When animal coloration is a poor match

Biologists usually pursue the adaptationist paradigm in trying to explain the functional significance of animal coloration. Here I collate instances in which coloration may be a poor match in the context of background matching, Batesian mimicry, aposematism, and colour polymorphisms. This can occur because of trade-offs with other functions, relaxed selection from predation, or colour trait neutrality. Also, mechanistic, pleiotropic and chance genetic effects can all result in a poor match to the background environment or to signaling efficiently. While biologists implicitly recognise these constraints placed on adaptive coloration, they rarely explicitly acknowledge the heterodox notion that coloration might be under weak selection or no selection at all. Unfortunately, it is difficult to show this definitively, as illustrated in an investigation into the function of colour polymorphisms in coconut crabs.

A comparison of animal color measurements using a commercially available digital color sensor and photograph analysis

An animal’s pelage, feather, or skin color can serve a variety of functions, so it is important to have multiple standardized methods for measuring color. One of the most common and reliable methods for measuring animal coloration is the use of standardized digital photographs of animals. New technology in the form of a commercially available handheld digital color sensor could provide an alternative to photography-based animal color measurements. To determine whether a digital color sensor could be used to measure animal coloration, we tested the ability of a digital color sensor to measure coloration of mammalian, avian, and lepidopteran museums specimens. We compared results from the sensor to measurements taken using traditional photography methods. Our study yielded significant differences between photography-based and digital color sensor measurements of brightness (light to dark) and colors along the green to red spectrum. There was no difference between photographs and the digital color sensor measurements for colors along the blue to yellow spectrum. The average difference in recorded color (ΔE) by the 2 methods was above the threshold at which humans can perceive a difference. There were significant correlations between the sensor and photographs for all measurements indicating that the sensor is an effective animal coloration measuring tool. However, the sensor’s small aperture and narrow light spectrum range designed for human-vision limit its value for ecological research. We discuss the conditions in which a digital color sensor can be an effective tool for measuring animal coloration in both laboratory settings and in the field.

Frequency dispersion reveals chromophore diversity and colour-tuning mechanism in parrot feathers

Variation in animal coloration is often viewed as the result of chemically distinct pigments conferring different hues. The role of molecular environment on hue tends to be overlooked as analyses are mostly performed on free pigments extracted from the integument. Here we analysed psittacofulvin pigments within parrot feathers to explore whether the in situ organization of pigments may have an effect on hue. Resonance Raman spectra from a red region of a yellow-naped amazon Amazona auropalliata tail feather show frequency dispersion, a phenomenon that is related to the presence of a range of molecular conformations (and multiple chromophores) in the pigment, whereas spectra from a yellow region on the same feather do not show the same evidence for multiple chromophores. Our findings are consistent with non-isomeric psittacofulvin pigments behaving as a single chromophore in yellow feather barbs, which implies that psittacofulvins are dispersed into a structurally disordered mixture in yellow feathers compared with red feathers. Frequency dispersion in red barbs may instead indicate that pigments are structurally organized through molecule–molecule interactions. Major differences in the hues of parrot feathers are thus associated with differences in the organization of pigments within feathers.

The perfection of mimicry: an information approach

We consider why imperfect deceptive mimics can persist when it appears to be in the predator's interest to discriminate finely between mimics and their models. One theory is that a receiver will accept being duped if the model and mimic overlap in appearance and the relative costs of attacking the model are high. However, a more fundamental explanation for the difficulty of discrimination is not based on perceptual uncertainty, but simply based on a lack of information. In particular, predators in the process of learning may cease sampling imperfect mimics entirely because the immediate pay-off and future value of information is low, allowing such mimics to persist. This outcome will be particularly likely when the model is relatively costly to attack and/or the discriminative rules the predator has to learn are complex. Information limitations neatly explain why predators tend to adopt discriminative rules based on single traits (such as stripe colour), rather than on combinations of traits (such as stripe order). They also explain why predators utilize certain salient discriminative traits while ignoring equally informative ones (a phenomenon known as overshadowing), and why imperfect mimics may be more common in phenotypically diverse prey communities. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

Human colour in mate choice and competition

The colour of our skin and clothing affects how others perceive us and how we behave. Human skin colour varies conspicuously with genetic ancestry, but even subtle changes in skin colour due to diet, blood oxygenation and hormone levels influence social perceptions. In this review, we describe the theoretical and empirical frameworks in which human colour is researched. We explore how subtle skin colour differences relate to judgements of health and attractiveness. Also, because humans are one of the few organisms able to manipulate their apparent colour, we review how cosmetics and clothing are implicated in courtship and competition, both inside the laboratory and in the real world. Research on human colour is in its infancy compared with human psychophysics and colour research in non-human animals, and hence we present best-practice guidelines for methods and reporting, which we hope will improve the validity and reproducibility of studies on human coloration. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

Genomics of coloration in natural animal populations

Animal coloration has traditionally been the target of genetic and evolutionary studies. However, until very recently, the study of the genetic basis of animal coloration has been mainly restricted to model species, whereas research on non-model species has been either neglected or mainly based on candidate approaches, and thereby limited by the knowledge obtained in model species. Recent high-throughput sequencing technologies allow us to overcome previous limitations, and open new avenues to study the genetic basis of animal coloration in a broader number of species and colour traits, and to address the general relevance of different genetic structures and their implications for the evolution of colour. In this review, we highlight aspects where genome-wide studies could be of major utility to fill in the gaps in our understanding of the biology and evolution of animal coloration. The new genomic approaches have been promptly adopted to study animal coloration although substantial work is still needed to consider a larger range of species and colour traits, such as those exhibiting continuous variation or based on reflective structures. We argue that a robust advancement in the study of animal coloration will also require large efforts to validate the functional role of the genes and variants discovered using genome-wide tools. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

Interspecific visual signalling in animals and plants: a functional classification

Organisms frequently gain advantages when they engage in signalling with individuals of other species. Here, we provide a functionally structured framework of the great variety of interspecific visual signals seen in nature, and then describe the different signalling mechanisms that have evolved in response to each of these functional requirements. We propose that interspecific visual signalling can be divided into six major functional categories: anti-predator, food acquisition, anti-parasite, host acquisition, reproductive and agonistic signalling, with each function enabled by several distinct mechanisms. We support our classification by reviewing the ecological and behavioural drivers of interspecific signalling in animals and plants, principally focusing on comparative studies that address large-scale patterns of diversity. Collating diverse examples of interspecific signalling into an organized set of functional and mechanistic categories places anachronistic behavioural and morphological labels in fresh context, clarifies terminology and redirects research effort towards understanding environmental influences driving interspecific signalling in nature. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

The current and future state of animal coloration research

Animal colour patterns are a model system for understanding evolution because they are unusually accessible for study and experimental manipulation. This is possible because their functions are readily identifiable. In this final paper of the symposium we provide a diagram of the processes affecting colour patterns and use this to summarize their functions and put the other papers in a broad context. This allows us to identify significant ‘holes’ in the field that only become obvious when we see the processes affecting colour patterns, and their interactions, as a whole. We make suggestions about new directions of research that will enhance our understanding of both the evolution of colour patterns and visual signalling but also illuminate how the evolution of multiple interacting traits works. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.