Visual camouflage evolves within the bounds of light's interaction with the surroundings and the sensory limits of its observers. Rapid temporal variation in lighting from weather and its interaction with objects within the visual scene alters the contrast of the spatio-chromatic features of both backgrounds and animals, the latter through self-shading and received shadows from their surroundings. Despite the apparent effect of lighting on object appearance, the enormity of interactions and the diversity of animal phenotypic solutions present challenges to investigating the combined effects of lighting and habitat structure on camouflage effectiveness and design. Genetic algorithms and mathematical animal pattern generation provide a potential solution to investigating this high-dimensional feature space. Here, an online artificial evolution experiment was used to examine the effect of lighting and habitat geometry on camouflage. Lighting and geometry changed which prey phenotypes evolved, and the predictive power of common camouflage metrics. Crucially, lighting condition systematically altered prey-targets' internal contrast and interacted with habitat geometry, affecting the evolved patterning, colour, and countershading. Our work demonstrates the importance of considering the relative geometry and lighting of an environment when determining the function of animal colouration and the adaptive value of camouflage. ### Competing Interest Statement The authors have declared no competing interest.

The color, geometry, and lighting of environments characterize their appearance to humans and other animals and, as a result, play an important role in the evolution of animal coloration. Yet despite this long-standing association, how changes in lighting from atmospheric conditions and three-dimensional geometry interact to alter the appearance of scenes and animals remains largely unexplored. To investigate these interactions, we quantified the appearance of natural backgrounds and standardized targets with color-calibrated photographs and 3-dimensional (3D) scans of 672 natural scenes taken under diffuse and direct lighting conditions. We find several instances where lighting and the local 3D environment systematically altered the colors and patterns of scenes and a target object, as well as their relationship with spatial scale. Shadows formed under direct lighting increased luminance and short-long wave (blue-yellow) contrast across spatial scales, especially at larger spatial scales for habitats with a greater 3D variation. Conversely, medium-long wave (green-red) information was highly stable to changes in lighting. Direct lighting and the 3D environment also influenced the directionality and orientation of patterns within the scenes and targets due to the formation of cast and self-shadows of different orientations. These analyses demonstrate the importance of considering the lighting and geometry of an environment when comparing the statistics of animals and their backgrounds. ### Competing Interest Statement The authors have declared no competing interest.

In many taxa, young can recognize their parents using olfactory cues. Yet this possibility has been overlooked in birds, because they were long assumed to have a poor sense of smell. While evidence is growing that birds use odours to communicate, olfactory parent recognition has only been documented in two altricial bird species. Whether chicks of precocial species use olfaction to recognize parents is currently unknown. Parent recognition is particularly important in precocial species, as chicks leave the nest shortly after hatching, and may lose contact with their parents and encounter other conspecific adults. We conducted Y-maze trials in the wild to test whether chicks of a precocial shorebird, the whitefronted plover, Anarhynchus marginatus, can recognize parents via olfaction. We tested first whether chicks show a preference for the odour (preen oil) of an unfamiliar adult over a control (no odour), and second whether chicks show a preference for the odour of a parent over that of an unfamiliar adult. Plover chicks spent as much time with the odour of an unfamiliar adult as with the control, and as much time with the odour of a parent as with that of an unfamiliar adult. Therefore, we found no evidence that chicks react to the preen oil odour of a conspecific adult, nor that they can discriminate a parent using preen oil odours. It may be that chicks of this species can discriminate parental and foreign odours but that our experiment failed to detect it, that they rely on other (e.g. auditory) cues, or that they do not need to discriminate between parents and foreign conspecific adults. (c) 2024 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

Abstract 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.

Camouflage is frequently used by animals for concealment and thereby improves survival. Typically, it is the animal's own colour and patterning that are expected to affect its detectability; however, the complexity of the background can also have an influence. Although there is a growing literature examining this, the underlying exact mechanism is unknown. In this study we addressed this issue by using humans as proxy ‘predators’ in a computer-based search task and monitoring their detection times for targets on varying backgrounds. By using artificial greyscale targets and backgrounds, we were able to isolate and manipulate the normally covarying factors that comprise ‘complexity’ in natural habitats. We show that reduced detection was explained not by greater information content (entropy) or higher variance in the background's features per se, but instead by reduced signal-to-noise ratio in the visual features that potentially distinguish target from background. This raises questions about when the term complexity should be used, and how observers learn the characteristics of a background.

Preen oil, the secretion from the uropygial gland of birds, may have a specific function in incubation. Consistent with this, during incubation, the chemical composition of preen oil is more likely to differ between sexes in species where only one sex incubates than in species where both sexes incubate. In this study, we tested the generality of this apparent difference, by investigating sex differences in the preen oil composition of a shorebird species, the Kentish plover (Anarhynchus, formerly Charadrius, alexandrinus). As both sexes incubate in this species, we predicted the absence of sex differences in preen oil composition during incubation. In the field, we sampled preen oil from nine females and 11 males during incubation, which we analysed with gas chromatography–mass spectrometry (GC–MS). Consistent with predictions, we found no sex difference in preen oil composition, neither in beta diversity (Bray-Curtis dissimilarities) nor in alpha diversity (Shannon index and number of substances). Based on these results, we cannot conclude whether preen oil has a function during incubation in Kentish plovers. Still, we discuss hypothetical roles, such as olfactory crypsis, protection against ectoparasites or olfactory intraspecific communication, which remain to be tested.

Prey animals typically try to avoid being detected and/or advertise to would-be predators that they should be avoided. Both anti-predator strategies primarily rely on colour to succeed, but the specific patterning used is also important. While the role of patterning in camouflage is relatively clear, the design features of aposematic patterns are less well understood. Here, we use a comparative approach to investigate how pattern use varies across a phylogeny of 268 species of cryptic and aposematic butterfly larvae, which also vary in social behaviour. We find that longitudinal stripes are used more frequently by cryptic larvae, and that patterns putatively linked to crypsis are more likely to be used by solitary larvae. By contrast, aposematic larvae are more likely to use horizontal bands and spots, but we find no differences in the use of individual pattern elements between solitary and gregarious aposematic species. However, solitary aposematic larvae are more likely to display multiple pattern elements, whereas those with no pattern are more likely to be gregarious. Our study advances our understanding of how pattern variation, coloration and social behaviour covary across lepidopteran larvae, and highlights new questions about how patterning affects larval detectability and predator responses to aposematic prey.

Parental care can enhance offspring survival but may impose significant costs to parents. The costs and benefits of care are key to understanding patterns of parental care, where parents can benefit by having their partner increase investment in care, while reducing their own effort. However, investigating the costs and benefits of parental care in wild populations is challenging. Here we use highly detailed behavioral observations in families of a small shorebird, where one parent frequently deserts its offspring, to explore the potential costs and benefits of desertion in a wild population. We first show that females desert their broods more frequently than males. Second, we investigate the benefits of this frequent female desertion in terms of additional mating opportunities, and the costs of desertion to females in terms of the growth and survival of deserted offspring. Our results indicate that female desertion is favored by a combination of remating benefits and a lack of costs to brood growth and survival, as abandoned male parents continue to provide care after desertion. Our results shed light on the costs and benefits underlying natural desertion strategies and suggest that female desertion is a fine-tuned behavior that responds to seasonally changing benefits of desertion.

'Dazzle coloration' describes a wide variety of high-contrast patterns allegedly providing protection against attack during motion. Previous research falls into three broad groups. First, studies using humans demonstrate that certain surface patterns can cause significant misperceptions in controlled laboratory conditions, although the effects are inconsistent in both direction and magnitude. Second, experiments on target capture or tracking also show effects that are strongly dependent upon the test paradigm. It has not been established that these laboratory findings generalize to other species, or to the real world. Third, mainly comparative studies build a case for longitudinal striping being involved in escape strategies in some squamate reptiles. We suggest that: (1) the concept of dazzle conflates a description of appearance with presumed function; ( 2) some effects attributed to dazzle have not been distinguished clearly from other mechanisms of protective coloration; and (3) confusion persists over the evidence necessary to attribute a dazzle function to markings. We refine the definition of dazzle to exclude appearance: dazzle is coloration that interferes with target interception, as a result of misperception of its speed, trajectory and/or range. Our review clarifies discussion of dazzle, and sets out a coherent and practical framework for future research.

Iridescence is a taxonomically widespread form of structural coloration that produces often intense hues that change with the angle of viewing. Its role as a signal has been investigated in multiple species, but recently, and counter-intuitively, it has been shown that it can function as camouflage. However, the property of iridescence that reduces detectability is, as yet, unclear. As viewing angle changes, iridescent objects change not only in hue but also in intensity, and many iridescent animals are also shiny or glossy; these "specular reflections," both from the target and background, have been implicated in crypsis. Here, we present a field experiment with natural avian predators that separate the relative contributions of color and gloss to the "survival" of iridescent and non-iridescent beetle-like targets. Consistent with previous research, we found that iridescent coloration, and high gloss of the leaves on which targets were placed, enhance survival. However, glossy targets survived less well than matt. We interpret the results in terms of signal-to-noise ratio: specular reflections from the background reduce detectability by increasing visual noise. While a specular reflection from the target attracts attention, a changeable color reduces the signal because, we suggest, normally, the color of an object is a stable feature for detection and identification.

Many species gain antipredator benefits by combining gregarious behavior with warning coloration, yet there is debate over which trait evolves first and which is the secondary adaptive enhancement. Body size can also influence how predators receive aposematic signals and potentially constrain the evolution of gregarious behavior. To our knowledge, the causative links between the evolution of gregariousness, aposematism, and larger body sizes have not been fully resolved. Here, using the most recently resolved butterfly phylogeny and an extensive new dataset of larval traits, we reveal the evolutionary interactions between important traits linked to larval gregariousness. We show that larval gregariousness has arisen many times across butterflies, and aposematism is a likely prerequisite for gregariousness to evolve. We also find that body size may be an important factor for determining the coloration of solitary, but not gregarious, larvae. Additionally, by exposing artificial larvae to wild avian predation, we show that undefended, cryptic larvae are heavily predated when aggregated but benefit from solitariness, whereas the reverse is true for aposematic prey. Our data reinforce the importance of aposematism for gregarious larval survival while identifying new questions about the roles of body size and toxicity in the evolution of grouping behavior.

Natural and sexual selection can be in conflict in driving the evolution of sexual ornamentation. Sexual selection typically favors detectability to potential mates, whereas natural selection penalizes detectability to avoid predators. Considering signal efficiency in addition to detectability, however, suggests that natural and sexual selection need not be antagonistic. In the visual domain, considerable evidence supports a “processing bias” by which people prefer images that match the spatial statistics of natural scenes, likely because the brain has evolved to process such scenes efficiently. A direct but untested prediction of this bias is that background-matching camouflage can be favored by natural and sexual selection. We conducted an online experiment where we show for the first time human preference for camouflaged patterning and confirm a preference for the average image statistics of natural scenes. Because many of the underlying visual mechanisms are shared across vertebrates, our results suggest that camouflage patterns can serve as evolutionary precursors of sexual signals. ### Competing Interest Statement The authors have declared no competing interest.

Abstract The nests of ground‐nesting birds rely heavily on camouflage for their survival, and predation risk, often linked to ecological changes from human activity, is a major source of mortality. Numerous ground‐nesting bird populations are in decline, so understanding the effects of camouflage on their nesting behavior is relevant to their conservation concerns. Habitat three‐dimensional (3D) geometry, together with predator visual abilities, viewing distance, and viewing angle, determine whether a nest is either visible, occluded, or too far away to detect. While this link is intuitive, few studies have investigated how fine‐scale geometry is likely to help defend nests from different predator guilds. We quantified nest visibility based on 3D occlusion, camouflage, and predator visual modeling in northern lapwings, Vanellus vanellus, on different land management regimes. Lapwings selected local backgrounds that had a higher 3D complexity at a spatial scale greater than their entire clutches compared to local control sites. Importantly, our findings show that habitat geometry—rather than predator visual acuity—restricts nest visibility for terrestrial predators and that their field habitats, perceived by humans as open, are functionally closed with respect to a terrestrial predator searching for nests on the ground. Taken together with lapwings' careful nest site selection, our findings highlight the importance of considering habitat geometry for understanding the evolutionary ecology and management of conservation sites for ground‐nesting birds.

Natural and sexual selection can be in conflict in driving the evolution of sexual ornamentation. Sexual selection typically favors detectability to potential mates, whereas natural selection penalizes detectability to avoid predators. Considering signal efficiency in addition to detectability, however, suggests that natural and sexual selection need not be antagonistic. In the visual domain, considerable evidence supports a "processing bias" by which people prefer images that match the spatial statistics of natural scenes, likely because the brain has evolved to process such scenes efficiently. A direct but untested prediction of this bias is that background-matching camouflage can be favored by natural and sexual selection. We conducted an online experiment where we show for the first time human preference for camouflaged patterning and confirm a preference for the average image statistics of natural scenes. Because many of the underlying visual mechanisms are shared across vertebrates, our results suggest that camouflage patterns can serve as evolutionary precursors of sexual signals.

Reporting of outcome variables by caregivers in welfare studies is commonplace but is open to subjective bias and so requires validation. Biases can occur in either direction: familiarity with an animal allows a deeper insight into welfare problems, but also can lead to reticence in admitting that an animal in one's care is experiencing problems. Here, we aim to validate owner-reporting of plumage condition of pet parrots, including those with self-inflicted feather-damaging behaviour (FDB), by comparing owners' scores of feather condition with those of two independent raters, blind to the owners' and each other's assessments. We surveyed pet parrot owners to collect data on basic demographics and feather condition, and requested four standardised photographs of birds. We received 259 responses (17% of the 1,521 people contacted); 78 sets of images of appropriate quality for assessment by raters were provided. Mean percentage agreement between owners' and raters' scores was mostly fair to substantial using Cohen's kappa; however, raters scored a greater proportion of feather damage than did owners. Overall, our results indicate owner-reporting of feather condition, including FDB, to be generally reliable and consistent with independent assessment of photographs. As the use of photographs can be limited by image quality, a failure to represent the long-term state of a parrot, and the potential for incorrect recording if assessed without relevant information (eg on moulting), this evidence that owner-reports can be reliable opens the door for larger-scale surveys of the extent of welfare-relevant problems.

All mammals experience different life stages as they develop, each of which is characterised by particular physical and behavioural changes. Despite the emergence of sophisticated behaviour analysis techniques, the ways in which social behaviour varies by life stage, and how this is influenced by an individual’s sex, is relatively understudied in most social mammals other than primates and elephants. Understanding the social requirements of mammals should be a central and critical component to their conservation, captive management and welfare. Here, we apply social network analysis techniques to understand how social behaviour differs with life stage in the giraffe, a gregarious fission–fusion mammal. We studied two wild populations of giraffes in Kenya and found that adolescents have significantly stronger associations with adolescents of their own sex first and foremost, then adults of their own sex. Other associations were significantly lower than would be expected, or non-significant. Our results suggest that adolescence in both male and female giraffes shares similar features to adolescence in other social mammal species. We discuss how the application of such knowledge might improve the management and welfare of captive giraffes.

Smell is a sensory modality that is rarely considered in birds, but evidence is mounting that olfaction is an important aspect of avian behaviour and ecology. The uropygial gland produces an odoriferous secretion (preen oil) that can differ seasonally and between the sexes. These differences are hypothesized to function in olfactory camouflage, i.e. minimizing detection by nest predators (olfactory crypsis hypothesis), and/or intraspecific olfactory communication, particularly during breeding (sex semiochemical hypothesis). However, evidence for seasonal and sex differences in preen oil is mixed, with some studies finding differences and others not, and direct evidence for the putative function(s) of seasonal variation and sex differences in preen oil remains limited. We conducted a systematic review of the evidence for such changes in preen oil chemical composition, finding seasonal differences in 95% of species (57/60 species in 35 studies) and sex differences in 47% of species (28/59 species in 46 studies). We then conducted phylogenetic comparative analyses using data from 59 bird species to evaluate evidence for both the olfactory crypsis and sex semiochemical hypotheses. Seasonal differences were more likely in the incubating than non-incubating sex in ground-nesting species, but were equally likely regardless of incubation strategy in non-ground-nesting species. This result supports the olfactory crypsis hypothesis, if ground nesters are more vulnerable to olfactorily searching predators than non-ground nesters. Sex differences were more likely in species with uniparental than biparental incubation and during breeding than non-breeding, consistent with both the olfactory crypsis and sex semiochemical hypotheses. At present, the data do not allow us to disentangle these two hypotheses, but we provide recommendations that will enable researchers to do so.

It has recently been found that iridescence, a taxonomically widespread form of animal coloration defined by a change in hue with viewing angle, can act as a highly effective form of camouflage. However, little is known about whether iridescence can confer a survival benefit to prey postdetection and, if so, which optical properties of iridescent prey are important for this putative protective function. Here, we tested the effects of both iridescence and surface gloss (i.e. specular reflection) on the attack behaviour of prey-naïve avian predators. Using real and artificial jewel beetle, Sternocera aequisignata, wing cases, we found that iridescence provides initial protection against avian predation by significantly reducing the willingness to attack. Importantly, we found that the main factor explaining this aversion is iridescence, not multiple colours per se, with surface gloss also having an independent effect. Our results are important because they demonstrate that even when prey are presented up close and against a mismatching background, iridescence may confer a survival benefit by inducing hesitation or even, as sometimes observed, an aversion response in attacking birds. Furthermore, this means that even postdetection, prey do not necessarily need to have secondary defences such as sharp spines or toxins for iridescence to have a protective effect. Taken together, our results suggest that reduced avian predation could facilitate the initial evolution of iridescence in many species of insects and that it is the defining feature of iridescence, its colour changeability, that is important for this effect.

Avoiding detection through camouflage is often key to survival. However, an animal's appearance is not the only factor affecting conspicuousness: background complexity also alters detectability. This has been experimentally demonstrated for both artificially patterned backgrounds in the laboratory and natural backgrounds in the wild, but only for targets that already match the background well. Do habitats of high visual complexity provide concealment to even relatively poorly camouflaged animals? Using artificial prey which differed in their degrees of background matching to tree bark, we were able to determine their survival, under bird predation, with respect to the natural complexity of the background. The latter was quantified using low-level vision metrics of feature congestion (or ‘visual clutter’) adapted for bird vision. Higher background orientation clutter (edges with varying orientation) reduced the detectability of all but the poorest background-matching camouflaged treatments; higher background luminance clutter (varying achromatic lightness) reduced average mortality for all treatments. Our results suggest that poorer camouflage can be mitigated by more complex backgrounds, with implications for both camouflage evolution and habitat preferences.

Evolutionary biologists frequently wish to measure the fitness of alternative phenotypes using behavioral experiments. However, many phenotypes are complex. One example is coloration: camouflage aims to make detection harder, while conspicuous signals (e.g., for warning or mate attraction) require the opposite. Identifying the hardest and easiest to find patterns is essential for understanding the evolutionary forces that shape protective coloration, but the parameter space of potential patterns (colored visual textures) is vast, limiting previous empirical studies to a narrow range of phenotypes. Here, we demonstrate how deep learning combined with genetic algorithms can be used to augment behavioral experiments, identifying both the best camouflage and the most conspicuous signal(s) from an arbitrarily vast array of patterns. To show the generality of our approach, we do so for both trichromatic (e.g., human) and dichromatic (e.g., typical mammalian) visual systems, in two different habitats. The patterns identified were validated using human participants; those identified as the best for camouflage were significantly harder to find than a tried-and-tested military design, while those identified as most conspicuous were significantly easier to find than other patterns. More generally, our method, dubbed the "Camouflage Machine," will be a useful tool for identifying the optimal phenotype in high dimensional state spaces.