripieri D, Stevens M (2013). Signaling in multiple modalities in male rhesus macaques: sex skin coloration and barks in relation to androgen levels, social status, and mating behavior. Behavioral Ecology and Sociobiology, 1-13.
Stevens M, Troscianko J, Marshall KLA, Finlay S (2013). What is camouflage through distractive markings a reply to Merilaita et al. (2013). Behavioral Ecology, 24(5).
2012
Merrill RM, Wallbank RWR, Bull V, Salazar PCA, Mallet J, Stevens M, Jiggins CD (2012). Disruptive ecological selection on a mating cue.
Proc Biol Sci,
279(1749), 4907-4913.
Abstract:
Disruptive ecological selection on a mating cue.
Adaptation to divergent ecological niches can result in speciation. Traits subject to disruptive selection that also contribute to non-random mating will facilitate speciation with gene flow. Such 'magic' or 'multiple-effect' traits may be widespread and important for generating biodiversity, but strong empirical evidence is still lacking. Although there is evidence that putative ecological traits are indeed involved in assortative mating, evidence that these same traits are under divergent selection is considerably weaker. Heliconius butterfly wing patterns are subject to positive frequency-dependent selection by predators, owing to aposematism and Müllerian mimicry, and divergent colour patterns are used by closely related species to recognize potential mates. The amenability of colour patterns to experimental manipulation, independent of other traits, presents an excellent opportunity to test their role during speciation. We conducted field experiments with artificial butterflies, designed to match natural butterflies with respect to avian vision. These were complemented with enclosure trials with live birds and real butterflies. Our experiments showed that hybrid colour-pattern phenotypes are attacked more frequently than parental forms. For the first time, we demonstrate disruptive ecological selection on a trait that also acts as a mating cue.
Abstract.
Author URL.
Hughes A, Stevens M, Tolhurst D (2012). Factors that affect tracking of occluded target motion.
PERCEPTION,
41, 118-118.
Author URL.
Spottiswoode CN, Stevens M (2012). Host-Parasite Arms Races and Rapid Changes in Bird Egg Appearance.
AMERICAN NATURALIST,
179(5), 633-648.
Author URL.
Spottiswoode CN, Stevens M (2012). Host-parasite arms races and rapid changes in bird egg appearance.
Am Nat,
179(5), 633-648.
Abstract:
Host-parasite arms races and rapid changes in bird egg appearance.
Coevolutionary arms races are a powerful force driving evolution, adaptation, and diversification. They can generate phenotypic polymorphisms that render it harder for a coevolving parasite or predator to exploit any one individual of a given species. In birds, egg polymorphisms should be an effective defense against mimetic brood parasites and are extreme in the African tawny-flanked prinia (Prinia subflava) and its parasite, the cuckoo finch (Anomalospiza imberbis). Here we use models of avian visual perception to analyze the appearance of prinia and cuckoo finch eggs from the same location over 40 years. We show that the two interacting populations have experienced rapid changes in egg traits. Egg colors of both species have diversified over time, expanding into avian color space as expected under negative frequency-dependent selection. Egg pattern showed signatures of both frequency-dependent and directional selection in different traits, which appeared to be evolving independently of one another. Host and parasite appear to be closely tracking one another's evolution, since parasites showed closer color mimicry of contemporaneous hosts. This correlational evidence suggests that hosts and parasites are locked in an ongoing arms race in egg appearance, driven by constant change in the selective advantage of different phenotypes, and that coevolutionary arms races can generate remarkably rapid phenotypic change.
Abstract.
Author URL.
Stevens M, Ruxton GD (2012). Linking the evolution and form of warning coloration in nature.
Proc Biol Sci,
279(1728), 417-426.
Abstract:
Linking the evolution and form of warning coloration in nature.
Many animals are toxic or unpalatable and signal this to predators with warning signals (aposematism). Aposematic appearance has long been a classical system to study predator-prey interactions, communication and signalling, and animal behaviour and learning. The area has received considerable empirical and theoretical investigation. However, most research has centred on understanding the initial evolution of aposematism, despite the fact that these studies often tell us little about the form and diversity of real warning signals in nature. In contrast, less attention has been given to the mechanistic basis of aposematic markings; that is, 'what makes an effective warning signal?', and the efficacy of warning signals has been neglected. Furthermore, unlike other areas of adaptive coloration research (such as camouflage and mate choice), studies of warning coloration have often been slow to address predator vision and psychology. Here, we review the current understanding of warning signal form, with an aim to comprehend the diversity of warning signals in nature. We present hypotheses and suggestions for future work regarding our current understanding of several inter-related questions covering the form of warning signals and their relationship with predator vision, learning, and links to broader issues in evolutionary ecology such as mate choice and speciation.
Abstract.
Author URL.
Hughes A, Troscianko J, Tolhurst D, Stevens M (2012). Prey movement and the mechanisms of dazzle coloration.
PERCEPTION,
41(10), 1271-1271.
Author URL.
2011
Stevens M, Merilaita S (2011). Animal Camouflage: an Introduction. In Stevens M, Merilaita S (Eds.) Animal Camouflage: Mechanisms & Function, Cambridge University Press, 1-16.
Stevens M (2011). Animal Camouflage: Mechanisms & Function. In Stevens M, Merilaita S (Eds.)
Animal Camouflage: Mechanisms & Function, Cambridge University Press.
Abstract:
Animal Camouflage: Mechanisms & Function
Abstract.
Stevens M, Merilaita S (2011). Animal Camouflage: Mechanisms and Function., Cambridge University Press.
Stevens M (2011). Avian vision and egg colouration: Concepts and measurements.
Avian Biology Research,
4(4), 168-184.
Abstract:
Avian vision and egg colouration: Concepts and measurements
Avian egg colours and patterns are spectacularly diverse and provide a wonderful system to study both the functional and mechanistic basis of animal colouration. In order to do this, it is essential to consider the vision of the appropriate receiver, particularly birds, and to use methods and models to quantify the colour patterns independent of subjective human assessment. In this paper, I summarise some of the key features of avian colour, luminance, and pattern vision, and provide an outline of the methods available to quantify visual signals, including their associated benefits and limitations. In doing so, I make links to various recent studies of avian eggshell colouration, and a range of other relevant studies. Overall, I argue that considering receiver vision and employing appropriate methods to quantify eggshell appearance is crucial to understanding the basis of avian egg diversity.
Abstract.
Stoddard MC, Stevens M (2011). Avian vision and the evolution of egg color mimicry in the common cuckoo.
Evolution,
65(7), 2004-2013.
Abstract:
Avian vision and the evolution of egg color mimicry in the common cuckoo.
Coevolutionary arms races are a potent force in evolution, and brood parasite-host dynamics provide classical examples. Different host-races of the common cuckoo, Cuculus canorus, lay eggs in the nests of other species, leaving all parental care to hosts. Cuckoo eggs often (but not always) appear to match remarkably the color and pattern of host eggs, thus reducing detection by hosts. However, most studies of egg mimicry focus on human assessments or reflectance spectra, which fail to account for avian vision. Here, we use discrimination and tetrachromatic color space modeling of bird vision to quantify egg background and spot color mimicry in the common cuckoo and 11 of its principal hosts, and we relate this to egg rejection by different hosts. Egg background color and luminance are strongly mimicked by most cuckoo host-races, and mimicry is better when hosts show strong rejection. We introduce a novel measure of color mimicry-"color overlap"-and show that cuckoo and host background colors increasingly overlap in avian color space as hosts exhibit stronger rejection. Finally, cuckoos with better background color mimicry also have better pattern mimicry. Our findings reveal new information about egg mimicry that would be impossible to derive by the human eye.
Abstract.
Author URL.
Merilaita S, Stevens M (2011). Crypsis Through Background Matching. In Stevens M, Merilaita S (Eds.) Animal Camouflage, Cambridge University Press, 17-33.
Lindstedt C, Eager H, Ihalainen E, Kahilainen A, Stevens M, Mappes J (2011). Direction and strength of selection by predators for the color of the aposematic wood tiger moth.
Behavioral Ecology,
22(3), 580-587.
Abstract:
Direction and strength of selection by predators for the color of the aposematic wood tiger moth
Conventionally, predation is assumed to select for conspicuousness and uniformity of warning signals in aposematic (i.e. chemically defended and warning signaling) prey because this enhances predators' initial and learned avoidance. On the other hand, it has been suggested that both variation in the background where the signal is displayed as well as variation in predators' probability to attack defended prey may favor intermediate signals or relax selection for signal monomorphism. We studied the direction and strength of selection for the hind wing color (orange vs. red) of female Parasemia plantaginis moths. Birds found the moths aversive and avoided them by sight both in laboratory and field experiments. A laboratory experiment with great tits showed that birds can discriminate between orange and red wings and attack red females less than orange females. This directional selection should decrease variation in hind wing color. However, the hind wing color did not significantly affect the "survival" of dead specimens under natural field conditions in a multipredator community. In addition, even though both orange and red were highly conspicuous against green leaves and silver birch trunks (backgrounds used in the studies), the magnitude of avian-perceived chromatic contrast (conspicuousness) differed against these backgrounds. Our results suggest that depending on the signal environment (background, predator community) directional selection for the warning signal monomorphism can be relaxed. © the Author 2011.
Abstract.
Higham JP, Hughes KD, Brent LJN, Dubuc C, Engelhardt A, Heistermann M, Maestriperi D, Santos LR, Stevens M (2011). Familiarity affects the assessment of female facial signals of fertility by free-ranging male rhesus macaques.
Proceedings of the Royal Society B: Biological Sciences,
278(1723), 3452-3458.
Abstract:
Familiarity affects the assessment of female facial signals of fertility by free-ranging male rhesus macaques
Animals signal their reproductive status in a range of sensory modalities. Highly social animals, such as primates, have access not only to such signals, but also to prior experience of other group members. Whether this experience affects how animals interpret reproductive signals is unknown. Here, we explore whether familiarity with a specific female affects a male's ability to assess that female's reproductive signals. We used a preferential looking procedure to assess signal discrimination in free-ranging rhesus macaques, a species in which female facial luminance covaries with reproductive status. We collected images of female faces throughout the reproductive cycle, and using faecal hormone analysis to determine ovulation, categorized images as coming from a female's pre-fertile, ovulating, or post-fertile period. Weprinted colour-calibrated stimuli of these faces, reproducing stimuli perceptually the same in colour and luminance to the original appearance of females. These images were presented to males who were either unfamiliar or familiar with stimuli females. Overall, males distinguished ovulatory from pre-ovulatory faces. However, a significant proportion of males did so only among males familiar with stimuli females. These experiments demonstrate that familiarity may increase a receiver's ability to use a social partner's signals to discern their reproductive status. © 2011 the Royal Society.
Abstract.
Spottiswoode CN, Stevens M (2011). How to evade a coevolving brood parasite: egg discrimination versus egg variability as host defences.
Proc Biol Sci,
278(1724), 3566-3573.
Abstract:
How to evade a coevolving brood parasite: egg discrimination versus egg variability as host defences.
Arms races between avian brood parasites and their hosts often result in parasitic mimicry of host eggs, to evade rejection. Once egg mimicry has evolved, host defences could escalate in two ways: (i) hosts could improve their level of egg discrimination; and (ii) negative frequency-dependent selection could generate increased variation in egg appearance (polymorphism) among individuals. Proficiency in one defence might reduce selection on the other, while a combination of the two should enable successful rejection of parasitic eggs. We compared three highly variable host species of the Afrotropical cuckoo finch Anomalospiza imberbis, using egg rejection experiments and modelling of avian colour and pattern vision. We show that each differed in their level of polymorphism, in the visual cues they used to reject foreign eggs, and in their degree of discrimination. The most polymorphic host had the crudest discrimination, whereas the least polymorphic was most discriminating. The third species, not currently parasitized, was intermediate for both defences. A model simulating parasitic laying and host rejection behaviour based on the field experiments showed that the two host strategies result in approximately the same fitness advantage to hosts. Thus, neither strategy is superior, but rather they reflect alternative potential evolutionary trajectories.
Abstract.
Author URL.
Stevens M, Searle WTL, Seymour JE, Marshall KLA, Ruxton GD (2011). Motion dazzle and camouflage as distinct anti-predator defenses.
BMC Biol,
9Abstract:
Motion dazzle and camouflage as distinct anti-predator defenses.
BACKGROUND: Camouflage patterns that hinder detection and/or recognition by antagonists are widely studied in both human and animal contexts. Patterns of contrasting stripes that purportedly degrade an observer's ability to judge the speed and direction of moving prey ('motion dazzle') are, however, rarely investigated. This is despite motion dazzle having been fundamental to the appearance of warships in both world wars and often postulated as the selective agent leading to repeated patterns on many animals (such as zebra and many fish, snake, and invertebrate species). Such patterns often appear conspicuous, suggesting that protection while moving by motion dazzle might impair camouflage when stationary. However, the relationship between motion dazzle and camouflage is unclear because disruptive camouflage relies on high-contrast markings. In this study, we used a computer game with human subjects detecting and capturing either moving or stationary targets with different patterns, in order to provide the first empirical exploration of the interaction of these two protective coloration mechanisms. RESULTS: Moving targets with stripes were caught significantly less often and missed more often than targets with camouflage patterns. However, when stationary, targets with camouflage markings were captured less often and caused more false detections than those with striped patterns, which were readily detected. CONCLUSIONS: Our study provides the clearest evidence to date that some patterns inhibit the capture of moving targets, but that camouflage and motion dazzle are not complementary strategies. Therefore, the specific coloration that evolves in animals will depend on how the life history and ontogeny of each species influence the trade-off between the costs and benefits of motion dazzle and camouflage.
Abstract.
Author URL.
Tanaka KD, Morimoto G, Stevens M, Ueda K (2011). Rethinking visual supernormal stimuli in cuckoos: Visual modeling of host and parasite signals.
Behavioral Ecology,
22(5), 1012-1019.
Abstract:
Rethinking visual supernormal stimuli in cuckoos: Visual modeling of host and parasite signals
Some parasitic cuckoo chicks display a vivid-colored gape to their host parents when begging for food. Their mouth color was once regarded as a supernormal stimulus, yet owing to a lack of experimental support, the idea has fallen out of favor. However, previous experiments were conducted without considering the vision of avian receivers. We compared the color and visibility of begging signals between chicks of a brood parasite, the Horsfield's hawk-cuckoo (Cuculus fugax), and that of its host, the red-flanked bluetail (Tarsiger cyanurus), considering bird vision. We investigated the mouth palate of host and parasite chicks, and a gape-colored skin patch on the wing of parasite chicks, which has previously been demonstrated to induce host parental feeding. We found that, in terms of stimulation of the birds' photoreceptors and visual discrimination thresholds, visibility of parasite signals, particularly of the wing-patch, was quantitatively greater than that of the host chick signal. Meanwhile, host and parasite signals were qualitatively different in the hue, which was driven mostly by greater ultraviolet reflectance of the parasite signals. Evidence from previous studies indicates that the visual attributes of the parasite signals may induce parental provisioning, suggesting that signal exaggeration of the parasite has evolved to stimulate hosts effectively in the dark nest environment. Overall, our results suggest that the color of hawk-cuckoo chicks' signaling traits can work as a supernormal stimulus, although host parental responses to exaggerated stimuli need to be tested experimentally. © 2011 the Author.
Abstract.
Langmore NE, Stevens M, Maurer G, Heinsohn R, Hall ML, Peters A, Kilner RM (2011). Visual mimicry of host nestlings by cuckoos.
Proceedings of the Royal Society B: Biological Sciences,
278(1717), 2455-2463.
Abstract:
Visual mimicry of host nestlings by cuckoos
Coevolution between antagonistic species has produced instances of exquisite mimicry. Among broodparasitic cuckoos, host defences have driven the evolution of mimetic eggs, but the evolutionary arms race was believed to be constrained from progressing to the chick stage, with cuckoo nestlings generally looking unlike host young. However, recent studies on bronze-cuckoos have confounded theoretical expectations by demonstrating cuckoo nestling rejection by hosts. Coevolutionary theory predicts reciprocal selection for visual mimicry of host young by cuckoos, although this has not been demonstrated previously. Here we show that, in the eyes of hosts, nestlings of three bronze-cuckoo species are striking visual mimics of the young of their morphologically diverse hosts, providing the first evidence that coevolution can select for visual mimicry of hosts in cuckoo chicks. Bronze-cuckoos resemble their own hosts more closely than other host species, but the accuracy of mimicry varies according to the diversity of hosts they exploit. © 2011 the Royal Society.
Abstract.
2010
Higham JP, Brent LJN, Dubuc C, Accamando AK, Engelhardt A, Gerald MS, Heistermann M, Stevens M (2010). Color signal information content and the eye of the beholder: a case study in the rhesus macaque.
Behavioral Ecology,
21(4), 739-746.
Abstract:
Color signal information content and the eye of the beholder: a case study in the rhesus macaque
Animal coloration has provided many classical examples of both natural and sexual selection. Methods to study color signals range from human assessment to models of receiver vision, with objective measurements commonly involving spectrometry or digital photography. However, signal assessment by a receiver is not objective but linked to receiver perception. Here, we use standardized digital photographs of female rhesus macaque (Macaca mulatta) face and hindquarter regions, combined with estimates of the timing of the female fertile phase, to assess how color varies with respect to this timing. We compare objective color measures (camera sensor responses) with models of rhesus vision (retinal receptor stimulation and visual discriminability). Due to differences in spectral separation between camera sensors and rhesus receptors, camera measures overestimated color variation and underestimated luminance variation compared with rhesus macaques. Consequently, objective digital camera measurements can produce statistically significant relationships that are probably undetectable to rhesus macaques, and hence biologically irrelevant, while missing variation in the measure that may be relevant. Discrimination modeling provided results that were most meaningful (as they were directly related to receiver perception) and were easiest to relate to underlying physiology. Further, this gave new insight into the function of such signals, revealing perceptually salient signal luminance changes outside of the fertile phase that could potentially enhance paternity confusion. Our study demonstrates how, even for species with similar visual systems to humans, models of vision may provide more accurate and meaningful information on the form and function of visual signals than objective color measures do. © the Author 2010.
Abstract.
Mappes J, Stevens M (2010). Information Use and Sensory Ecology. In Westneat D, Fox C (Eds.)
Evolutionary Behavioral Ecology, OUP USA, 148-161.
Abstract:
Information Use and Sensory Ecology
Abstract.
Stoddard MC, Stevens M (2010). Pattern mimicry of host eggs by the common cuckoo, as seen through a bird's eye.
Proc Biol Sci,
277(1686), 1387-1393.
Abstract:
Pattern mimicry of host eggs by the common cuckoo, as seen through a bird's eye.
Cuckoo-host interactions provide classical examples of coevolution. Cuckoos place hosts under selection to detect and reject foreign eggs, while host defences result in the evolution of host-egg mimicry in cuckoos. Despite a long history of research, egg pattern mimicry has never been objectively quantified, and so its coevolution with host defences has not been properly assessed. Here, we use digital image analysis and modelling of avian vision to quantify the level of pattern mimicry in eight host species of the common cuckoo Cuculus canorus and their respective cuckoo host-races. We measure a range of pattern attributes, including marking size, diversity in size, contrast, coverage and dispersion. This new technique reveals hitherto unnoticed sophistication in egg pattern mimicry. We show that various features of host egg pattern are mimicked by the eggs of their respective cuckoo host-races, and that cuckoos have evolved better pattern mimicry for host species that exhibit stronger egg rejection. Pattern differs relatively more between eggs of different host species than between their respective cuckoo host-races. We suggest that cuckoos may have more 'average' markings in order to be able to use subsidiary hosts. Our study sheds new light on cuckoo-host coevolution and illustrates a new technique for quantifying animal markings with respect to the relevant animal visual system.
Abstract.
Author URL.
Stevens M (2010). Sensory ecology, evolution, and behavior. Current Zoology, 56(3).
Shultziner D, Stevens T, Stevens M, Stewart BA, Hannagan RJ, Saltini-Semerari G (2010). The causes and scope of political egalitarianism during the Last Glacial: a multi-disciplinary perspective.
Biology and Philosophy,
25(3), 319-346.
Abstract:
The causes and scope of political egalitarianism during the Last Glacial: a multi-disciplinary perspective
This paper reviews and synthesizes emerging multi-disciplinary evidence toward understanding the development of social and political organization in the Last Glacial. Evidence for the prevalence and scope of political egalitarianism is reviewed and the biological, social, and environmental influences on this mode of human organization are further explored. Viewing social and political organization in the Last Glacial in a much wider, multi-disciplinary context provides the footing for coherent theory building and hypothesis testing by which to further explore human political systems. We aim to overcome the claim that our ancestors' form of social organization is untestable, as well as counter a degree of exaggeration regarding possibilities for sedentism, population densities, and hierarchical structures prior to the Holocene with crucial advances from disparate disciplines. © 2010 Springer Science+Business Media B.V.
Abstract.
Sandre SL, Stevens M, Mappes J (2010). The effect of predator appetite, prey warning coloration and luminance on predator foraging decisions.
Behaviour,
147(9), 1121-1143.
Abstract:
The effect of predator appetite, prey warning coloration and luminance on predator foraging decisions
Aposematic prey advertise their defence to visually hunting predators using conspicuous warning colouration. Established theory predicts that aposematic signals should evolve towards increased conspicuousness and similarity to enhance predator education. Contrary to theoretical expectations, there is often considerable within- and between-species variation in aposematic signals of animals sharing the same ecological niche, phylogeny and predators. This may be explained by varying responses of predators that weaken the selection pressure for a consistent signal. By presenting painted mealworm larvae as prey to great tits as predators we tested if different aposematic colour patterns have different values as a means of initial protection and learnt avoidance from predators, and how widely birds generalise their learnt avoidance to other colour patterns. We also investigated how the colour and luminance of the pattern elements affect predator attack decisions. Finally, we studied if hunger affects the predators' reaction to differently coloured prey. We found that similarity in colour was not crucial to the survival of aposematic prey, since learnt avoidance was not influenced by colour, and predators remembered and generalised widely in their learnt avoidance to other colours. We found that initial avoidance was, however, apparently influenced by luminance contrast. Interestingly, the predators' level of hunger was more important than the colour of the aposematic signal in determining birds' decisions to attack chemically-defended insect larvae. We discuss the implications of visual properties of prey colour pattern and predator appetite for the evolution of insect defences and warning signals. In addition we propose a methodological approach to effectively control for predator appetite in laboratory experiments. © 2010 Koninklijke Brill NV, Leiden.
Abstract.
Spottiswoode CN, Stevens M (2010). Visual modeling shows that avian host parents use multiple visual cues in rejecting parasitic eggs.
Proc Natl Acad Sci U S A,
107(19), 8672-8676.
Abstract:
Visual modeling shows that avian host parents use multiple visual cues in rejecting parasitic eggs.
One of the most striking outcomes of coevolution between species is egg mimicry by brood parasitic birds, resulting from rejection behavior by discriminating host parents. Yet, how exactly does a host detect a parasitic egg? Brood parasitism and egg rejection behavior provide a model system for exploring the relative importance of different visual cues used in a behavioral task. Although hosts are discriminating, we do not know exactly what cues they use, and to answer this it is crucial to account for the receiver's visual perception. Color, luminance ("perceived lightness") and pattern information have never been simultaneously quantified and experimentally tested through a bird's eye. The cuckoo finch Anomalospiza imberbis and its hosts show spectacular polymorphisms in egg appearance, providing a good opportunity for investigating visual discrimination owing to the large range of patterns and colors involved. Here we combine field experiments in Africa with modeling of avian color vision and pattern discrimination to identify the specific visual cues used by hosts in making rejection decisions. We found that disparity between host and foreign eggs in both color and several aspects of pattern (dispersion, principal marking size, and variability in marking size) were important predictors of rejection, especially color. These cues correspond exactly to the principal differences between host and parasitic eggs, showing that hosts use the most reliable available cues in making rejection decisions, and select for parasitic eggs that are increasingly mimetic in a range of visual attributes.
Abstract.
Author URL.
2009
Stevens M, Merilaita S (2009). Animal camouflage: current issues and new perspectives.
Philos Trans R Soc Lond B Biol Sci,
364(1516), 423-427.
Author URL.
Langmore NE, Stevens M, Maurer G, Kilner RM (2009). Are dark cuckoo eggs cryptic in host nests?.
Animal Behaviour,
78(2), 461-468.
Abstract:
Are dark cuckoo eggs cryptic in host nests?
The coevolutionary arms race between cuckoos and their hosts has famously yielded cuckoo eggs that evade host recognition and rejection by mimicking the appearance of the host's own clutch. But not all coevolutionary interactions between cuckoos and hosts have followed the same pathway. Several host species do not show egg rejection even when the cuckoo's egg looks entirely unlike their own. For example, hosts of some Australian bronze-cuckoos, Chalcites spp. routinely accept olive-brown cuckoo eggs that look very different from the speckled white eggs of their own clutch. Here we investigate the hypothesis that these cuckoo eggs are cryptic, which might explain why hosts do not remove them from their clutch. First, we use a phylogenetic analysis to show that dark bronze-cuckoo eggs are not ancestral, but instead have evolved in a group that parasitizes hosts with dark nests exclusively. Second, we show that dark bronze-cuckoo eggs are laid by two species that parasitize hosts with relatively dark nests, whereas a congeneric bronze-cuckoo species parasitizing host nests with greater ambient light levels lays a mimetic egg. Finally, we use a model of avian visual processing to show that the dark eggs of Gould's bronze-cuckoo C. russatus are cryptic in dark host nests. Our results support the hypothesis that some bronze-cuckoo species and their hosts have pursued an alternative coevolutionary trajectory, which has resulted in the evolution of cryptic, rather than mimetic, cuckoo eggs. Crown Copyright © 2009.
Abstract.
Stevens M, Merilaita S (2009). Defining disruptive coloration and distinguishing its functions.
Philos Trans R Soc Lond B Biol Sci,
364(1516), 481-488.
Abstract:
Defining disruptive coloration and distinguishing its functions.
Disruptive coloration breaks up the shape and destroys the outline of an object, hindering detection. The principle was first suggested approximately a century ago, but, although research has significantly increased, the field remains conceptually unstructured and no unambiguous definition exists. This has resulted in variable use of the term, making it difficult to formulate testable hypotheses that are comparable between studies, slowing down advancement in this field. Related to this, a range of studies do not effectively distinguish between disruption and other forms of camouflage. Here, we give a formal definition of disruptive coloration, reorganize a range of sub-principles involved in camouflage and argue that five in particular are specifically related to disruption: differential blending; maximum disruptive contrast; disruption of surface through false edges; disruptive marginal patterns; and coincident disruptive coloration. We discuss how disruptive coloration can be optimized, how it can relate to other forms of camouflage markings and where future work is particularly needed.
Abstract.
Author URL.
Stevens M, Winney IS, Cantor A, Graham J (2009). Outline and surface disruption in animal camouflage.
Proc Biol Sci,
276(1657), 781-786.
Abstract:
Outline and surface disruption in animal camouflage.
Camouflage is an important strategy in animals to prevent predation. This includes disruptive coloration, where high-contrast markings placed at an animal's edge break up the true body shape. Successful disruption may also involve non-marginal markings found away from the body outline that create 'false edges' more salient than the true body form ('surface disruption'). However, previous work has focused on breaking up the true body outline, not on surface disruption. Furthermore, while high contrast may enhance disruption, it is untested where on the body different contrasts should be placed for maximum effect. We used artificial prey presented to wild avian predators in the field, to determine the effectiveness of surface disruption, and of different luminance contrast placed in different prey locations. Disruptive coloration was no more effective when comprising high luminance contrast per se, but its effectiveness was dramatically increased with high-contrast markings placed away from the body outline, creating effective surface disruption. A model of avian visual edge processing showed that surface disruption does not make object detection more difficult simply by creating false edges away from the true body outline, but its effect may also be based on a different visual mechanism. Our study has implications for whether animals can combine disruptive coloration with other 'conspicuous' signalling strategies.
Abstract.
Author URL.
Stevens M, Stoddard MC, Higham JP (2009). Studying primate color: Towards visual system-dependent methods.
International Journal of Primatology,
30(6), 893-917.
Abstract:
Studying primate color: Towards visual system-dependent methods
Primates exhibit a striking diversity of colors and patterns in their pelage and skin markings, used in functions as diverse as camouflage to sexual signaling. In studying primate colors, it is important to adopt approaches not based on human assessment wherever possible, and that preferably take account of the visual system of the appropriate receiver(s). Here, we outline some of the main techniques for recording the colors exhibited and encountered by primates, including the use of digital photography and reflectance spectrometry. We go on to discuss the main approaches for analyzing the data obtained, including those not linked to a particular visual system, such as direct analyses of reflectance spectra. We argue that researchers should strive for analyses based on the visual system of the relevant receiver, and outline some of the main modeling approaches that can be used, such as color space and discrimination threshold modeling. By analyzing color measures with respect to specific visual systems, field studies can link behavioral ecology to the visual and cognitive sciences, and move toward descriptions of signal information content that incorporate elements of receiver psychology. This in turn should lead to a greater understanding of the detection and interpretation of signals by receivers, and hence their likely use in decision making. © 2009 Springer Science+Business Media, LLC.
Abstract.
Stevens M, Cantor A, Graham J, Winney IS (2009). The function of animal 'eyespots': Conspicuousness but not eye mimicry is key.
Current Zoology,
55(5), 319-326.
Abstract:
The function of animal 'eyespots': Conspicuousness but not eye mimicry is key
Many animals are marked with conspicuous circular features often called 'eyespots', which intimidate predators, preventing or halting an attack. It has long been assumed that eyespots work by mimicking the eyes of larger animals, but recent experiments have indicated that conspicuousness and contrast is important in eyespot function, and not eye mimicry. We undertake two further experiments to distinguish between the conspicuousness and mimicry hypotheses, by using artificial prey presented to wild avian predators in the field. In experiment 1, we test if eyespot effectiveness depends on the marking shape (bar or circle) and arrangement (eye-like and non- eye-like positions). We find no difference between shapes or arrangement; all spots were equally effective in scaring birds. In experiment 2, we test if the often yellow and black colors of eyespots mimic the eyes of birds of prey. We find no effect of shape, and no advantage to yellow and black spots over non-eye-like but equally conspicuous colors. The consistent finding is that eyespot function lies in being a conspicuous signal to predators, and not necessarily due to eye mimicry. © 2009 Current Zoology.
Abstract.
Stevens M, Castor-Perry SA, Price JRF (2009). The protective value of conspicuous signals is not impaired by shape, size, or position asymmetry.
Behavioral Ecology,
20(1), 96-102.
Abstract:
The protective value of conspicuous signals is not impaired by shape, size, or position asymmetry
Various conspicuous signals in nature promote initial and learned avoidance by predators. It is widely thought that such signals are most effective when highly symmetrical in features such as size and shape, supported by recent laboratory experiments with domestic chicks and artificial prey. However, no study has investigated the effect of asymmetry on conspicuous signals in a natural setting, where viewing distances, angles, predator species, and light conditions vary and where predators encounter prey sequentially rather than simultaneously. We undertook 2 field experiments with artificial gray-scale prey, marked with a pair of white markings presented to wild avian predators, to test the effect of asymmetry on the survival value of conspicuous signals in the field. Experiment 1 had treatments with symmetrical spots or with spots asymmetrical in area between 5 and 50%. All marked treatments survived better than unmarked controls, but there was no benefit of being symmetrical. Experiment 2 tested the effect of possessing markings asymmetrical for shape or position and any additive effect of these 2 features. Again, symmetry conferred no benefit and targets with markings asymmetrical for position and/or shape survived equally well as those with symmetrical arrangements. These findings indicate that asymmetry in warning signals may not be costly to prey in nature or be of less importance compared with other features of the signal, such as color and overall size. © the Author 2008. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved.
Abstract.
2008
Stevens M, Hardman CJ, Stubbins CL (2008). Conspicuousness, not eye mimicry, makes "eyespots" effective antipredator signals.
Behavioral Ecology,
19(3), 525-531.
Abstract:
Conspicuousness, not eye mimicry, makes "eyespots" effective antipredator signals
Many animals bear colors and patterns to reduce the risk of predation from visually hunting predators, including warning colors, camouflage, and mimicry. In addition, various species possess paired circular features often called "eyespots," which may intimidate or startle predators preventing or postponing an attack. Most explanations for how eyespots work assert that they mimic the eyes of the predators own enemies. However, recent work has indicated that spots may reduce the risk of predation based purely on how conspicuous they are to a predator's visual system. Here, we use a field technique involving artificial prey marked with stimuli of various shapes, numbers, and sizes, presented to avian predators in the field, to distinguish between the eye mimicry and conspicuousness theories. In 3 experiments, we find that the features which make effective antipredator wing markings are large size and higher numbers of spots. Stimuli with circles survived no better than those marked with other conspicuous shapes such as bars, and changing the spatial construction of the spots to increase the level of eye mimicry had no effect on the protective value of the spots. These experiments support other recent work indicating that conspicuousness, and not eye mimicry, is important in promoting avoidance behavior in predators and that eyespots on real animals need not necessarily, as most accounts claim, mimic the eyes of other animals. © the Author 2008. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved.
Abstract.
Stevens M, Yule DH, Ruxton GD (2008). Dazzle coloration and prey movement.
Proc Biol Sci,
275(1651), 2639-2643.
Abstract:
Dazzle coloration and prey movement.
Many traits in animals reduce the rate of attack from visually hunting predators, including camouflage, warning signals and mimicry. In addition, some animal markings may reduce the likelihood that an attack ends in successful capture. These might include dazzle markings, high-contrast patterns that make the estimation of speed and trajectory difficult. However, until now, no study has experimentally tested whether some markings may achieve such an effect. We developed a computer 'game' where human 'predators' have to capture computer-generated prey moving across a background. In two experiments, we find that although uniform camouflaged targets were among the hardest to capture, so were a range of high-contrast conspicuous patterns, such as bands and zigzags. Prey were also more difficult to capture against more heterogeneous than uniform backgrounds, and at faster speeds of movement. As such, we find the first experimental evidence that conspicuous patterns, similar to those found in a wide range of real animals, make the capture of moving prey more challenging. Various anti-predator markings may work prey during motion, and some animals may combine such dazzle patterns with other functions, such as camouflage, thermoregulation, sexual and warning signals.
Abstract.
Author URL.
Stevens M, Graham J, Winney IS, Cantor A (2008). Testing Thayer's hypothesis: can camouflage work by distraction?.
Biol Lett,
4(6), 648-650.
Abstract:
Testing Thayer's hypothesis: can camouflage work by distraction?
One of the oldest theories of animal camouflage predicts that apparently conspicuous markings enhance concealment. Such 'distraction' marks are hypothesized to work by drawing the viewer's attention away from salient features, such as the body outline, that would otherwise reveal the animal. If distraction marks enhance concealment, then they offer a route for animals to combine camouflage markings with conspicuous signalling strategies, such as warning signals. However, the theory has never been tested and remains controversial. By using camouflaged artificial prey presented to wild avian predators, we test whether distractive markings enhance concealment. In contrast to predictions, we find that markings, both circular and irregular shapes, increase predation compared with unmarked targets. Markings became increasingly costly as their contrast against the prey increased. Our experiments failed to find any empirical support for the hypothesis that distraction markings are an important aspect of camouflage in animals.
Abstract.
Author URL.
Caro T, Merilaita S, Stevens M (2008). The Colours of Animals: from Wallace to the Present Day. I. Cryptic Colouration. In Smith CH, Beccaloni G (Eds.)
Natural Selection and Beyond: the Intellectual Legacy of Alfred Russel Wallace, OUP Oxford, 125-143.
Abstract:
The Colours of Animals: from Wallace to the Present Day. I. Cryptic Colouration
Abstract.
Stevens M, Stubbins CL, Hardman CJ (2008). The anti-predator function of 'eyespots' on camouflaged and conspicuous prey.
Behavioral Ecology and Sociobiology,
62(11), 1787-1793.
Abstract:
The anti-predator function of 'eyespots' on camouflaged and conspicuous prey
Animals utilise various strategies to reduce the risk of predation, including camouflage, warning colours and mimicry, and many of these protective signals promote avoidance behaviour in predators. For example, various species possess paired circular 'eyespots', which startle or intimidate predators, preventing or halting an attack. However, little is known of how the efficacy of such signals relates to the context in which they are found, and no studies have tested the relative effectiveness of anti-predator signals when on otherwise camouflaged and conspicuous prey. We find that the protective value of conspicuous wing spots, placed on artificial moth-like targets presented to wild birds in the field, is strongly affected by the attributes of the prey 'animal' on which they are found. Wing spots reduced predation when on conspicuous prey but were rendered ineffective when on otherwise camouflaged targets, indeed they increased the risk of predation compared to non-marked camouflaged controls. These results demonstrate how different anti-predator strategies may interact, and that protective signals can switch from being beneficial to costly under different contexts. © 2008 Springer-Verlag.
Abstract.
2007
Houston AI, Stevens M, Cuthill IC (2007). Animal camouflage: Compromise or specialize in a 2 patch-type environment?.
Behavioral Ecology,
18(4), 769-775.
Abstract:
Animal camouflage: Compromise or specialize in a 2 patch-type environment?
Many animals possess camouflage markings that reduce the risk of detection by visually hunting predators. A key aspect of camouflage involves mimicking the background against which the animal is viewed. However, most animals experience a wide variety of backgrounds and cannot change their external appearance to match each selectively. We investigate whether such animals should adopt camouflage specialized with respect to one background or adopt a compromise between the attributes of multiple backgrounds. We do this using a model consisting of predators that hunt prey in patches of 2 different types, where prey adopt the camouflage that minimizes individual risk of predation. We show that the optimal strategy of the prey is affected by a number of factors, including the relative frequencies of the patch types, the travel time of predators between patches, the mean prey number in each patch type, and the trade-off function between the levels of crypsis in the patch types. We find evidence that both specialist and compromise strategies of prey camouflage are favored under different model parameters, indicating that optimal concealment may not be as straightforward as previously thought. © the Author 2007. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved.
Abstract.
Rowland HM, Speed MP, Ruxton GD, Edmunds M, Stevens M, Harvey IF (2007). Countershading enhances cryptic protection: an experiment with wild birds and artificial prey.
Animal Behaviour,
74(5), 1249-1258.
Abstract:
Countershading enhances cryptic protection: an experiment with wild birds and artificial prey
Of the many traits seen in cryptic prey animals, countershading (darker pigmentation on those surfaces exposed to the most lighting) is one of the commonest, and paradoxically one of the least understood. Countershading has been hypothesized to enhance crypsis by shadow-obliteration, in which lighter coloration on the undersides compensates for increased shadow in these regions, thus reducing detection by visually hunting predators. We tested the hypothesis that countershading enhances crypsis in two experiments with artificial prey presented to free-living birds. In the first experiment, artificial prey were presented on lawns to a range of bird species. In the second experiment, the prey were presented on green boards to individual blackbirds, Turdus merula. In both experiments countershaded prey had significantly lower levels of predation than controls. Our results show that countershading can enhance cryptic protection and has important implications for the evolutionary ecology of prey defences. © 2007 the Association for the Study of Animal Behaviour.
Abstract.
Cuthill IC, Troscianko TS, Kibblewhite A, King O, Stevens M (2007). Edge enhancement in disruptive camouflage.
PERCEPTION,
36(9), 1399-1399.
Author URL.
Stevens M, Hopkins E, Hinde W, Adcock A, Connolly Y, Troscianko T, Cuthill IC (2007). Field experiments on the effectiveness of 'eyespots' as predator deterrents.
Animal Behaviour,
74(5), 1215-1227.
Abstract:
Field experiments on the effectiveness of 'eyespots' as predator deterrents
Many potential prey species possess features that falsely convey a risk to the predator. One form of visual signal hypothesized to have this function are the circular features commonly found on the wings of diurnal lepidopterans: 'eyespots'. Eyespots have been found to reduce predation by intimidating predators before an attack or startling them once an attack has been initiated, and have been proposed to work because predators falsely classify the markings as the eyes of the predators' own enemies. However, this argument lacks unambiguous experimental support because the data are consistent with the hypothesis that eyespots are effective merely because they are highly conspicuous. High contrast concentric circles are liable to be highly effective in stimulating the centre-surround arrangement of receptive fields typical of the vertebrate retina. Here, we use a field technique involving artificial moth-like stimuli, with or without different forms of grey-scale eyespots, to test these two opposing hypotheses. Across five experiments, the consistent effect is that predation risk is reduced by patterns that have high internal pattern contrast and also contrast highly with the target background, irrespective of the specific pattern arrangement. Circles are more effective than less eye-like shapes, but this constitutes only limited support for the eye-mimicry hypothesis because such patterns will also be highly conspicuous to animals with circular receptive fields in their retinae. We conclude that, contrary to popular belief, 'eyespots' can be effective predator deterrents without mimicking eyes. © 2007 the Association for the Study of Animal Behaviour.
Abstract.
Stevens M, Cuthill IC (2007). Hidden messages: Are ultraviolet signals a special channel in avian communication?.
BioScience,
57(6), 501-507.
Abstract:
Hidden messages: Are ultraviolet signals a special channel in avian communication?
It is widely known that birds are sensitive to ultraviolet (UV) light, and that they use UV reflecting signals in choosing mates. This led to the proposal that UV signals in birds may represent private channels of communication hidden from predators, because most mammalian predators of birds are unlikely to see UV light. This idea has held great sway with researchers and the public, sustained no doubt by human fascination with an area of communication invisible to us. However, the primary predators of songbirds are often birds of prey that can see UV light, shedding doubt on the idea that UV reflecting patterns represent "special" signals. A range of recent studies, including mate-choice experiments, models of visual processing, and comparative analyses, have claimed to provide support for and against the theory that UV signals are special. We summarize the evidence for and against this idea and conclude that, while further work is required, current evidence generally does not favor this hypothesis. We finish with suggestions for future work to settle the controversy. © 2007 American Institute of Biological Sciences.
Abstract.
Stevens M (2007). Predator perception and the interrelation between different forms of protective coloration.
Proc Biol Sci,
274(1617), 1457-1464.
Abstract:
Predator perception and the interrelation between different forms of protective coloration.
Animals possess a range of defensive markings to reduce the risk of predation, including warning colours, camouflage, eyespots and mimicry. These different strategies are frequently considered independently, and with little regard towards predator vision, even though they may be linked in various ways and can be fully understood only in terms of predator perception. For example, camouflage and warning coloration need not be mutually exclusive, and may frequently exploit similar features of visual perception. This paper outlines how different forms of protective markings can be understood from predator perception and illustrates how this is fundamental in determining the mechanisms underlying, and the interrelation between, different strategies. Suggestions are made for future work, and potential mechanisms discussed in relation to various forms of defensive coloration, including disruptive coloration, eyespots, dazzle markings, motion camouflage, aposematism and mimicry.
Abstract.
Author URL.
Stevens M, Párraga CA, Cuthill IC, Partridge JC, Troscianko TS (2007). Using digital photography to study animal coloration.
Biological Journal of the Linnean Society,
90(2), 211-237.
Abstract:
Using digital photography to study animal coloration
In understanding how visual signals function, quantifying the components of those patterns is vital. With the ever-increasing power and availability of digital photography, many studies are utilizing this technique to study the content of animal colour signals. Digital photography has many advantages over other techniques, such as spectrometry, for measuring chromatic information, particularly in terms of the speed of data acquisition and its relatively cheap cost. Not only do digital photographs provide a method of quantifying the chromatic and achromatic content of spatially complex markings, but also they can be incorporated into powerful models of animal vision. Unfortunately, many studies utilizing digital photography appear to be unaware of several crucial issues involved in the acquisition of images, notably the nonlinearity of many cameras' responses to light intensity, and biases in a camera's processing of the images towards particular wavebands. In the present study, we set out step-by-step guidelines for the use of digital photography to obtain accurate data, either independent of any particular visual system (such as reflection values), or for particular models of nonhuman visual processing (such as that of a passerine bird). These guidelines include how to: (1) linearize the camera's response to changes in light intensity; (2) equalize the different colour channels to obtain reflectance information; and (3) produce a mapping from camera colour space to that of another colour space (such as photon catches for the cone types of a specific animal species). © 2007 the Linnean Society of London.
Abstract.
2006
Stevens M, Cuthill IC (2006). Disruptive coloration, crypsis and edge detection in early visual processing.
Proc Biol Sci,
273(1598), 2141-2147.
Abstract:
Disruptive coloration, crypsis and edge detection in early visual processing.
Many animals use concealing markings to reduce the risk of predation. These include background pattern matching (crypsis), where the coloration matches a random sample of the background and disruptive patterns, whose effectiveness has been hypothesized to lie in breaking up the body into a series of apparently unrelated objects. We have previously established the effectiveness of disruptive coloration against avian predators, using artificial moth-like stimuli with colours designed to match natural backgrounds as perceived by birds. Here, we investigate the mechanism by which disruptive patterns reduce detectability, using a computational vision model of edge detection applied to photographs of our experimental stimuli, calibrated for bird colour vision. We show that, disruptive coloration is effective by exploiting edge detection algorithms that we use to model early visual processing. Thus, 'false' edges are detected within the body rather than at its periphery, so inhibiting successful detection of the animal's body outline.
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Author URL.
Stevens M, Cuthill IC, Windsor AMM, Walker HJ (2006). Disruptive contrast in animal camouflage.
Proc Biol Sci,
273(1600), 2433-2438.
Abstract:
Disruptive contrast in animal camouflage.
Camouflage typically involves colour patterns that match the background. However, it has been argued that concealment may be achieved by strategic use of apparently conspicuous markings. Recent evidence supports the theory that the presence of contrasting patterns placed peripherally on an animal's body (disruptive coloration) provides survival advantages. However, no study has tested a key prediction from the early literature that disruptive coloration is effective even when some colour patches do not match the background and have a high contrast with both the background and adjacent pattern elements (disruptive contrast). We test this counter-intuitive idea that conspicuous patterns might aid concealment, using artificial moth-like targets with pattern elements designed to match or mismatch the average luminance (lightness) of the trees on which they were placed. Disruptive coloration was less effective when some pattern elements did not match the background luminance. However, even non-background-matching disruptive patterns reduced predation relative to equivalent non-disruptive patterns or to unpatterned controls. Therefore, concealment may still be achieved even when an animal possesses markings not found in the background. Disruptive coloration may allow animals to exploit backgrounds on which they are not perfectly matched, and to possess conspicuous markings while still retaining a degree of camouflage.
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Stevens M, Cuthill IC, Alejandro Párraga C, Troscianko T (2006). The effectiveness of disruptive coloration as a concealment strategy.
Prog Brain Res,
155, 49-64.
Abstract:
The effectiveness of disruptive coloration as a concealment strategy.
Our understanding of camouflage has been developing for over 100 years. Several underlying principles have emerged. Background pattern matching, or crypsis, is insufficient to conceal objects because of edge information. Other strategies exist to disrupt the continuity of extended edges. These strategies are reviewed. We pay particular attention to the theory of disruptive coloration, which predicts that high-contrast elements located at the object edge will mask the perception of a target as belonging to a certain category of object, in spite of the fact that the edge elements are independently visible. Although this strategy has long been assumed to be effective, there has been a lack of supportive data involving the perception of targets by nonhuman animals. We present evidence, from a field study, in support of the notion that disruptive coloration reduces the chances of bird predation of artificial "moths."
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Cuthill IC, Stevens M, Windsor AMM, Walker HJ (2006). The effects of pattern symmetry on detection of disruptive and background matching coloration. Behavioral Ecology, 17(5), 828-832.
2005
Troscianko T, Cuthill IC, Stevens M, Graham L, Richardson S, Parraga CA (2005). Detection of cryptic targets in avian vision: a field study.
PERCEPTION,
34, 47-47.
Author URL.
Cuthill IC, Stevens M, Sheppard J, Maddocks T, Párraga CA, Troscianko TS (2005). Disruptive coloration and background pattern matching.
Nature,
434(7029), 72-74.
Abstract:
Disruptive coloration and background pattern matching.
Effective camouflage renders a target indistinguishable from irrelevant background objects. Two interrelated but logically distinct mechanisms for this are background pattern matching (crypsis) and disruptive coloration: in the former, the animal's colours are a random sample of the background; in the latter, bold contrasting colours on the animal's periphery break up its outline. The latter has long been proposed as an explanation for some apparently conspicuous coloration in animals, and is standard textbook material. Surprisingly, only one quantitative test of the theory exists, and one experimental test of its effectiveness against non-human predators. Here we test two key predictions: that patterns on the body's outline should be particularly effective in promoting concealment and that highly contrasting colours should enhance this disruptive effect. Artificial moth-like targets were exposed to bird predation in the field, with the experimental colour patterns on the 'wings' and a dead mealworm as the edible 'body'. Survival analysis supported the predictions, indicating that disruptive coloration is an effective means of camouflage, above and beyond background pattern matching.
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Stevens M (2005). The role of eyespots as anti-predator mechanisms, principally demonstrated in the Lepidoptera.
Biol Rev Camb Philos Soc,
80(4), 573-588.
Abstract:
The role of eyespots as anti-predator mechanisms, principally demonstrated in the Lepidoptera.
Eyespots are found in a variety of animals, in particular lepidopterans. The role of eyespots as antipredator mechanisms has been discussed since the 19th Century, with two main hypotheses invoked to explain their occurrence. The first is that large, centrally located eyespots intimidate predators by resembling the eyes of the predators' own enemies; the second, though not necessarily conflicting, hypothesis is that small, peripherally located eyespots function as markers to deflect the attacks of predators to non-vital regions of the body. A third possibility is also proposed; that eyespots intimidate predators merely because they are novel or rarely encountered salient features. These hypotheses are reviewed, with special reference given to avian predators, since these are likely to be the principal visually hunting predators of the lepidopterans considered. Also highlighted is the necessity to consider the potential influence of sexual selection on lepidopteran wing patterns, and the genetics and development of eyespot formation.
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Stevens M, Cuthill IC (2005). The unsuitability of HTML-based colour charts for estimating animal colours--a comment on Berggren and Merilä (2004).
Front Zool,
2Abstract:
The unsuitability of HTML-based colour charts for estimating animal colours--a comment on Berggren and Merilä (2004).
BACKGROUND: a variety of techniques are used to study the colours of animal signals, including the use of visual matching to colour charts. This paper aims to highlight why they are generally an unsatisfactory tool for the measurement and classification of animal colours and why colour codes based on HTML (really RGB) standards, as advocated in a recent paper, are particularly inappropriate. There are many theoretical arguments against the use of colour charts, not least that human colour vision differs markedly from that of most other animals. However, the focus of this paper is the concern that, even when applied to humans, there is no simple 1:1 mapping from an RGB colour space to the perceived colours in a chart (the results are both printer- and illumination-dependent). We support our criticisms with data from colour matching experiments with humans, involving self-made, printed colour charts. RESULTS: Colour matching experiments with printed charts involving 11 subjects showed that the choices made by individuals were significantly different between charts that had exactly the same RGB values, but were produced from different printers. Furthermore, individual matches tended to vary under different lighting conditions. Spectrophotometry of the colour charts showed that the reflectance spectra of the charts varied greatly between printers and that equal steps in RGB space were often far from equal in terms of reflectance on the printed charts. CONCLUSION: in addition to outlining theoretical criticisms of the use of colour charts, our empirical results show that: individuals vary in their perception of colours, that different printers produce strikingly different results when reproducing what should be the same chart, and that the characteristics of the light irradiating the surface do affect colour perception. Therefore, we urge great caution in the use of colour charts to study animal colour signals. They should be used only as a last resort and in full knowledge of their limitations, with specially produced charts made to high industry standards.
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2004
Stevens M, Cuthill IC, Sheppard J, Maddocks T, Troscianko T, Parraga CA (2004). Disruptive colouration and background pattern matching in insect crypsis.
PERCEPTION,
33, 35-36.
Author URL.
2003
Franks NR, Dornhaus A, Fitzsimmons JP, Stevens M (2003). Speed versus accuracy in collective decision making.
Proc Biol Sci,
270(1532), 2457-2463.
Abstract:
Speed versus accuracy in collective decision making.
We demonstrate a speed versus accuracy trade-off in collective decision making. House-hunting ant colonies choose a new nest more quickly in harsh conditions than in benign ones and are less discriminating. The errors that occur in a harsh environment are errors of judgement not errors of omission because the colonies have discovered all of the alternative nests before they initiate an emigration. Leptothorax albipennis ants use quorum sensing in their house hunting. They only accept a nest, and begin rapidly recruiting members of their colony, when they find within it a sufficient number of their nest-mates. Here we show that these ants can lower their quorum thresholds between benign and harsh conditions to adjust their speed-accuracy trade-off. Indeed, in harsh conditions these ants rely much more on individual decision making than collective decision making. Our findings show that these ants actively choose to take their time over judgements and employ collective decision making in benign conditions when accuracy is more important than speed.
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