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Of all hypotheses advanced for why zebras have stripes, avoidance ofbiting fly attack receives by far the most support, yet the mechanisms bywhich stripes thwart landings are not yet understood. A logical and popularhypothesis is that stripes interfere with optic flow patterns needed by flyinginsects to execute controlled landings. This could occur through disruptingthe radial symmetry of optic flow via the aperture effect (i.e. generation offalse motion cues by straight edges), or through spatio-temporal aliasing(i.e. misregistration of repeated features) of evenly spaced stripes. By recordingand reconstructing tabanid fly behaviour around horses wearingdifferently patterned rugs, we could tease out these hypotheses using realistictarget stimuli. We found that flies avoided landing on, flew faster near,and did not approach as close to striped and checked rugs compared togrey. Our observations that flies avoided checked patterns in a similarway to stripes refutes the hypothesis that stripes disrupt optic flow via theaperture effect, which critically demands parallel striped patterns. Ourdata narrow the menu of fly-equid visual interactions that form the basisfor the extraordinary coloration of zebras.

Research on animal coloration is a vibrant area of biology currently involving evolutionary biologists, behavioural ecologists, psychologists, optical physicists, visual ecologists, geneticists and anthropologists. The proliferation of recent work requires that we take stock of the field, aiming to identify major themes, questions and future directions. This was the goal of the year-long Wissenschaftskolleg zu Berlin ‘focus group’ (2015–2016), in which many of the authors in this issue participated either as fellows or visitors. The result is this 19-chapter theme issue, in which we pinpoint the breakthroughs and challenges in animal coloration research, focusing on production, perception, function and evolution. We also explore animal coloration research as it applies to humans. This theme issue is by no means exhaustive but our goal has been to summarize …

Weaponry is ubiquitous in male ungulates and is driven by intrasexual selection, but the mystery surrounding its sporadic presence in females remains unsolved. Female horns are often smaller and shaped differently to male horns, suggesting a different function; indeed, hypotheses explaining the presence of female horns include competition for food, male mollification and defence against predators. Here we use comparative phylogenetic analyses to show that females are significantly more likely to bear horns in bovids that are conspicuous due to large body size and living in open habitats than inconspicuous species living in closed habitats or that are small. An inability to rely on crypsis or take refuge in deep vegetation has apparently driven the evolution of horns for defence against predators in female bovids, a finding supported by many field observations. Typically, exceptions are small species where females are territorial (e.g. duikers) and use horns in intrasexual contests. Furthermore, we suggest that conspicuousness and territoriality hypotheses may explain other instances of female cranial weaponry (i.e. antlers and ossicones) in other horned ruminants. Our phylogenetic reconstruction indicates that the primary function of horns in females is linked to antipredator defence in most clades, but occasionally to intrasexual competition in others.

While basic research on animal coloration is the theme of this special edition, here we highlight its applied significance for industry, innovation and society. Both the nanophotonic structures producing stunning optical effects and the colour perception mechanisms in animals are extremely diverse, having been honed over millions of years of evolution for many different purposes. Consequently, there is a wealth of opportunity for biomimetic and bioinspired applications of animal coloration research, spanning colour production, perception and function. Fundamental research on the production and perception of animal coloration is contributing to breakthroughs in the design of new materials (cosmetics, textiles, paints, optical coatings, security labels) and new technologies (cameras, sensors, optical devices, robots, biomedical implants). In addition, discoveries about the function of animal colour are influencing sport, fashion, the military and conservation. Understanding and applying knowledge of animal coloration is now a multidisciplinary exercise. Our goal here is to provide a catalyst for new ideas and collaborations between biologists studying animal coloration and researchers in other disciplines. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

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’.

Here I survey, collate and synthesize contrasting coloration in 5000 species of terrestrial mammals focusing on black and white pelage. After briefly reviewing alternative functional hypotheses for coloration in mammals, I examine nine colour patterns and combinations on different areas of the body and for each mammalian taxon to try to identify the most likely evolutionary drivers of contrasting coloration. Aposematism and perhaps conspecific signalling are the most consistent explanations for black and white pelage in mammals; background matching may explain white pelage. Evidence for contrasting coloration is being involved in crypsis through pattern blending, disruptive coloration or serving other functions, such as signalling dominance, lures, reducing eye glare or in temperature regulation has barely moved beyond anecdotal stages of investigation. Sexual dichromatism is limited in this taxon and its basis is unclear. Astonishingly, the functional significance of pelage coloration in most large charismatic black and white mammals that were new to science 150 years ago still remains a mystery.