The current and future state of animal coloration research

The integrative biology of decoy coloration in lizards

Anti-predator coloration is a widespread phenomenon that includes such tactics as aposematism, crypsis and mimicry. Most research on colorful anti-predator traits has focused on these well-known and well-studied tactics. One anti-predator trait that is found in diverse taxa yet is still poorly understood is anti-predator decoys. New advances in phylogenetics, genomics and molecular biology have allowed for an enriched understanding of the integrative biology of this type of anti-predator trait, particularly in lizards. While anti-predator decoy coloration is present in multiple taxa across the animal tree of life, it is particularly prevalent among lizards, which use both color and behavior to attract predator attention to the tail, which is often autotomizable and can regrow. In this Review, we discuss the integrative biology of decoy coloration in lizards, including the role of color, ontogenetic variation, gene expression and genomics. We begin by reviewing the mechanisms of the production of decoy coloration in lizards then discuss the function and macroevolution of decoy coloration. Finally, we suggest potentially fruitful avenues for future research on anti-predator decoys in lizards and other animal taxa.

GTP cyclohydrolase II (gch2) and axanthism in ball pythons: A new vertebrate model for pterin-based pigmentation

Pterin pigments are responsible for many of the bright colors observed across the animal kingdom. However, unlike melanin, the genetics of pterin-based pigmentation has received relatively little attention in animal coloration studies. Here, we investigate a lineage of axanthic ball pythons (Python regius) found in captivity as a model system to study pterin pigmentation in vertebrates. By crowdsourcing shed skin samples from commercial breeders and applying a case-control study design, we used whole-genome pool sequencing (pool-seq) and variant annotation. We identified a premature stop codon in the gene GTP cyclohydrolase II (gch2), which is associated with the axanthic phenotype. GCH2 catalyzes the first rate-limiting step in riboflavin biosynthesis. This study provides the first identification of an axanthism-associated gene in vertebrates and highlights the utility of ball pythons as a model to study pterin-based pigmentation.

Parasitism as a potential driver of aposematism

Aposematic animals couple unprofitability to predators, such as toxicity, with a warning signal, such as conspicuous coloration. Although toxicity and coloration can provide effective protection against predation, these traits also play a major role in the context of parasitism. Many of the color pigments used by aposematic animals are components of anti-infection immunity. Moreover, toxic compounds are used by conspicuous animals as defenses against parasites and pathogens. Parasites and pathogens not only pose selection on coloration through immunity and toxicity, they also play a major role in sexual selection, with mate choice often depending on conspicuous coloration and anti-infection toxicity. Consequently, parasitism is likely an important component in the evolution of traits that provide predator protection through aposematism.

Environmental Gradients in Lizard Colouration

Environmental pressures shape animal colouration, facilitating adaptation to local conditions. However, the extent to which climatic gradients drive colour variation in a species across its distributional range remains unclear. Here, we tested whether the dorsal colouration of Lusitanian wall lizards (Podarcis lusitanicus) varies spatially in response to environmental gradients across its distribution in the north-western Iberian Peninsula. We estimated dorsal colour brightness (i.e., lightness) from multispectral photographs of 463 animals, originating from 21 locations distributed across the species range. We studied direct and indirect (mediated by body mass) relationships between environmental variables and the lightness of lizards, by piecewise structural equation modelling. We simultaneously tested predictions from Gloger's (darker colouration in warm and humid environments), thermal melanism (darker colouration in colder environments), photoprotection (darker colouration in areas with higher intensity of solar radiation) and Bergmann's (larger body size in colder environments) hypotheses. We found that the lightness of lizards best follows predictions of Gloger's hypothesis for humidity, but not supporting the photoprotection hypothesis, independent of the populations' shared ancestry and geographic location. We found no support for direct thermal melanism, as temperature was not directly associated with lightness. Instead, the indirect effect of temperature on lightness through body size was detected. Consistent with Bergmann's hypothesis, lizards in colder regions tended to be larger and darker. Our study indicates that the evolution of lizard dorsal colouration is driven by variable climatic factors. Experimental tests are necessary to assess the mechanisms driving climatic effects on colouration across diverse environments, advancing beyond the simplistic correlations suggested by ecogeographic hypotheses.

How life became colourful: colour vision, aposematism, sexual selection, flowers, and fruits

Plants and animals are often adorned with potentially conspicuous colours (e.g. red, yellow, orange, blue, purple). These include the dazzling colours of fruits and flowers, the brilliant warning colours of frogs, snakes, and invertebrates, and the spectacular sexually selected colours of insects, fish, birds, and lizards. Such signals are often thought to utilize pre-existing sensitivities in the receiver's visual systems. This raises the question: what was the initial function of conspicuous colouration and colour vision? Here, we review the origins of colour vision, fruit, flowers, and aposematic and sexually selected colouration. We find that aposematic colouration is widely distributed across animals but relatively young, evolving only in the last ~150 million years (Myr). Sexually selected colouration in animals appears confined to arthropods and chordates, and is also relatively young (generally <100 Myr). Colourful flowers likely evolved ~200 million years ago (Mya), whereas colourful fruits/seeds likely evolved ~300 Mya. Colour vision (sensu lato) appears to be substantially older, and likely originated ~400-500 Mya in both arthropods and chordates. Thus, colour vision may have evolved long before extant lineages with fruit, flowers, aposematism, and sexual colour signals. We also find that there appears to have been an explosion of colour within the last ~100 Myr, including >200 origins of aposematic colouration across nine animal phyla and >100 origins of sexually selected colouration among arthropods and chordates.

A Ball Python Colour Morph Implicates MC1R in Melanophore-Xanthophore Distribution and Pattern Formation

Reptiles showcase an extensive array of skin colours and patterns, yet little is known about the genetics of reptile colouration. Here, we investigate the genetic basis of the Clown colour morph found in captive-bred ball pythons (Python regius) to study skin pigmentation and patterning in snakes. We obtained samples by crowdsourcing shed skin from commercial breeders and hobbyists. We applied a case-control design, whole-genome pool sequencing, variant annotation, histological analyses, and electron microscopy imaging. We identified a missense mutation in a transmembrane region of the melanocortin-1 receptor (MC1R) associated with the Clown phenotype. In classic avian and mammalian model species, MC1R is known for controlling the type and amount of melanin produced. In contrast, our results suggest that MC1R signalling might play a key role in pattern formation in ball pythons, affecting xanthophore-melanophore distribution. This work highlights the varied functions of MC1R across different vertebrate lineages and promotes a novel model system to study reptile colouration.

Current evidence of climate-driven colour change in insects and its impact on sexual signals

The colours of insects function in intraspecific communication such as sexual signalling, interspecific communication such as protection from predators, and in physiological processes, such as thermoregulation. The expression of melanin-based colours is temperature-dependent and thus likely to be impacted by a changing climate. However, it is unclear how climate change drives changes in body and wing colour may impact insect physiology and their interactions with conspecifics (e.g. mates) or heterospecific (e.g. predators or prey). The aim of this review is to synthesise the current knowledge of the consequences of climate-driven colour change on insects. Here, we discuss the environmental factors that affect insect colours, and then we outline the adaptive mechanisms in terms of phenotypic plasticity and microevolutionary response. Throughout we discuss the impact of climate-related colour change on insect physiology, and interactions with con-and-heterospecifics.

Using high-resolution microscopy data to generate realistic structures for electromagnetic FDTD simulations from complex biological models

Finite-difference time-domain (FDTD) electromagnetic simulations are a computational method that has seen much success in the study of biological optics; however, such simulations are often hindered by the difficulty of faithfully replicating complex biological microstructures in the simulation space. Recently, we designed simulations to calculate the trajectory of electromagnetic light waves through realistically reconstructed retinal photoreceptors and found that cone photoreceptor mitochondria play a substantial role in shaping incoming light. In addition to vision research and ophthalmology, such simulations are broadly applicable to studies of the interaction of electromagnetic radiation with biological tissue. Here, we present our method for discretizing complex 3D models of cellular structures for use in FDTD simulations using MEEP, the MIT Electromagnetic Equation Propagation software, including subpixel smoothing at mesh boundaries. Such models can originate from experimental imaging or be constructed by hand. We also include sample code for use in MEEP. Implementation of this algorithm in new code requires understanding of 3D mathematics and may require several weeks of effort, whereas use of our sample code requires knowledge of MEEP and C++ and may take up to a few hours to prepare a model of interest for 3D FDTD simulation. In all cases, access to a facility supercomputer with parallel processing capabilities is recommended. This protocol offers a practical solution to a significant challenge in the field of computational electrodynamics and paves the way for future advancements in the study of light interaction with biological structures.

Analysing biological colour patterns from digital images: An introduction to the current toolbox

Understanding the numerous roles that colouration serves in the natural world has remained a central focus in many evolutionary and ecological studies. However, to accurately characterise and then compare colours or patterns among individuals or species has been historically challenging. In recent years, there have been a myriad of new resources developed that allow researchers to characterise biological colours and patterns, specifically from digital imagery. However, each resource has its own strengths and weaknesses, answers a specific question and requires a detailed understanding of how it functions to be used properly. These nuances can make navigating this emerging field rather difficult. Herein, we evaluate several new techniques for analysing biological colouration, with a specific focus on digital images. First, we introduce fundamental background knowledge about light and perception to be considered when designing and implementing a study of colouration. We then show how numerous modifications can be made to images to ensure consistent formatting prior to analysis. After, we describe many of the new image analysis approaches and their respective functions, highlighting the type of research questions that they can address. We demonstrate how these various techniques can be brought together to examine novel research questions and test specific hypotheses. Finally, we outline potential future directions in colour pattern studies. Our goal is to provide a starting point and pathway for researchers wanting to study biological colour patterns from digital imagery.

The Evolutionary Importance of Intraspecific Variation in Sexual Communication Across Sensory Modalities

The evolution of sexual communication is critically important in the diversity of arthropods, which are declining at a fast pace worldwide. Their environments are rapidly changing, with increasing chemical, acoustic, and light pollution. To predict how arthropod species will respond to changing climates, habitats, and communities, we need to understand how sexual communication systems can evolve. In the past decades, intraspecific variation in sexual signals and responses across different modalities has been identified, but never in a comparative way. In this review, we identify and compare the level and extent of intraspecific variation in sexual signals and responses across three different modalities, chemical, acoustic, and visual, focusing mostly on insects. By comparing causes and possible consequences of intraspecific variation in sexual communication among these modalities, we identify shared and unique patterns, as well as knowledge needed to predict the evolution of sexual communication systems in arthropods in a changing world.

Triphenyltin Influenced Carotenoid-Based Coloration in Coral Reef Fish, Amphiprion ocellaris, by Disrupting Carotenoid Metabolism

Triphenyltin (TPT), a kind of persistent pollutant, is prevalent in the aquatic environment and could pose a threat to coral reef fish. However, little is known about the toxicity of TPT on coral reef fish, especially regarding the representative characteristics of body coloration. Therefore, this study chose the clownfish (Amphiprion ocellaris) in order to investigate the effects of TPT exposure on its carotenoid-based body coloration under the environmentally relevant concentrations (0, 1, 10 and 100 ng/L). After TPT exposure for 60 d, the carotenoid contents were decreased and histological damage in the liver was found, shown as nuclear pyknosis and shift, lipid deposition and fibrotic tissue hyperplasia. Liver transcriptomic analysis showed that TPT exposure interfered with oxidative phosphorylation and fatty acid metabolism pathways, which related to carotenoids uptake and metabolism. Furthermore, TPT exposure led to oxidative damage in the liver, which is responsible for the changes in the antioxidant capacity of enzymes, including GSH, MDA, POD, CAT and T-SOD. TPT exposure also affected the genes (Scarb1, CD36, Stard3 and Stard5) related to carotenoid absorption and transport, as well as the genes (GstP1 and Bco2) related to carotenoid deposition and decomposition. Taken together, our results demonstrate that TPT influenced carotenoid-based coloration in coral reef fish by disrupting carotenoid metabolism, which complements the ecotoxicological effects and toxic mechanisms of TPT and provides data for the body color biology of coral reef fishes.

Ultrastructural analysis of throat dermal tissue and chromatophore components in the threespine stickleback (Gasterosteus aculeatus)

The threespine stickleback (Gasterosteus aculeatus) is an important model for studying the evolution of nuptial coloration, but histological analyses of color are largely lacking. Previous analyses of one nuptial coloration trait, orange-red coloration along the body, have indicated carotenoids are the main pigment producing this color. In addition, recent gene expression studies found variation in the correlates of throat coloration between the sexes and between populations, raising the possibility of variation in the mechanisms underlying superficially similar coloration. We used transmission electron microscopy (TEM) to investigate the histological correlates of color in the throat dermal tissue of threespine stickleback from Western North America, within and between sexes, populations, and ecotypes. Ultrastructural analysis revealed carotenoid-containing erythrophores to be the main chromatophore component associated with orange-red coloration in both males and females across populations. In individuals where some darkening of the throat tissue was present, with no obvious orange-red coloration, erythrophores were not detected. Melanophore presence was more population-specific in expression, including being the only chromatophore component detected in a population of darker fish. We found no dermal chromatophore units within colorless throat tissue. This work confirms the importance of carotenoids and the erythrophore in producing orange-red coloration across sexes, as well as melanin within the melanophore in producing darkened coloration, but does not reveal broad histological differences among populations with similar coloration.

Colouration matters in dull toads: ultraviolet adornment for ladies and agrochemicals fading effects

Integument colouration can influence many aspects of fitness, and is under strong sexual selection. Amphibians often express sexual dichromatism, and ultra-violet (UV) colouration is usually biased toward males as a sexual signal. As an honest signal, colouration is related to several individual traits, but can also be related to environmental factors such as anthropogenic pollutants, to which amphibians are highly sensitive. In this study, we investigated sexual dichromatism and UV reflectance covering a large visual spectrum (wavelength ranging from 300 to 700 nm) on different body areas (throat, ventral and dorsal areas), in a widespread amphibian species, the spiny toad (Bufo spinosus). Then, we tested the impact of chronic exposure to two widespread herbicides (glyphosate's primary metabolite [AMPA] and Nicosulfuron) on their colouration. We found a strong but unexpected sexual dichromatism with females reflecting more in the UV spectrum (throat and ventral area) than males, suggesting these body parts might be critical in intra-specific signalling. Females with higher ventral UV reflectance were in better body condition, suggesting an honest signal role of UV reflectance which could influence male choice. Throat colouration was further differentially influenced by agrochemicals according to sexes. In AMPA-exposed males, throat was more saturated in yellow-orange than in control males, and Nicosulfuron exposure decreased the throat's reflectance hue in females, which can bear consequences on mate attractiveness. Future studies need to investigate the underlying mechanisms that are altered by agrochemical exposure.

Development shapes the evolutionary diversification of rodent stripe patterns

Vertebrate groups have evolved strikingly diverse color patterns. However, it remains unknown to what extent the diversification of such patterns has been shaped by the proximate, developmental mechanisms that regulate their formation. While these developmental mechanisms have long been inaccessible empirically, here we take advantage of recent insights into rodent pattern formation to investigate the role of development in shaping pattern diversification across rodents. Based on a broad survey of museum specimens, we first establish that various rodents have independently evolved diverse patterns consisting of longitudinal stripes, varying across species in number, color, and relative positioning. We then interrogate this diversity using a simple model that incorporates recent molecular and developmental insights into stripe formation in African striped mice. Our results suggest that, on the one hand, development has facilitated pattern diversification: The diversity of patterns seen across species can be generated by a single developmental process, and small changes in this process suffice to recapitulate observed evolutionary changes in pattern organization. On the other hand, development has constrained diversification: Constraints on stripe positioning limit the scope of evolvable patterns, and although pattern organization appears at first glance phylogenetically unconstrained, development turns out to impose a cryptic constraint. Altogether, this work reveals that pattern diversification in rodents can in part be explained by the underlying development and illustrates how pattern formation models can be leveraged to interpret pattern evolution.

Eocene aposematic patterns persist in modern European Lycidae beetles despite the absence of co-mimics

Ancient aposematic signals might have evolved under different ecological circumstances. Using European Cenozoic amber and phylogenetic reconstruction, we evaluated the evolution of net-winged beetle aposematism. We describe Priabonian Hiekeolycus winkleri sp. nov. from Baltic amber, review known fossil species, and suggest earlier high diversity and morphological conservativeness of European Lycidae since the Eocene. We hypothesize the presence of red and black/red aposematic patterns in Eocene Europe. The analyses suggest the Oligocene to Miocene dispersal of additional species from East Asia and their advergence to autochthonous patterns. Recently dispersed lycids have retained similarities with their East Asian relatives. Net-winged beetles are rare in Europe after the Quaternary climatic oscillations, and we hypothesize a currently relaxed selection for shared aposematic signals. Neophobia, and eventually inborn rejection of brightly colored prey, putatively preserved ancient aposematism under changing conditions. Evidence from paleontology and phylogenetics can provide insight into the long-term persistence of old adaptations under changing conditions.

Piebaldism and chromatophore development in reptiles are linked to the tfec gene

Reptiles display great diversity in color and pattern, yet much of what we know about vertebrate coloration comes from classic model species such as the mouse and zebrafish.1,2,3,4 Captive-bred ball pythons (Python regius) exhibit a remarkable degree of color and pattern variation. Despite the wide range of Mendelian color phenotypes available in the pet trade, ball pythons remain an overlooked species in pigmentation research. Here, we investigate the genetic basis of the recessive piebald phenotype, a pattern defect characterized by patches of unpigmented skin (leucoderma). We performed whole-genome sequencing and used a case-control approach to discover a nonsense mutation in the gene encoding the transcription factor tfec, implicating this gene in the leucodermic patches in ball pythons. We functionally validated tfec in a lizard model (Anolis sagrei) using the gene editing CRISPR/Cas9 system and TEM imaging of skin. Our findings show that reading frame mutations in tfec affect coloration and lead to a loss of iridophores in Anolis, indicating that tfec is required for chromatophore development. This study highlights the value of captive-bred ball pythons as a model species for accelerating discoveries on the genetic basis of vertebrate coloration.

Offspring plumage coloration as a condition-dependent signal in the blue tit

In many species, offspring display conspicuous coloration already early in life, even though they might be very vulnerable to predation at this stage. However, most attention has been drawn to the conspicuous plumage displayed by adult individuals in a sexual context, while other signaling functions have been explored much less. Here, we investigated whether the yellow breast plumage of blue tit (Cyanistes caeruleus) nestlings shows patterns of condition dependence and hence signals individual quality, as has been described for adult birds. During three consecutive breeding seasons, we, therefore, explored the association between nestling body mass and three color components of the yellow breast plumage (i.e., UV chroma, carotenoid chroma, and total brightness), considering both within and among nest effects. Variation in carotenoid chroma was not related to body mass. However, UV chroma and total brightness varied with body mass on an among-nest level, suggesting that they might signal aspects of genetic quality or parental rearing capacity. Interestingly, we also found a within-nest effect of body mass on total brightness, suggesting that this is a good candidate for a condition-dependent signal within the family. Thus, other family members could rely on brightness to adjust their behavioral strategies, such as feeding behavior in parents. Our study thus reveals that certain color components of the yellow breast plumage might signal different aspects of offspring quality, and they might have a correlated signaling value across life-history stages.

A Mini-Review on Reflectins, from Biochemical Properties to Bio-Inspired Applications

Some cephalopods (squids, octopuses, and cuttlefishes) produce dynamic structural colors, for camouflage or communication. The key to this remarkable capability is one group of specialized cells called iridocytes, which contain aligned membrane-enclosed platelets of high-reflective reflectins and work as intracellular Bragg reflectors. These reflectins have unusual amino acid compositions and sequential properties, which endows them with functional characteristics: an extremely high reflective index among natural proteins and the ability to answer various environmental stimuli. Based on their unique material composition and responsive self-organization properties, the material community has developed an impressive array of reflectin- or iridocyte-inspired optical systems with distinct tunable reflectance according to a series of internal and external factors. More recently, scientists have made creative attempts to engineer mammalian cells to explore the function potentials of reflectin proteins as well as their working mechanism in the cellular environment. Progress in wide scientific areas (biophysics, genomics, gene editing, etc.) brings in new opportunities to better understand reflectins and new approaches to fully utilize them. The work introduced the composition features, biochemical properties, the latest developments, future considerations of reflectins, and their inspiration applications to give newcomers a comprehensive understanding and mutually exchanged knowledge from different communities (e.g., biology and material).

Colour scales with climate in North American ratsnakes: a test of the thermal melanism hypothesis using community science images

Animal colour is a complex trait shaped by multiple selection pressures that can vary across geography. The thermal melanism hypothesis predicts that darker coloration is beneficial to animals in colder regions because it allows for more rapid solar absorption. Here, we use community science images of three closely related species of North American ratsnakes (genus Pantherophis) to examine if climate predicts colour variation across range-wide scales. We predicted that darker individuals are found in colder regions and higher elevations, in accordance with the thermal melanism hypothesis. Using an unprecedented dataset of over 8000 images, we found strong support for temperature as a key predictor of darker colour, supporting thermal melanism. We also found that elevation and precipitation are predictive of colour, but the direction and magnitude of these effects were more variable across species. Our study is the first to quantify colour variation in Pantherophis ratsnakes, highlighting the value of community science images for studying range-wide colour variation.

Star finches Neochmia ruficauda have a visual preference for white dot patterns: a possible case of trypophilia

Many animals have polka dot patterns on their body surface, some of which are known to have signalling functions; however, their evolutionary origins remain unclear. Dot patterns can trigger a fear response (trypophobia) in humans and are known to function as aposematic signals in non-human animals, suggesting that dots may deserve attention for biological reasons. Interestingly in many birds, plumage dot patterns serve for social/sexual signalling. To understand their evolution, we have focused on the sensory bias hypothesis, which predicts the role of pre-existing sensory preference driven by natural selection in shaping signal design. Our previous phylogenetic comparative study supported the hypothesis and showed that diet-driven visual preference promoted the evolution of plumage patterns, as there was an evolutionary correlation between termite-eating (white roundish gregarious prey) and the presence of plumage dot patterns in species of the family Estrildidae. This suggests that these species possess an intrinsic preference for dots. To test this, we compared the responses of an Estrildid species with dot plumage pattern (star finch Neochmia ruficauda) towards simultaneously presented monochrome-printed white dot vs white stripe patterns under both food-deprived and -supplied conditions. Overall, star finches preferred dots to stripes. They showed foraging-like behaviours almost only toward dots when hungry and gazed at dots frequently even when food was available, suggesting both hunger-related and hunger-neutral dot preferences. These results are rather surprising, given how strongly the subjects were attracted to abstract dot patterns without organic structure, but provided good support for the sensory bias hypothesis.

Heterozygote advantage and pleiotropy contribute to intraspecific color trait variability

The persistence of intrapopulation phenotypic variation typically requires some form of balancing selection because drift and directional selection eventually erode genetic variation. Heterozygote advantage remains a classic explanation for the maintenance of genetic variation in the face of selection. However, examples of heterozygote advantage, other than those associated with disease resistance, are rather uncommon. Across most of its distribution, males of the aposematic moth Arctia plantaginis have two hindwing phenotypes determined by a heritable one locus-two allele polymorphism (genotypes: WW/Wy = white morph, yy = yellow morph). Using genotyped moths, we show that the presence of one or two copies of the yellow allele affects several life-history traits. Reproductive output of both males and females and female mating success are negatively affected by two copies of the yellow allele. Females carrying one yellow allele (i.e., Wy) have higher fertility, hatching success, and offspring survival than either homozygote, thus leading to strong heterozygote advantage. Our results indicate strong female contribution especially at the postcopulatory stage in maintaining the color polymorphism. The interplay between heterozygote advantage, yellow allele pleiotropic effect, and morph-specific predation pressure may exert balancing selection on the color locus, suggesting that color polymorphism may be maintained through complex interactions between natural and sexual selection.

Color in motion: Generating 3-dimensional multispectral models to study dynamic visual signals in animals

Analyzing color and pattern in the context of motion is a central and ongoing challenge in the quantification of animal coloration. Many animal signals are spatially and temporally variable, but traditional methods fail to capture this dynamism because they use stationary animals in fixed positions. To investigate dynamic visual displays and to understand the evolutionary forces that shape dynamic colorful signals, we require cross-disciplinary methods that combine measurements of color, pattern, 3-dimensional (3D) shape, and motion. Here, we outline a workflow for producing digital 3D models with objective color information from museum specimens with diffuse colors. The workflow combines multispectral imaging with photogrammetry to produce digital 3D models that contain calibrated ultraviolet (UV) and human-visible (VIS) color information and incorporate pattern and 3D shape. These “3D multispectral models” can subsequently be animated to incorporate both signaler and receiver movement and analyzed in silico using a variety of receiver-specific visual models. This approach—which can be flexibly integrated with other tools and methods—represents a key first step toward analyzing visual signals in motion. We describe several timely applications of this workflow and next steps for multispectral 3D photogrammetry and animation techniques.

Environmental conditions and male quality traits simultaneously explain variation of multiple colour signals in male lizards

Male lizards often display multiple pigment‐based and structural colour signals which may reflect various quality traits (e.g. performance, parasitism), with testosterone (T) often mediating these relationships. Furthermore, environmental conditions can explain colour signal variation by affecting processes such as signal efficacy, thermoregulation and camouflage. The relationships between colour signals, male quality traits and environmental factors have often been analysed in isolation, but simultaneous analyses are rare. Thus, the response of multiple colour signals to variation in all these factors in an integrative analysis remains to be investigated.

Here, we investigated how multiple colour signals relate to their information content, examined the role of T as a potential mediator of these relationships and how environmental factors explain colour signal variation.

We performed an integrative study to examine the covariation between three colour signals (melanin‐based black, carotenoid‐based yellow–orange and structural UV), physiological performance, parasitism, T levels and environmental factors (microclimate, forest cover) in male common lizards Zootoca vivipara from 13 populations.

We found that the three colour signals conveyed information on different aspects of male condition, supporting a multiple message hypothesis. T influenced only parasitism, suggesting that T does not directly mediate the relationships between colour signals and their information content. Moreover, colour signals became more saturated in forested habitats, suggesting an adaptation to degraded light conditions, and became generally brighter in mesic conditions, in contradiction with the thermal melanism hypothesis.

We show that distinct individual quality traits and environmental factors simultaneously explain variations of multiple colour signals with different production modes. Our study therefore highlights the complexity of colour signal evolution, involving various sets of selective pressures acting at the same time, but in different ways depending on colour production mechanism.

Computer vision for assessing species color pattern variation from web-based community science images

Openly available community science digital vouchers provide a wealth of data to study phenotypic change across space and time. However, extracting phenotypic data from these resources requires significant human effort. Here, we demonstrate a workflow and computer vision model for automatically categorizing species color pattern from community science images. Our work is focused on documenting the striped/unstriped color polymorphism in the Eastern Red-backed Salamander (Plethodon cinereus). We used an ensemble convolutional neural network model to analyze this polymorphism in 20,318 iNaturalist images. Our model was highly accurate (∼98%) despite image heterogeneity. We used the resulting annotations to document extensive niche overlap between morphs, but wider niche breadth for striped morphs at the range-wide scale. Our work showcases key design principles for using machine learning with heterogeneous community science image data to address questions at an unprecedented scale.

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We built a deep learning model to group color morphs from community science images

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Our model achieved 98% accuracy for classifying striped and unstriped salamanders

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We used our model to classify >20,000 images and built morph-specific niche models

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We then determined if Red-backed salamanders niche partition at a range-wide scale

The exploitation of sexual signals by predators: a meta-analysis

Sexual signals are often central to reproduction, and their expression is thought to strike a balance between advertising to mates and avoiding detection by predatory eavesdroppers. Tests of the predicted predation costs have produced mixed results, however. Here we synthesized 187 effects from 78 experimental studies in a meta-analytic test of two questions; namely, whether predators, parasites and parasitoids express preferences for the sexual signals of prey, and whether sexual signals increase realized predation risk in the wild. We found that predators and parasitoids express strong and consistent preferences for signals in forced-choice contexts. We found a similarly strong overall increase in predation on sexual signallers in the wild, though here it was modality specific. Olfactory and acoustic signals increased the incidence of eavesdropping relative to visual signals, which experienced no greater risk than controls on average. Variation in outcome measures was universally high, suggesting that contexts in which sexual signalling may incur no cost, or even reduce the incidence of predation, are common. Our results reveal unexpected complexity in a central viability cost to sexual signalling, while also speaking to applied problems in invasion biology and pest management where signal exploitation holds promise for bio-inspired solutions.

First detection of biofluorescence in a deep-sea anglerfish

Biofluorescence has been observed in a variety of fishes, but is rare in deep-sea environments where light from the surface cannot reach. Here, we document biofluorescence in an oceanic anglerfish, the Pacific footballfish. Green biofluorescence was observed in small spots on the distal surface of the esca. While the wavelength of bioluminescent light is unknown for this species, it is possible that light produced by this species also results in biofluorescent emission that may create a more complex lure for attracting prey or mates.

Context-dependent coloration of prey and predator decision making in contrasting light environments

A big question in behavioral ecology is what drives diversity of color signals. One possible explanation is that environmental conditions, such as light environment, may alter visual signaling of prey, which could affect predator decision-making. Here, we tested the context-dependent predator selection on prey coloration. In the first experiment, we tested detectability of artificial visual stimuli to blue tits (Cyanistes caeruleus) by manipulating stimulus luminance and chromatic context of the background. We expected the presence of the chromatic context to facilitate faster target detection. As expected, blue tits found targets on chromatic yellow background faster than on achromatic grey background whereas in the latter, targets were found with smaller contrast differences to the background. In the second experiment, we tested the effect of two light environments on the survival of aposematic, color polymorphic wood tiger moth (Arctia plantaginis). As luminance contrast should be more detectable than chromatic contrast in low light intensities, we expected birds, if they find the moths aversive, to avoid the white morph which is more conspicuous than the yellow morph in low light (and vice versa in bright light). Alternatively, birds may attack first moths that are more detectable. We found birds to attack yellow moths first in low light conditions, whereas white moths were attacked first more frequently in bright light conditions. Our results show that light environments affect predator foraging decisions, which may facilitate context-dependent selection on visual signals and diversity of prey phenotypes in the wild.

Black bear colour polymorphism through a fragmented Snell’s window

The white colour morph of the black bear (Ursus americanus kermodei) occurring on islands on the coast of British Columbia, western Canada, captures more salmon (Oncorhynchus spp.) than does the black morph and is hypothesized to have reduced contrast against the sky from the visual perspective of the salmon. We tested this hypothesis in a natural salmon stream by recording the number and proximity of chum salmon (Oncorhynchus keta) approaches (N = 1617 fish, 91 trials) towards life-size bear models differing in body and leg coloration under a mixed forest-sky canopy. Although salmon approached the white models at a much higher rate than black models, consistent with camouflage, we found greater abrupt evasions to the black models, largely independent of their contrast against the above-surface or below-surface backgrounds. Upward-facing sub-surface video-imaging through the rippled water-air interface indicated major visual fragmentation of the model’s integrity. We suggest that increased evasiveness to black models reflects an evolutionary response due to 3+ million years of trophic interaction between salmon and bears, and that the major differences between calm vs. rippled conditions through the optical cone (Snell’s window) at the water-air interface remains a largely unexplored theme in assessing foraging preferences and adaptive coloration within and among species using the water-air interface.

Sensory-based quantification of male colour patterns in Trinidadian guppies reveals no support for parallel phenotypic evolution in multivariate trait space

Parallel evolution, in which independent populations evolve along similar phenotypic trajectories, offers insights into the repeatability of adaptive evolution. Here, we revisit a classic example of parallelism, that of repeated evolution of brighter males in the Trinidadian guppy (Poecilia reticulata). In guppies, colonisation of low predation habitats is associated with emergence of 'more colourful' phenotypes since predator-induced viability selection for crypsis weakens while sexual selection by female preference for conspicuousness remains strong. Our study differs from previous investigations in three respects. First, we adopted a multivariate phenotyping approach to characterise parallelism in multitrait space. Second, we used ecologically-relevant colour traits defined by the visual systems of the two selective agents (i.e., guppy, predatory cichlid). Third, we estimated population genetic structure to test for adaptive (parallel) evolution against a model of neutral phenotypic divergence. We find strong phenotypic differentiation that is inconsistent with a neutral model but very limited support for the predicted pattern of greater conspicuousness at low predation. Effects of predation regime on each trait were in the expected direction, but weak, largely nonsignificant, and explained little among-population variation. In multitrait space, phenotypic trajectories of lineages colonising low from high predation regimes were not parallel. Our results are consistent with reduced predation risk facilitating adaptive differentiation, potentially by female choice, but suggest that this proceeds in independent directions of multitrait space across lineages. Pool-sequencing data also revealed SNPs showing greater differentiation than expected under neutrality, among which some are found in genes contributing to colour pattern variation, presenting opportunities for future genetic study.

Comparative transcriptomics of albino and warningly-coloured caterpillars

Coloration is perhaps one of the most prominent adaptations for survival and reproduction of many taxa. Coloration is of particular importance for aposematic species, which rely on their coloring and patterning acting as a warning signal to deter predators. Most research has focused on the evolution of warning coloration by natural selection. However, little information is available for color mutants of aposematic species, particularly at the genomic level. Here, I compare the transcriptomes of albino mutant caterpillars of the aposematic wood tiger moth (Arctia plantaginis) to those of their full sibs having their distinctive orange-black warning coloration. The results showed >290 differentially expressed genes genome-wide. Genes involved in the immune system, structural constituents of cuticular, and immunity were mostly downregulated in the albino caterpillars. Surprisingly, higher expression was observed in core melanin genes from albino caterpillars, suggesting that melanin synthesis may be disrupted in terminal ends of the pathway during its final conversion. Taken together, these results suggest that caterpillar albinism may not be due to a depletion of melanin precursor genes. In contrast, the albino condition may result from the combination of faulty melanin conversion late in its synthesis and structural deficiencies in the cuticular preventing its deposition. The results are discussed in the context of how albinism may impact individuals of aposematic species in the wild.

Meta-analytic evidence for quantitative honesty in aposematic signals

The combined use of noxious chemical defences and conspicuous warning colours is a ubiquitous anti-predator strategy. That such signals advertise the presence of defences is inherent to their function, but their predicted potential for quantitative honesty-the positive scaling of signal salience with the strength of protection-is the subject of enduring debate. Here, we systematically synthesized the available evidence to test this prediction using meta-analysis. We found evidence for a positive correlation between warning colour expression and the extent of chemical defences across taxa. Notably, this relationship held at all scales; among individuals, populations and species, though substantial between-study heterogeneity remains unexplained. Consideration of the design of signals revealed that all visual features, from colour to contrast, were equally informative of the extent of prey defence. Our results affirm a central prediction of honesty-based models of signal function and narrow the scope of possible mechanisms shaping the evolution of aposematism. They suggest diverse pathways to the encoding and exchange of information, while highlighting the need for deeper knowledge of the ecology of chemical defences to enrich our understanding of this widespread anti-predator adaptation.

Sexual selection on jumping spider color pattern: investigation with a new quantitative approach

How animals assess information encoded in individual color patches have been extensively studied, yet the role of both individual color patches and gross color pattern (i.e., the combination of color patches) remains understudied. We tested the functioning of both individual color patches and gross color pattern in sexual selection using the jumping spider Siler semiglaucus as a study system. We first quantified sexual dimorphism in S. semiglaucus in both individual patches and gross color pattern using the newly developed quantitative color pattern analysis (QCPA) framework. After detecting sexual differences in color coverage and pattern contrast, we manipulated the abdomen color pattern of males and had them engage in both female mate choice and male contest trials. Females spent more time watching males with lower pattern contrast and greater red coverage during mate assessment, suggesting that they evaluate information from both individual patches and gross color pattern of males. However, male color pattern had no significant effect on the outcomes of male contests. Thus, we suggest that the observed sexual color pattern dimorphism evolved primarily through female mate choice in S. semiglaucus. This is the first study to use QCPA framework to quantify sexual dimorphism in within-pattern conspicuousness from an intraspecific perspective in invertebrates. Our study also highlights the importance of both individual color patches and gross color pattern in sexual selection.

Conspicuousness, phylogenetic structure, and origins of Müllerian mimicry in 4000 lycid beetles from all zoogeographic regions

Biologists have reported on the chemical defences and the phenetic similarity of net-winged beetles (Coleoptera: Lycidae) and their co-mimics. Nevertheless, our knowledge has remained fragmental, and the evolution of mimetic patterns has not been studied in the phylogenetic context. We illustrate the general appearance of ~ 600 lycid species and ~ 200 co-mimics and their distribution. Further, we assemble the phylogeny using the transcriptomic backbone and ~ 570 species. Using phylogenetic information, we closely scrutinise the relationships among aposematically coloured species, the worldwide diversity, and the distribution of aposematic patterns. The emitted visual signals differ in conspicuousness. The uniform coloured dorsum is ancestral and was followed by the evolution of bicoloured forms. The mottled patterns, i.e. fasciate, striate, punctate, and reticulate, originated later in the course of evolution. The highest number of sympatrically occurring patterns was recovered in New Guinea and the Andean mountain ecosystems (the areas of the highest abundance), and in continental South East Asia (an area of moderate abundance but high in phylogenetic diversity). Consequently, a large number of co-existing aposematic patterns in a single region and/or locality is the rule, in contrast with the theoretical prediction, and predators do not face a simple model-like choice but cope with complex mimetic communities. Lycids display an ancestral aposematic signal even though they sympatrically occur with differently coloured unprofitable relatives. We show that the highly conspicuous patterns evolve within communities predominantly formed by less conspicuous Müllerian mimics and, and often only a single species displays a novel pattern. Our work is a forerunner to the detailed research into the aposematic signalling of net-winged beetles.

Population genetic structure underlying the geographic variation in beetle structural colour with multiple transition zones

We studied the population genetic structure underlying the geographic variation in the structural colour of the geotrupid dung beetle, Phelotrupes auratus, which exhibits metallic body colours of different reflectance wavelengths perceived as red, green and indigo. These forms occur parapatrically in an area of Japan. The colour variation was not related to variation in climatic factors. Using single nucleotide polymorphisms (SNPs) from restriction-site-associated DNA sequences, we discriminated five groups of populations (west/red, south/green, south/indigo, south/red and east/red) by a combination of genetic clusters (west, south and east) and three colour forms. There were three transition zones for the colour forms: two between the red and green forms were hybrid zones with steep genetic clines, which implies the existence of barriers to gene flow between regions with different colours. The remaining transition zone between the green and indigo forms lacked genetic differentiation, despite the evident colour changes, which implies regionally specific selection on the different colours. In a genomewide association study, we identified four SNPs that were associated with the red/green or indigo colour and were not linked with one another, which implies that the coloration was controlled by multiple loci, each affecting the expression of a different colour range. These loci may have controlled the transitions between different combinations of colours. Our study demonstrates that geographic colour variation within a species can be maintained by nonuniform interactions among barriers to gene flow, locally specific selection on different colours, and the effects of different colour loci.

Changing How Biologists View Flowers-Color as a Perception Not a Trait

Studying flower color evolution can be challenging as it may require several different areas of expertise, ranging from botany and ecology through to understanding color sensing of insects and thus how they perceive flower signals. Whilst studies often view plant-pollinator interactions from the plant's perspective, there is growing evidence from psychophysics studies that pollinators have their own complex decision making processes depending on their perception of color, viewing conditions and individual experience. Mimicry of rewarding flowers by orchids is a fascinating system for studying the pollinator decision making process, as rewarding model flowering plants and mimics can be clearly characterized. Here, we focus on a system where the rewardless orchid Eulophia zeyheriana mimics the floral color of Wahlenbergia cuspidata (Campanulaceae) to attract its pollinator species, a halictid bee. Using recently developed psychophysics principles, we explore whether the color perception of an insect observer encountering variable model and mimic flower color signals can help explain why species with non-rewarding flowers can exist in nature. Our approach involves the use of color discrimination functions rather than relying on discrimination thresholds, and the use of statistical distributions to model intraspecific color variations. Results show that whilst an experienced insect observer can frequently make accurate discriminations between mimic and rewarding flowers, intraspecific signal variability leads to overlap in the perceived color, which will frequently confuse an inexperienced pollinator. This new perspective provides an improved way to incorporate pollinator decision making into the complex field of plant-pollinator interactions.

Visual acuity and egg spatial chromatic contrast predict egg rejection behavior of American robins

Color and spatial vision is critical for recognition and discrimination tasks affecting fitness, including finding food and mates, and recognizing offspring. For example, as a counter defense to avoid the cost of raising the unrelated offspring of obligate interspecific avian brood parasites, many host species routinely view, recognize and remove the foreign egg(s) from their nests. Recent research has shown that host species visually attend to both chromatic and spatial pattern features of eggs; yet how hosts simultaneously integrate these features together when recognizing eggs remains an open question. Here, we tested egg rejection responses of American robins (Turdus migratorius) using a range of 3D-printed model eggs covered with blue and yellow checkered patterns differing in relative square sizes. We predicted that robins would reject a model egg if they could visually resolve the blue and yellow squares as separate features, or accept it if the squares blended together and appeared similar in color to the natural blue-green color of robin eggs as perceived by the avian visual system. As predicted, the probability of robins rejecting a model egg increased with greater sizes of its blue and yellow squares. Our results suggest that chromatic visual acuity and viewing distance have the potential to limit the ability of a bird to recognize a foreign egg in its nest, thus providing a limitation to host egg recognition that obligate interspecific avian brood parasites may exploit.

Prey with hidden colour defences benefit from their similarity to aposematic signals

Some camouflaged animals hide colour signals and display them only transiently. These hidden colour signals are often conspicuous and are used as a secondary defence to warn or startle predators (deimatic displays) and/or to confuse them (flash displays). The hidden signals used in these displays frequently resemble typical aposematic signals, so it is possible that prey with hidden signals have evolved to employ colour patterns of a form that predators have previously learned to associate with unprofitability. Here, we tested this hypothesis by conducting two experiments that examined the effect of predator avoidance learning on the efficacy of deimatic and flash displays. We found that the survival benefits of both deimatic and flash displays were substantially higher against predators that had previously learned to associate the hidden colours with unprofitability than against naive predators. These findings help explain the phenological patterns we found in 1568 macro-lepidopteran species on three continents: species with hidden signals tend to occur later in the season than species without hidden signals.

Benefits of insect colours: a review from social insect studies

Insect colours assist in body protection, signalling, and physiological adaptations. Colours also convey multiple channels of information. These channels are valuable for species identification, distinguishing individual quality, and revealing ecological or evolutionary aspects of animals' life. During recent years, the emerging interest in colour research has been raised in social hymenopterans such as ants, wasps, and bees. These insects provide important ecosystem services and many of those are model research organisms. Here we review benefits that various colour types give to social insects, summarize practical applications, and highlight further directions. Ants might use colours principally for camouflage, however the evolutionary function of colour in ants needs more attention; in case of melanin colouration there is evidence for its interrelation with thermoregulation and pathogen resistance. Colours in wasps and bees have confirmed linkages to thermoregulation, which is increasingly important in face of global climate change. Besides wasps use colours for various types of signalling. Colour variations of well chemically defended social insects are the mimetic model for unprotected organisms. Despite recent progress in molecular identification of species, colour variations are still widely in use for species identification. Therefore, further studies on variability is encouraged. Being closely interconnected with physiological and biochemical processes, insect colouration is a great source for finding new ecological indicators and biomarkers. Due to novel digital imaging techniques, software, and artificial intelligence there are emerging possibilities for new advances in this topic. Further colour research in social insects should consider specific features of sociality.

Water pollution affects fish community structure and alters evolutionary trajectories of invasive guppies (Poecilia reticulata)

Anthropogenic habitat alterations have the potential to affect both, ecological dynamics of communities and populations, as well as evolutionary processes within populations. Invasive species may benefit from anthropogenic disturbance, such as water pollution, to which they sometimes seem more resistant than native ones. They also allow investigating evolutionary divergence among populations occurring along pollution gradients. We assessed fish communities at 55 sampling sites in the degraded and heavily overstocked Mutara Rangelands of north-eastern Rwanda (upper Nile drainage), which receive pollution from domestic wastewater and cattle dung. Diverse fish communities became apparent that included invasive guppies (Poecilia reticulata, Poeciliidae), and canonical correspondence analyses found significant differentiation of community structures along several environmental parameters (condensed into principal components), including pollution-effects. As predicted, generalized linear models found guppies to have a higher likelihood of occurrence at polluted sites. Local abundances of guppies, however, decreased at polluted sites. Since guppies are color-polymorphic, and color patterns have a heritable basis, they allow inferences regarding both pollution-induced suppression of male ornamentation (e.g., through xenestrogens) and evolutionary population divergence. We thus quantified different ornament types (numbers and percent body surface cover). ANCOVAs uncovered several weak (based on effect strengths), but statistically significant pollution-effects and interactions with other environmental parameters. The direction of several interaction effects was similar for blue/black and red/orange ornaments, while white/iridescent ornaments responded dissimilarly. As responses differed between ornament types, they likely reflect evolutionary divergence due to site-specific alterations of selective regimes rather than developmental inhibition of male secondary sexual characters. We propose that pollution affects local fitness landscapes resulting, e.g., from predation and mate competition (as a function of local abundances), altogether driving evolutionary divergence of sexually selected traits. This study highlights how human activities not only impact ecological dynamics, but-mediated by altered Eco-Evo dynamics-might change the evolutionary trajectories of populations.

Phenotypic flexibility in background-mediated color change in sticklebacks

Phenotypic flexibility may incur a selective advantage in changing and heterogeneous environments, and is increasingly recognized as an integral aspect of organismal adaptation. Despite the widespread occurrence and potential importance of rapid and reversible background-mediated color change for predator avoidance, knowledge gaps remain regarding its adaptive value, repeatability within individuals, phenotypic correlates, and whether its expression is context dependent. We used manipulative experiments to investigate these issues in two fish species, the three-spined stickleback (Gasterosteus aculeatus) and nine-spined stickleback (Pungitius pungitius). We sequentially exposed individuals to dark and light visual background treatments, quantified color change from video recordings, and examined associations of color change with phenotypic dimensions that can influence the outcome of predator-prey interactions. G. aculeatus expressed a greater degree of color change compared to P. pungitius. In G. aculeatus, the color change response was repeatable within individuals. Moreover, the color change response was independent of body size but affected by sex and boldness, with males and bolder individuals changing less. Infection by the parasite Schistocephalus solidus did not affect the degree of color change, but it did modulate its association with sex and boldness. G. aculeatus adjusted the expression of color change in response to predation risk, with enhanced color change expression in individuals exposed to either simulated attacks, or olfactory cues from a natural predator. These results provide novel evidence on repeatability, correlated traits, and context dependence in the color change response and highlight how a suite of factors can contribute to individual variation in phenotypic flexibility.

Wild hummingbirds discriminate nonspectral colors

Many animals have the potential to discriminate nonspectral colors. For humans, purple is the clearest example of a nonspectral color. It is perceived when two color cone types in the retina (blue and red) with nonadjacent spectral sensitivity curves are predominantly stimulated. Purple is considered nonspectral because no monochromatic light (such as from a rainbow) can evoke this simultaneous stimulation. Except in primates and bees, few behavioral experiments have directly examined nonspectral color discrimination, and little is known about nonspectral color perception in animals with more than three types of color photoreceptors. Birds have four color cone types (compared to three in humans) and might perceive additional nonspectral colors such as UV+red and UV+green. Can birds discriminate nonspectral colors, and are these colors behaviorally and ecologically relevant? Here, using comprehensive behavioral experiments, we show that wild hummingbirds can discriminate a variety of nonspectral colors. We also show that hummingbirds, relative to humans, likely perceive a greater proportion of natural colors as nonspectral. Our analysis of plumage and plant spectra reveals many colors that would be perceived as nonspectral by birds but not by humans: Birds' extra cone type allows them not just to see UV light but also to discriminate additional nonspectral colors. Our results support the idea that birds can distinguish colors throughout tetrachromatic color space and indicate that nonspectral color perception is vital for signaling and foraging. Since tetrachromacy appears to have evolved early in vertebrates, this capacity for rich nonspectral color perception is likely widespread.

Sexual signaling pattern correlates with habitat pattern in visually ornamented fishes

Sexual signal design is an evolutionary puzzle that has been partially solved by the hypothesis of sensory drive. Framed in signal detection theory, sensory drive posits that the attractiveness of a signal depends on its detectability, measured as contrast with the background. Yet, cognitive scientists have shown that humans prefer images that match the spatial statistics of natural scenes. The explanation is framed in information theory, whereby attractiveness is determined by the efficiency of information processing. Here, we apply this framework to animals, using Fourier analysis to compare the spatial statistics of body patterning in ten species of darters (Etheostoma spp.) with those of their respective habitats. We find a significant correlation between the spatial statistics of darter patterns and those of their habitats for males, but not for females. Our results support a sensory drive hypothesis that recognizes efficient information processing as a driving force in signal evolution.

Avian egg and nestling detection in the wild: should we rely on visual models or behavioural experiments?

The fields of avian egg and nestling colour detection have rapidly advanced owing to the application of visual models, which have allowed assessing of evolutionary questions considering receiver perception. Here, I first review the literature aiming to identify patterns of avian visual model usage. Second, I elaborate on limitations in the application of the receptor-noise limited perceptual (RNL hereafter) model. A systematic literature review revealed that the RNL model was the most used approach (81% of studies) in the field, and that most studies (76%) were concerned with classic evolutionary questions in avian brood parasitism. Some known limitations of the RNL model deal with model assumptions and parameterization, or, a poor consideration of post-detection neural processes. Others, however, are specific of the fields of egg and nestling discrimination and deal with the highly variable nature of ambient light at the nests, the complex colour design of eggs and nestlings, the multi-dimensional nature of perception, and the possible implication of learning. I, therefore, conclude that visual models should be used with caution to establish inference about egg and nestling discrimination, and rather be used to provide reasonable hypotheses which need to be validated with behavioural experiments. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.