Individual colour patches as multicomponent signals

Regulation of red fluorescent light emission in a cryptic marine fish

Introduction

Animal colouration is a trade-off between being seen by intended, intra- or inter-specific receivers while not being seen by the unintended. Many fishes solve this problem by adaptive colouration. Here, we investigate whether this also holds for fluorescent pigments. In those aquatic environments in which the ambient light is dominated by bluish light, red fluorescence can generate high-contrast signals. The marine, cryptic fish Tripterygion delaisi inhabits such environments and has a bright red-fluorescent iris that can be rapidly up- and down-regulated. Here, we described the physiological and cellular mechanism of this phenomenon using a neurostimulation treatment with KCl and histology.

Results

KCl-treatment revealed that eye fluorescence regulation is achieved through dispersal and aggregation of black-pigmented melanosomes within melanophores. Histology showed that globular, fluorescent iridophores on the anterior side of the iris are grouped and each group is encased by finger-like extensions of a single posterior melanophore. Together they form a so-called chromatophore unit. By dispersal and aggregation of melanosomes into and out of the peripheral membranous extensions of the melanophore, the fluorescent iridophores are covered or revealed on the anterior (outside) of the iris.

Conclusion

T. delaisi possesses a well-developed mechanism to control the fluorescent emission from its eyes, which may be advantageous given its cryptic lifestyle. This is the first time chromatophore units are found to control fluorescent emission in marine teleost fishes. We expect other fluorescent fish species to use similar mechanisms in the iris or elsewhere in the body. In contrast to a previously described mechanism based on dendritic fluorescent chromatophores, chromatophore units control fluorescent emission through the cooperation between two chromatophore types: an emitting and an occluding type. The discovery of a second mechanism for fluorescence modulation strengthens our view that fluorescence is a relevant and adaptive component of fish colouration.

Pigmentation in Xiphophorus: an emerging system in ecological and evolutionary genetics

The genus Xiphophorus has great potential to contribute to the study of vertebrate pigmentation and elucidating the relative influence of ecology, physiology, and behavior on evolution at the molecular level. More importantly, the association between pigmentation and a functional oncogene offers the potential to understand the evolution and maintenance of cancer-causing genetic elements. Using criteria laid out recently in the literature, I demonstrate the power of the Xiphophorus system for studying pigment evolution through integrative organismal biology. Using the most recent phylogeny, the phylogenetic distribution of several important pigmentation loci are reevaluated. I then review support for existing hypotheses of the functional importance of pigmentation. Finally, new observations and hypotheses regarding some of the characteristics of pigment patterns in natural populations and open questions and future directions in the study of the evolution of these traits are discussed.

Gradient evolution of body colouration in surface- and cave-dwelling Poecilia mexicana and the role of phenotype-assortative female mate choice

Ecological speciation assumes reproductive isolation to be the product of ecologically based divergent selection. Beside natural selection, sexual selection via phenotype-assortative mating is thought to promote reproductive isolation. Using the neotropical fish Poecilia mexicana from a system that has been described to undergo incipient ecological speciation in adjacent, but ecologically divergent habitats characterized by the presence or absence of toxic H2S and darkness in cave habitats, we demonstrate a gradual change in male body colouration along the gradient of light/darkness, including a reduction of ornaments that are under both inter- and intrasexual selection in surface populations. In dichotomous choice tests using video-animated stimuli, we found surface females to prefer males from their own population over the cave phenotype. However, female cave fish, observed on site via infrared techniques, preferred to associate with surface males rather than size-matched cave males, likely reflecting the female preference for better-nourished (in this case: surface) males. Hence, divergent selection on body colouration indeed translates into phenotype-assortative mating in the surface ecotype, by selecting against potential migrant males. Female cave fish, by contrast, do not have a preference for the resident male phenotype, identifying natural selection against migrants imposed by the cave environment as the major driver of the observed reproductive isolation.

Precise colocalization of interacting structural and pigmentary elements generates extensive color pattern variation in Phelsuma lizards

BACKGROUND

Color traits in animals play crucial roles in thermoregulation, photoprotection, camouflage, and visual communication, and are amenable to objective quantification and modeling. However, the extensive variation in non-melanic pigments and structural colors in squamate reptiles has been largely disregarded. Here, we used an integrated approach to investigate the morphological basis and physical mechanisms generating variation in color traits in tropical day geckos of the genus Phelsuma.

RESULTS

Combining histology, optics, mass spectrometry, and UV and Raman spectroscopy, we found that the extensive variation in color patterns within and among Phelsuma species is generated by complex interactions between, on the one hand, chromatophores containing yellow/red pteridine pigments and, on the other hand, iridophores producing structural color by constructive interference of light with guanine nanocrystals. More specifically, we show that 1) the hue of the vivid dorsolateral skin is modulated both by variation in geometry of structural, highly ordered narrowband reflectors, and by the presence of yellow pigments, and 2) that the reflectivity of the white belly and of dorsolateral pigmentary red marks, is increased by underlying structural disorganized broadband reflectors. Most importantly, these interactions require precise colocalization of yellow and red chromatophores with different types of iridophores, characterized by ordered and disordered nanocrystals, respectively. We validated these results through numerical simulations combining pigmentary components with a multilayer interferential optical model. Finally, we show that melanophores form dark lateral patterns but do not significantly contribute to variation in blue/green or red coloration, and that changes in the pH or redox state of pigments provide yet another source of color variation in squamates.

CONCLUSIONS

Precisely colocalized interacting pigmentary and structural elements generate extensive variation in lizard color patterns. Our results indicate the need to identify the developmental mechanisms responsible for the control of the size, shape, and orientation of nanocrystals, and the superposition of specific chromatophore types. This study opens up new perspectives on Phelsuma lizards as models in evolutionary developmental biology.

Possible involvement of cone opsins in distinct photoresponses of intrinsically photosensitive dermal chromatophores in tilapia Oreochromis niloticus

Dermal specialized pigment cells (chromatophores) are thought to be one type of extraretinal photoreceptors responsible for a wide variety of sensory tasks, including adjusting body coloration. Unlike the well-studied image-forming function in retinal photoreceptors, direct evidence characterizing the mechanism of chromatophore photoresponses is less understood, particularly at the molecular and cellular levels. In the present study, cone opsin expression was detected in tilapia caudal fin where photosensitive chromatophores exist. Single-cell RT-PCR revealed co-existence of different cone opsins within melanophores and erythrophores. By stimulating cells with six wavelengths ranging from 380 to 580 nm, we found melanophores and erythrophores showed distinct photoresponses. After exposed to light, regardless of wavelength presentation, melanophores dispersed and maintained cell shape in an expansion stage by shuttling pigment granules. Conversely, erythrophores aggregated or dispersed pigment granules when exposed to short- or middle/long-wavelength light, respectively. These results suggest that diverse molecular mechanisms and light-detecting strategies may be employed by different types of tilapia chromatophores, which are instrumental in pigment pattern formation.

How hollow melanosomes affect iridescent colour production in birds

Developmental constraints and trade-offs can limit diversity, but organisms have repeatedly evolved morphological innovations that overcome these limits by expanding the range and functionality of traits. Iridescent colours in birds are commonly produced by melanin-containing organelles (melanosomes) organized into nanostructured arrays within feather barbules. Variation in array type (e.g. multilayers and photonic crystals, PCs) is known to have remarkable effects on plumage colour, but the optical consequences of variation in melanosome shape remain poorly understood. Here, we used a combination of spectrophotometric, experimental and theoretical methods to test how melanosome hollowness--a morphological innovation largely restricted to birds--affects feather colour. Optical analyses of hexagonal close-packed arrays of hollow melanosomes in two species, wild turkeys (Meleagris gallopavo) and violet-backed starlings (Cinnyricinclus leucogaster), indicated that they function as two-dimensional PCs. Incorporation of a larger dataset and optical modelling showed that, compared with solid melanosomes, hollow melanosomes allow birds to produce distinct colours with the same energetically favourable, close-packed configurations. These data suggest that a morphological novelty has, at least in part, allowed birds to achieve their vast morphological and colour diversity.

The phylogenetic significance of colour patterns in marine teleost larvae

Ichthyologists, natural-history artists, and tropical-fish aquarists have described, illustrated, or photographed colour patterns in adult marine fishes for centuries, but colour patterns in marine fish larvae have largely been neglected. Yet the pelagic larval stages of many marine fishes exhibit subtle to striking, ephemeral patterns of chromatophores that warrant investigation into their potential taxonomic and phylogenetic significance. Colour patterns in larvae of over 200 species of marine teleosts, primarily from the western Caribbean, were examined from digital colour photographs, and their potential utility in elucidating evolutionary relationships at various taxonomic levels was assessed. Larvae of relatively few basal marine teleosts exhibit erythrophores, xanthophores, or iridophores (i.e. nonmelanistic chromatophores), but one or more of those types of chromatophores are visible in larvae of many basal marine neoteleosts and nearly all marine percomorphs. Whether or not the presence of nonmelanistic chromatophores in pelagic marine larvae diagnoses any major teleost taxonomic group cannot be determined based on the preliminary survey conducted, but there is a trend toward increased colour from elopomorphs to percomorphs. Within percomorphs, patterns of nonmelanistic chromatophores may help resolve or contribute evidence to existing hypotheses of relationships at multiple levels of classification. Mugilid and some beloniform larvae share a unique ontogenetic transformation of colour pattern that lends support to the hypothesis of a close relationship between them. Larvae of some tetraodontiforms and lophiiforms are strikingly similar in having the trunk enclosed in an inflated sac covered with xanthophores, a character that may help resolve the relationships of these enigmatic taxa. Colour patterns in percomorph larvae also appear to diagnose certain groups at the interfamilial, familial, intergeneric, and generic levels. Slight differences in generic colour patterns, including whether the pattern comprises xanthophores or erythrophores, often distinguish species. The homology, ontogeny, and possible functional significance of colour patterns in larvae are discussed. Considerably more investigation of larval colour patterns in marine teleosts is needed to assess fully their value in phylogenetic reconstruction.

Ten unanswered questions in multimodal communication

The study of multimodal communication has become an active and vibrant field. This special issue of Behavioral Ecology and Sociobiology brings together new developments in this rapidly expanding area. In this final contribution to the special issue, I look to the future and discuss ten questions in need of further work, touching on issues ranging from theoretical modeling and the evolution of behavior to molecular mechanisms and the development of behavior. In particular, I emphasize that the use of multimodal communication allows animals to switch between sensory channels when one channel becomes too noisy, and suggest that a better understanding of this process may help us both to understand the evolution of multisensory signaling and to predict the success of species facing environmental changes that affect signaling channels, such as urbanization and climate change. An expanded section is included on the effects of climate change on animal communication across sensory channels, urging researchers to pursue this topic due to the rapidity with which the environment is currently transforming.

Negative correlation between nuptial throat colour and blood parasite load in male European green lizards supports the Hamilton-Zuk hypothesis

During female mate choice, conspicuous male sexual signals are used to infer male quality and choose the best sire for the offspring. The theory of parasite-mediated sexual selection (Hamilton-Zuk hypothesis) presumes that parasite infection can influence the elaboration of sexual signals: resistant individuals can invest more energy into signal expression and thus advertise their individual quality through signal intensity. By preferring these males, females can provide resistance genes for their offspring. Previous research showed that nuptial throat colour of male European green lizard, Lacerta viridis, plays a role in both inter- and intrasexual selections as a condition-dependent multiple signalling system. The aim of this study was to test the predictions of the Hamilton-Zuk hypothesis on male European green lizards. By blood sampling 30 adult males during the reproductive season, we found members of the Haemogregarinidae family in all but one individual (prevalence = 96%). The infection intensity showed strong negative correlation with the throat and belly colour brightness in line with the predictions of the Hamilton-Zuk hypothesis. In addition, we found other correlations between infection intensity and other fitness-related traits, suggesting that parasite load has a remarkable effect on individual fitness. This study shows that throat patch colour of the European green lizards not only is a multiple signalling system but also possibly acts as an honest sexual signal of health state in accordance with the Hamilton-Zuk hypothesis.

Genetics and evolution of colour patterns in reptiles

The study of coloration in the polyphyletic reptilians has flourished in the last two decades, in particular with respect to the underlying genetics of colour traits, the function of colours in social interactions, and ongoing selection on these traits in the wild. The taxonomic bias, however, is profound: at this level of resolution almost all available information is for diurnal lizards. Therefore, we focus on case studies, for which there are as complete causal sequences of colour evolution as possible, from phenotypic expression of variation in colour, to ongoing selection in the wild. For work prior to 1992 and for a broader coverage of reptilian coloration we refer the readers to Cooper and Greenburg's (Biology of the Reptilia, 1992) review. There are seven major conclusions we would like to emphasise: (a) visual systems in diurnal lizards are broadly conserved but among the wider range of reptiles in general, there is functionally important variation in the number and type of photoreceptors, spectral tuning of photopigments and optical properties of the eye; (b) coloration in reptiles is a function of complex interactions between structural and pigmentary components, with implications for both proximate control and condition dependence of colour expression; (c) studies of colour-variable species have enabled estimates of heritability of colour and colour patterns, which often show a simple Mendelian pattern of inheritance; (d) colour-polymorphic lizard species sometimes, but not always, show striking differences in genetically encoded reproductive tactics and provide useful models for studying the evolution and maintenance of polymorphism; (e) both male and female colours are sometimes, but not always, a significant component of socio-sexual signalling, often based on multiple traits; (f) evidence for effects of hormones and condition on colour expression, and trade-offs with immunocompetence and parasite load, is variable; (g) lizards show fading of colours in response to physiological stress and ageing and are hence likely to be appropriate models for work on the interactions between handicaps, indicator traits, parasitology and immunoecology.

Analyzing visual signals as visual scenes

The study of visual signal design is gaining momentum as techniques for studying signals become more sophisticated and more freely available. In this paper we discuss methods for analyzing the color and form of visual signals, for integrating signal components into visual scenes, and for producing visual signal stimuli for use in psychophysical experiments. Our recommended methods aim to be rigorous, detailed, quantitative, objective, and where possible based on the perceptual representation of the intended signal receiver(s). As methods for analyzing signal color and luminance have been outlined in previous publications we focus on analyzing form information by discussing how statistical shape analysis (SSA) methods can be used to analyze signal shape, and spatial filtering to analyze repetitive patterns. We also suggest the use of vector-based approaches for integrating multiple signal components. In our opinion elliptical Fourier analysis (EFA) is the most promising technique for shape quantification but we await the results of empirical comparison of techniques and the development of new shape analysis methods based on the cognitive and perceptual representations of receivers. Our manuscript should serve as an introductory guide to those interested in measuring visual signals, and while our examples focus on primate signals, the methods are applicable to quantifying visual signals in most taxa.

Independent sources of condition dependency and multiple pathways determine a composite trait: lessons from carotenoid-based plumage colouration

Many colour ornaments are composite traits consisting of at least four components, which themselves may be more complex, determined by independent evolutionary pathways, and potentially being under different environmental control. To date, little evidence exists that several different components of colour elaboration are condition dependent and no direct evidence exists that different ornamental components are affected by different sources of variation. For example, in carotenoid-based plumage colouration, one of the best-known condition-dependent ornaments, colour elaboration stems from both condition-dependent pigment concentration and structural components. Some environmental flexibility of these components has been suggested, but specifically which and how they are affected remains unknown. Here, we tested whether multiple colour components may be condition dependent, by using a comprehensive 3 × 2 experimental design, in which we carotenoid supplemented and immune challenged great tit nestlings (Parus major) and quantified effects on different components of colouration. Plumage colouration was affected by an interaction between carotenoid availability and immune challenge. Path analyses showed that carotenoid supplementation increased plumage saturation via feather carotenoid concentration and via mechanisms unrelated to carotenoid deposition, while immune challenge affected feather length, but not carotenoid concentration. Thus, independent condition-dependent pathways, affected by different sources of variation, determine colour elaboration. This provides opportunities for the evolution of multiple signals within components of ornamental traits. This finding indicates that the selective forces shaping the evolution of different components of a composite trait and the trait's signal content may be more complex than believed so far, and that holistic approaches are required for drawing comprehensive evolutionary conclusions.

By-product information can stabilize the reliability of communication

Although communication underpins many biological processes, its function and basic definition remain contentious. In particular, researchers have debated whether information should be an integral part of a definition of communication and how it remains reliable. So far the handicap principle, assuming signal costs to stabilize reliable communication, has been the predominant paradigm in the study of animal communication. The role of by-product information produced by mechanisms other than the communicative interaction has been neglected in the debate on signal reliability. We argue that by-product information is common and that it provides the starting point for ritualization as the process of the evolution of communication. Second, by-product information remains unchanged during ritualization and enforces reliable communication by restricting the options for manipulation and cheating. Third, this perspective changes the focus of research on communication from studying signal costs to studying the costs of cheating. It can thus explain the reliability of signalling in many communication systems that do not rely on handicaps. We emphasize that communication can often be informative but that the evolution of communication does not cause the evolution of information because by-product information often predates and stimulates the evolution of communication. Communication is thus a consequence but not a cause of reliability. Communication is the interplay of inadvertent, informative traits and evolved traits that increase the stimulation and perception of perceivers. Viewing communication as a complex of inadvertent and derived traits facilitates understanding of the selective pressures shaping communication and those shaping information and its reliability. This viewpoint further contributes to resolving the current controversy on the role of information in communication.

One trait, many signals: different information on male quality is enclosed within the same trait in a blenny fish

Colourful ornaments are traditionally evaluated as one trait. However, they could consist of several components, such as total size, colour intensity and extension, each possibly bearing its own message about one aspect of quality. Males of the blenny Salaria pavo exhibit a colourful head crest and solely care for eggs. During the breeding season, the head crest shows a yellow colouration, the intensity and relative extension of which are independent of crest size. Here, we show that: (1) carotenoids are responsible for the head crest yellow patch; (2) activating the immune system by injecting the bacterial antigen lipopolysaccharides affects both the intensity and extent of the yellow colouration; and (3) females assess males on the basis of colour patch expression. However, the response of the yellow patch to the immune challenge was dependent on head crest size. Indeed, males with a larger head crest reacted better to the simulated infection, sustaining a level of yellow patch close to pre-challenge size.

Sexual selection and the evolution of complex color patterns in dragon lizards

Many species have elaborate and complex coloration and patterning, which often differ between the sexes. Sexual selection may increase the size or intensity of color patches (elaboration) in one sex or drive the evolution of novel signal elements (innovation). The latter potentially increases color pattern complexity. Color pattern complexity may also be influenced by ecological factors related to predation and environment; however, very few studies have investigated the effects of both sexual and natural selection on color pattern complexity across species. We used a phylogenetic comparative approach to examine these effects in 85 species and subspecies of Australian dragon lizards (family Agamidae). We quantified color pattern complexity by adapting the Shannon-Wiener diversity index. There were clear sex differences in color pattern complexity, which were positively correlated with both sexual dichromatism and sexual size dimorphism, consistent with the idea that sexual selection plays a significant role in the evolution of color pattern complexity. By contrast, we found little evidence of a link between environmental factors and color pattern complexity on body regions exposed to predators. Our results suggest that sexual selection rather than natural selection has led to increased color pattern complexity in males.

Environmental and genetic effects on pigment-based vs. structural component of yellow feather colouration

Background

Carotenoid plumage is of widespread use in bird communication. Carotenoid-based feather colouration has recently been shown to be dependent on both pigment concentration and feather structure. If these two components are determined differently, one plumage patch may potentially convey different aspects of individual quality.

Methodology/Principal Findings

We evaluated the effects of genetic and environmental factors on carotenoid-based yellow breast colouration of Great Tit (Parus major) nestlings. By partial cross-fostering, we separated the genetic and pre-natal vs. post-natal parental effects on both the structural and the pigment-based component of carotenoid-based plumage colouration. We also simultaneously manipulated the post-hatching environment by brood size manipulation. The structural component of nestling colouration reflected features of female colouration. On the other hand, the pigment-based component was more affected by rearing conditions presumably representing food quality. While the structural component was related to both origin- and environment-related factors, the pigment-based component seemed to be environment-dependent only. These results support the notion that pigment-based and structural components of feather colouration are determined differently.

Conclusions/Significance

Chromatic and achromatic components of carotenoid-based feather colouration reflected different aspects of individual quality and history, and thus may potentially form a multicomponent signal.

Temperature, but not available energy, affects the expression of a sexually selected ultraviolet (UV) colour trait in male European green lizards

BACKGROUND

Colour signals are widely used in intraspecific communication and often linked to individual fitness. The development of some pigment-based (e.g. carotenoids) colours is often environment-dependent and costly for the signaller, however, for structural colours (e.g. ultraviolet [UV]) this topic is poorly understood, especially in terrestrial ectothermic vertebrates.

METHODOLOGY/PRINCIPAL FINDINGS

In a factorial experiment, we studied how available energy and time at elevated body temperature affects the annual expression of the nuptial throat colour patch in male European green lizards (Lacerta viridis) after hibernation and before mating season. In this species, there is a female preference for males with high throat UV reflectance, and males with high UV reflectance are more likely to win fights. We found that (i) while food shortage decreased lizards' body condition, it did not affect colour development, and (ii) the available time for maintaining high body temperature affected the development of UV colour without affecting body condition or other colour traits.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that the expression of a sexually selected structural colour signal depends on the time at elevated body temperature affecting physiological performance but not on available energy gained from food per se in an ectothermic vertebrate. We suggest that the effect of high ambient temperature on UV colour in male L. viridis makes it an honest signal, because success in acquiring thermally favourable territories and/or effective behavioural thermoregulation can both be linked to individual quality.

Geographic and intrapopulational variation in colour and patterns of an aposematic toad, Melanophryniscus rubriventris (Amphibia, Anura, Bufonidae)

The aposematic toad genus Melanophryniscus is a polymorphic group with noticeable variation in colour and pattern. Here, we objectively evaluate variation in spectral reflectance and quantify variation in patterning within and among populations of the species M. rubriventris in NW Argentina. We conducted spectrophotometric analysis on 69 individuals and recorded dorsal and ventral pattern morphs of 727 individuals from six populations. We found high divergence in the reflectance spectra, the extent of brightly coloured areas, and the skin alkaloid profiles with no correlation among coloration varieties, alkaloid profiles, and the geographic distance between populations. Our analyses imply subdivision of sampled populations groupings based mostly on different dorsal colorations. Our results also reveal that populations with very similar patterns may differ markedly in colour and vice versa. It is striking that these aposematic toads show a pronounced variation in colour and patterning among and within populations showing individuals with a conspicuous bright dorsal colouration but also morphs with a rather cryptic black or drab colouration. However, the known presence of several alkaloids classes in all populations suggests that all morphs might be equally unpalatable.

Association of Mc1r variants with ecologically relevant phenotypes in the European ocellated lizard, Lacerta lepida

A comprehensive knowledge on the genetic basis of coloration is crucial to understand how new colour phenotypes arise and how they contribute to the emergence of new species. Variation in melanocortin-1 receptor (Mc1r), a gene that has been reported as a target for repeated evolution in a wide range of vertebrate taxa, was assessed in European ocellated lizards (Lacerta lepida) to search for associations with melanin-based colour phenotypes. Lacerta lepida subspecies' distribution is associated with the three major bio-climatic regions in the Iberian Peninsula. A nonconserved and derived substitution (T162I) was associated with the L. l. nevadensis phenotype (prevalence of brown scales). Another substitution (S172C) was associated with the presence of black scales in both L. l. lepida and L. l. iberica, but no mutations were found to be associated with the higher proportion of black in L. l. iberica. Extensive genotyping of Mc1r along the contact zone between L. l. nevadensis and L. l. lepida revealed low gene flow (only two hybrids detected). The implications of these findings are discussed in the context of previous knowledge about the evolutionary history of ocellated lizards.

Why do Calypte hummingbirds "sing" with both their tail and their syrinx? An apparent example of sexual sensory bias

Courtship displays frequently include complex signals that females use to pick a mate. Male Costa's hummingbirds (Calypte costae) generate two acoustic signals during courtship: a vocal song produced close to a female and a dive-sound produced during a courtship dive. The song and dive-sound sound similar, and both were assumed to be produced vocally by the syrinx. Here, we show that they are not; whereas the song is produced by the syrinx, the dive-sound is produced by high-frequency fluttering of the outermost tail feathers. The Anna's hummingbird (Calypte anna), sister to the Costa's, also sings a vocal song and produces a dive-sound with the wings and outermost tail feathers that sounds similar to a portion of the song. The interspecific match in signal form between the two species is not as strong as the intraspecific match. Phylogenetic reconstruction indicates that the dive-sounds may have evolved first, suggesting that the song may have evolved to mimic the dive-sound. We propose the "sexual sensory bias" hypothesis as an explanation for the match in form between the song and the dive-sound within each species, in which we suggest that new sexual signals can arise in response to preexisting female preferences for older sexual signals.

Flexibility in the colouration of the meninx (brain covering) in the guppy (Poecilia reticulata): investigations of potential function

Many organisms can change the apparent colour of their bodies by altering the aggregation of pigment in chromatophores in a process known as physiological colour change. In this study, we investigate a previously unstudied example of physiological colour change, from clear to black, of a brain covering, or meninx, in the guppy ( Poecilia reticulata Peters, 1859). UV protection in bright light was our primary hypothesis for the function of the meningeal colour, with a cost of increased conspicuousness to avian predators selecting for plasticity in the trait. An alternate hypothesis was that this flexible trait could be a physiological by-product of stress. Thus, we investigated the response of meningeal colour to light, stress, and simulated predator attacks, and also whether the black meninx affected conspicuousness to potential predators. Meningeal response to higher light levels did not differ from baseline responses. However, we did find that stress induced a sex-biased, rapid darkening of the meninx; this darkening then declined over time. These results suggest that meningeal blackness could be used as a novel, noninvasive indicator of stress level in guppies. We found no evidence for a role of predation in meningeal colour: meninx colour did not respond to the presence of a predator model and human “predators” detected similar numbers of guppies with black meninges and guppies with clear meninges.

A protean palette: colour materials and mixing in birds and butterflies

While typically classified as either 'structural' or 'pigmentary', bio-optical tissues of terrestrial animals are rarely homogeneous and typically contain both a structural material such as keratin or chitin and one or more pigments. These base materials interact physically and chemically to create colours. Combinations of structured base materials and embedded pigment molecules often interact optically to produce unique colours and optical properties. Therefore, to understand the mechanics and evolution of bio-optical tissues it is critical to understand their material properties, both in isolation and in combination. Here, we review the optics and evolution of coloured tissues with a focus on their base materials, using birds and butterflies as exemplar taxa owing to the strength of our current knowledge of colour production in these animals. We first review what is known of their base materials, and then discuss the consequences of these interactions from an optical perspective. Finally, we suggest directions for future research on colour optics and evolution that will be invaluable as we move towards a fuller understanding of colour in the natural world.

An intracellular antioxidant determines the expression of a melanin-based signal in a bird

To understand how traits used in animal communication evolved and are maintained as honest signals, we need to understand the mechanisms that prevent cheating. It has been proposed that honest signaling is guaranteed by the costs associated with the signal expression. However, the nature of these costs is still under debate. Melanin-based signals are intriguing because their expression seems to be tightly controlled by genes and the resource involved (i.e. melanin) seems to be not limited. However, in vertebrates, low levels of a key intracellular antioxidant (i.e. glutathione) are needed to promote melanogenesis. We propose that melanin-based ornaments can signal the ability to cope with oxidative stress because those individuals with low enough levels of glutathione, such as those required for melanin production, should manage well the whole of the antioxidant machinery in order to maintain a certain oxidative status. We analysed the expression of a melanin-based signal: the well-known black stripe of the great tit (Parus major). Great tit nestlings were injected with a specific inhibitor of glutathione production (DL-buthionine-S,R-sulfoximine; BSO) throughout their development. BSO effectively decreased intracellular glutathione levels without apparent side effects on growth or body condition. Instead, treated nestlings developed black breast stripes 70-100% larger than controls. Moreover, treated nestlings also compensated the decrease in glutathione levels by increasing the levels of circulating antioxidants. Results indicate that melanin-based signals can be at least partially permeable to environmental influences such as those associated to oxidative stress. They also reveal a potential handicap associated to the expression of this kind of signals. Finally, although other contributing factors could have been present, our findings emphasize the role of oxidative stress in shaping the evolution of animal signals in general and, in particular, those produced by pigments.

Expression of breeding coloration in European Green Lizards (Lacerta viridis): variation with morphology and tick infestation

According to the hypothesis of parasite-mediated sexual selection, for a communication system to work reliably, parasites should reduce the showiness of sexual signals of their host. In this study, we examined whether the expression of breeding coloration in free-ranging adult European Green Lizards ( Lacerta viridis (Laurenti, 1768)) is linked with infestation by their common ectoparasite Ixodes ricinus (L., 1758) (Acari: Ixodidae). We found that tick infestation was higher in males than in females. Males showing relatively heavier body for their tail length (predominantly males with regenerated tails) and relatively thinner tail base experienced higher infestation rates. In turn, relatively heavier females for their snout–vent length were less tick infested. Although some components of throat and chest coloration varied significantly with relative tail length, tail-base thickness, body mass, and head size, a measure of male throat and female chest color saturation seemed independent of lizard morphology. After correcting for the effects of morphology on skin coloration and tick load, the saturation of blue throat color in male lizards decreased with increasing level of tick infestation. In contrast, yellow chest color saturation increased with residual tick numbers in females. Considering presumably different signaling functions of male and female lizard coloration, our work suggests that tick infestation might represent a handicap for Green Lizards.