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For camouflage to succeed, an individual has to pass undetected, unrecognized or untargeted, and hence it is the processing of visual information that needs to be deceived. Camouflage is therefore an adaptation to the perception and cognitive mechanisms of another animal. Although this has been acknowledged for a long time, there has been no unitary account of the link between visual perception and camouflage. Viewing camouflage as a suite of adaptations to reduce the signal-to-noise ratio provides the necessary common framework. We review the main processes in visual perception and how animal camouflage exploits these. We connect the function of established camouflage mechanisms to the analysis of primitive features, edges, surfaces, characteristic features and objects (a standard hierarchy of processing in vision science). Compared to the commonly used research approach based on established camouflage mechanisms, we argue that our approach based on perceptual processes targeted by camouflage has several important benefits: specifically, it enables the formulation of more precise hypotheses and addresses questions that cannot even be identified when investigating camouflage only through the classic approach based on the patterns themselves. It also promotes a shift from the appearance to the mechanistic function of animal coloration. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

Background matching is the most familiar and widespread camouflage strategy: avoiding detection by having a similar colour and pattern to the background. Optimizing background matching is straightforward in a homogeneous environment, or when the habitat has very distinct sub-types and there is divergent selection leading to polymorphism. However, most backgrounds have continuous variation in colour and texture, so what is the best solution? Not all samples of the background are likely to be equally inconspicuous, and laboratory experiments on birds and humans support this view. Theory suggests that the most probable background sample (in the statistical sense), at the size of the prey, would, on average, be the most cryptic. We present an analysis, based on realistic assumptions about low-level vision, that estimates the distribution of background colours and visual textures, and predicts the best camouflage. We present data from a field experiment that tests and supports our predictions, using artificial moth-like targets under bird predation. Additionally, we present analogous data for humans, under tightly controlled viewing conditions, searching for targets on a computer screen. These data show that, in the absence of predator learning, the best single camouflage pattern for heterogeneous backgrounds is the most probable sample.

Countershading, the widespread tendency of animals to be darker on the side that receives strongest illumination, has classically been explained as an adaptation for camouflage: obliterating cues to 3D shape and enhancing background matching. However, there have only been two quantitative tests of whether the patterns observed in different species match the optimal shading to obliterate 3D cues, and no tests of whether optimal countershading actually improves concealment or survival. We use a mathematical model of the light field to predict the optimal countershading for concealment that is specific to the light environment and then test this prediction with correspondingly patterned model “caterpillars” exposed to avian predation in the field. We show that the optimal countershading is strongly illumination-dependent. A relatively sharp transition in surface patterning from dark to light is only optimal under direct solar illumination; if there is diffuse illumination from cloudy skies or shade, the pattern provides no advantage over homogeneous background-matching coloration. Conversely, a smoother gradation between dark and light is optimal under cloudy skies or shade. The demonstration of these illumination-dependent effects of different countershading patterns on predation risk strongly supports the comparative evidence showing that the type of countershading varies with light environment.

While one has evolved and the other been consciously created, animal and military camouflage are expected to show many similar design principles. Using a unique database of calibrated photographs of camouflage uniform patterns, processed using texture and colour analysis methods from computer vision, we show that the parallels with biology are deeper than design for effective concealment. Using two case studies we show that, like many animal colour patterns, military camouflage can serve multiple functions. Following the dissolution of the Warsaw Pact, countries that became more Western-facing in political terms converged on NATO patterns in camouflage texture and colour. Following the break-up of the former Yugoslavia, the resulting states diverged in design, becoming more similar to neighbouring countries than the ancestral design. None of these insights would have been obtained using extant military approaches to camouflage design, which focus solely on concealment. Moreover, our computational techniques for quantifying pattern offer new tools for comparative biologists studying animal coloration. This article is part of the themed issue ‘Animal coloration: production, perception, function and application'.

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.

The evolution of defensive coloration is dependent upon a complex combination of variables related to the prey itself, the environmental context, and predator vision/behavior. Some animals reflect ultraviolet (UV) light, which is visible to predators such as birds. Here, we report the first case of UV reflectance by a tadpole (Ololygon machadoi, Hylidae) and we investigate whether it influences tadpole detection/predation by a bird (Saltator similis, Passeriformes) on backgrounds of three different colors (two blending – yellow and dark – and one not blending – blue – to tadpole colors). We also tested whether a previous experience of the birds with the prey species would influence the search/attack effort. Although many predation attempts were recorded, birds did not consume many tadpoles. Birds attempted to/preyed upon more tadpoles on dark backgrounds in the presence rather than the absence of the UV portion of the spectrum; attempt/predation on the other backgrounds (yellow, blue) did not change between the UV conditions. The differential contrast of dark and yellow tadpole body parts against specific backgrounds may aid to disruptive properties. The interruption of tadpole body contour was probably enough to prevent detection on yellow and blue backgrounds based on low achromatic contrasts of yellow and dark body parts against such backgrounds, respectively. On dark backgrounds, however, an increase in the chromatic contrast of yellow bars caused by UV reflectance may have reached a threshold and increased tadpole detectability. Birds spent more time searching for tadpoles and visited more frequently the trays in the first than in the second trial, and several tadpoles were regurgitated after ingestion, suggesting that the existence of a potential aposematic role of the UV reflectance cannot be disregarded. Birds also spent more time inspecting blue backgrounds, which may relate to their unfamiliarity with such backgrounds. UV reflectance in O. machadoi tadpoles unveals a potential adaptative value of this feature for the larval stage, and our results indicate influence on tadpole detection by birds. The effects of such adaptation in natural habitats and its occurrence in other species deserve further studies.

Animal warning signals show remarkable diversity, yet subjectively appear to share certain visual features that make defended prey stand out and look different from more cryptic palatable species. For example, many (but far from all) warning signals involve high contrast elements, such as stripes and spots, and often involve the colours yellow and red. How exactly do aposematic species differ from non‐aposematic ones in the eyes (and brains) of their predators? Here, we develop a novel computational modelling approach, to quantify prey warning signals and establish what visual features they share. First, we develop a model visual system, made of artificial neurons with realistic receptive fields, to provide a quantitative estimate of the neural activity in the first stages of the visual system of a predator in response to a pattern. The system can be tailored to specific species. Second, we build a novel model that defines a ‘neural signature’, comprising quantitative metrics that measure the strength of stimulation of the population of neurons in response to patterns. This framework allows us to test how individual patterns stimulate the model predator visual system. For the predator–prey system of birds foraging on lepidopteran prey, we compared the strength of stimulation of a modelled avian visual system in response to a novel database of hyperspectral images of aposematic and undefended butterflies and moths. Warning signals generate significantly stronger activity in the model visual system, setting them apart from the patterns of undefended species. The activity was also very different from that seen in response to natural scenes. Therefore, to their predators, lepidopteran warning patterns are distinct from their non‐defended counterparts and stand out against a range of natural backgrounds. For the first time, we present an objective and quantitative definition of warning signals based on how the pattern generates population activity in a neural model of the brain of the receiver. This opens new perspectives for understanding and testing how warning signals have evolved, and, more generally, how sensory systems constrain signal design.

Most animals need to move, and motion will generally break camouflage. In many instances, most of the visual field of a predator does not fall within a high-resolution area of the retina and so, when an undetected prey moves, that motion will often be in peripheral vision. We investigate how this can be exploited by prey, through different patterns of movement, to reduce the accuracy with which the predator can locate a cryptic prey item when it subsequently orients towards a target. The same logic applies for a prey species trying to localize a predatory threat. Using human participants as surrogate predators, tasked with localizing a target on peripherally viewed computer screens, we quantify the effects of movement (duration and speed) and target pattern. We show that, while motion is certainly detrimental to camouflage, should movement be necessary, some behaviours and surface patterns reduce that cost. Our data indicate that the phenotype that minimizes localization accuracy is unpatterned, having the mean luminance of the background, does not use a startle display prior to movement, and has short (below saccadic latency), fast movements.

An animal’s vulnerability to predators can be influenced by its behavior, morphology, body size, coloration, habitat preferences, and palatability. We tested whether the coloration of Bokermannohyla saxicola and Scinax machadoi tadpoles affects their survival when exposed to local visually oriented predators at a site in southeastern Brazil. We tested three aquatic invertebrates (Aeshnidae, Belostoma sp., Lethocerus sp.) and birds as tadpole predators. We predicted that predation rates would differ depending on the substrate where the tadpoles positioned themselves (light or dark), hypothesizing that each tadpole would use preferentially a background that conferred camouflage and that predation levels would be lower on such backgrounds compared to others. B. saxicola had higher survivorship than S. machadoi on light backgrounds at some instances, in accordance with its crypsis hypothesis. However, B. saxicola tadpoles did not use light backgrounds more often than dark ones. S. machadoi coloration looked disruptive on both light and dark backgrounds, and tadpoles showed no preference or differences in survival rates between these backgrounds. Predation rates did not differ between the two species in a way that could confirm a previous hypothesis of aposematic/mimetic coloration for S. machadoi tadpoles. Our results show that colorations that appear to function to impair visual detection may play this role at some circumstances but not others. Tadpole colorations may have evolved in another context, in which avoiding visual detection by predators was a stronger selective pressure. In a context with lower predation pressure from visually oriented predators, the expected background choice behavior for increased camouflage may not be strongly selected for.