Carotenoid-based Coloration Research Articles

Carotenoids bolster immunity during moult in a wild songbird with sexually selected plumage coloration

Carotenoid-based colours in animals are valuable models for testing theories of sexual selection and life-history trade-offs because the pigments used in coloration are chemically tractable in the diet and in the body, where they serve multiple purposes (e.g. health enhancement, photoprotection). An important assumption underlying the hypothesized signalling value of carotenoid coloration is that there is a trade-off in carotenoid pigment allocation, such that not all individuals can meet the physiological/morphological demands for carotenoids (i.e. carotenoids are limited) and that only those who have abundant supplies or fewer demands become the most colourful. Studies of carotenoid trade-offs in colourful animals have been limited largely to domesticated species, which may have undergone artificial selection that changed the historical/natural immunomodulatory roles of carotenoids, to young animals lacking carotenoid-based signals or to species displaying carotenoid-based skin and bare parts. We studied the health benefits of carotenoids during moult in house finches (Carpodacus mexicanus), which display sexually selected, carotenoid-based plumage coloration. We manipulated dietary carotenoid availability during both winter (nonmoult) and autumn (moult) in captive males and females and found that carotenoid-supplemented birds mounted stronger immune responses (to phytohemagglutinin injection and to a bacterial inoculation in blood) than control birds only during moult. This study provides experimental, seasonal support for a fundamental tenet of Lozano's ‘carotenoid trade-off’ hypothesis and adds to a growing list of animal species that benefit immunologically from ingesting higher dietary carotenoid levels.

Bird population declines due to radiation exposure at Chernobyl are stronger in species with pheomelanin-based coloration

Eumelanin and pheomelanin are the most common pigments providing color to the integument of vertebrates. While pheomelanogenesis requires high levels of a key intracellular antioxidant (glutathione, GSH), eumelanogenesis is inhibited by GSH. This implies that species that possess the molecular basis to produce large amounts of pheomelanin might be more limited in coping with environmental conditions that generate oxidative stress than species that produce eumelanin. Exposure to ionizing radiation produces free radicals and depletes antioxidant resources. GSH is particularly susceptible to radiation, so that species with large proportions of pheomelanic integument may be limited by the availability of GSH to combat oxidative stress and may thus suffer more from radiation effects. We tested this hypothesis in 97 species of birds censused in areas with varying levels of radioactive contamination around Chernobyl. After controlling for the effects of carotenoid-based color, body mass and similarity among taxa due to common phylogenetic descent, the proportion of pheomelanic plumage was strongly negatively related to the slope estimates of the relationship between abundance and radiation levels, while no effect of eumelanic color proportion was found. This represents the first report of an effect of the expression of melanin-based coloration on the capacity to resist the effects of ionizing radiation. Population declines were also stronger in species that exhibit carotenoid-based coloration and have large body mass. The magnitude of population declines had a relatively high phylogenetic signal, indicating that certain groups of birds, especially non-corvid passeriforms, are particularly susceptible to suffer from the effects of radioactive contamination due to phylogenetic inertia.

Age, sex and beauty: methodological dependence of age- and sex-dichromatism in the great tit Parus major

Carotenoid-based colour expression is frequently involved in sexual dichromatism, particularly in bird plumage, suggesting a role in sexual selection. Despite much work on expression of the carotenoid-based ventral plumage coloration of the great tit (Parus major), which represents a popular model in evolution and ecology, a consensus on even the most basic demographic patterns of variation (e.g. age and sex differences) is lacking. This may reflect the use of variable methods for analysing colour variation, although what is not clear, either in this case or in general, is the extent to which these alternative methods are equally effective at describing age- and sex-related dichromatism. Using data obtained over 4 years from a large sample of free-ranging great tits, we examined how colour-scoring methodology influences estimates of age- and sex-related dichromatism. We compare: (1) principal components analysis-derived scores; (2) tristimulus colour variables; (3) a visual model-independent, carotenoid-focussed colour score; and (4) two colour scoring methods based on avian visual models, examining how they assess colour variation with respect to age and sex to determine how methodology may influence results. We demonstrate clear age- and sex-dependent expression of this colour trait, both in our own data and in meta-analyses of results from great tit populations across Europe, and discuss the merits of the various colour scores, which yield very different estimates of the extent of age- and sex-dependent dichromatism. We show variation is likely to be visible to conspecifics and propose a novel, visual model-derived scoring system for describing variation in carotenoid-based colour patches, where the perceived signal is divided into independent chromatic and achromatic components, in line with current understanding of visual perception. The present study highlights the impact of colour-scoring methodology and shows that, as novel measures continue to be developed, researchers should consider carefully how they quantify colour expression. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 777–796.

Does Breeding Ornamentation Signal Genetic Quality in Arctic charr, Salvelinus alpinus?

Genetic theories of sexual selection predict that most ornamental secondary sexual traits provide reliable indication of the genetic quality of their bearers. Accordingly, also the offspring of mates with elaborate mating display should perform better than those of less conspicuous counterparts. In this study, we used Arctic charr (Salvelinus alpinus) as a model species to investigate whether the variation in a carotenoid-based red breeding coloration (a sexually dichromatic trait) in different sexes would reflect differences in individual genetic variability, one measure of individual quality, and/or indirectly, be manifested in variation in the offspring’s early viability and growth. We created maternal half-sibling families by artificially fertilizing the eggs with milt from bright- and pale-coloured males and then held the resulting progenies under identical hatchery conditions. The expression of red coloration among parental fish was not associated with their genetic diversity estimates in either sex nor did offspring sired by bright males consistently differ in terms of embryo survival or endogenous growth efficiency from offspring sired by pale males. By contrast, maternal effects were notably strong and, additionally, the degree of female coloration was negatively linked to their reproductive potential. The more intensely coloured females had a smaller relative fecundity and they also produced offspring of lower viability, implying a significant trade-off in resource allocation between ornamentation and offspring. Our results indicate that the red breeding ornamentation of Arctic charr is likely to be informative rather among females than males when the reproductive quality is predicted on grounds of the number of offspring produced. Nevertheless, this study does not support the direct selection hypothesis in explaining the evolution of female ornamentation, but rather suggests that the less intense coloration of female charr compared to males may reflect inter-sexual differences in the trade-off between natural and sexual selection.

Atypical Teratoid/Rhabdoid Tumors in Adult Patients: CT and MR Imaging Features

Primary AT/RT is a rare highly malignant tumor of the CNS, usually occurring in children younger than 5 years of age. The objective of this study was to characterize the CT and MR imaging findings in a series of 5 adult patients with pathologically proved AT/RT. All 5 AT/RTs were supratentorial. In 2 patients who underwent nonenhanced CT, the tumors appeared isoattenuated, and 1 of the 2 tumors contained calcifications. Solid portions of the tumors on MR imaging were isointense on T1-weighted, T2-weighted, and FLAIR images, and 1 case showed restricted diffusion on DWI. The tumors also demonstrated a bandlike rim of strong enhancement surrounding a central cystic area on contrast-enhanced T1-weighted imaging. One tumor was associated with destruction of the calvaria. Although AT/RTs can have nonspecific findings, the tumors in our series were large and isointense on T1-weighted, T2-weighted, and FLAIR images with central necrosis and prominent rim enhancement.

Age-Related Differences in a Carotenoid-Based Coloration Trait are Due to Within-Individual Changes in Great TitsParus major

Adult birds usually display brighter plumage coloration than younger individuals. This difference may be either due to selection against less coloured yearlings or to an increase in coloration after their first complete moult. In this study, we examined age-related differences in carotenoid-based yellow ventral coloration of Great Tits Parus major. Cross-sectional analyses at the population level showed that older males are in general greener and more saturated in colour than yearling individuals. Longitudinal analyses using birds captured in subsequent seasons confirmed within-individual changes in coloration with age. Such increase in coloration might be the result of the development of better foraging skills with age- and hence, the individual ability to obtain enough carotenoid-rich sources at the time of moult-, or consequence of other factors indirectly associated with age. However, we cannot rule out the possibility that other factors (such as directional selection against less coloured individuals) operate additionally at the population level.

Carotenoid-based bill colour is an integrative signal of multiple parasite infection in blackbird

In the study of parasite-mediated sexual selection, there has been controversial evidence for the prediction that brighter males should have fewer parasites. Most of these studies have focused on one parasite species. Our aim was to investigate the expression of carotenoid-based coloured signals in relation to patterns of multiple parasite infections, to determine whether colour reflects parasite load of all parasite species, or whether different relationships might be found when looking at each parasite species independently. We investigated the relationship between bill colour, body mass and plasma carotenoids and parasite load (feather chewing lice, blood parasite Plasmodium sp., intestinal parasites cestodes and coccidia) in the blackbird (Turdus merula). Bill colour on its own appeared to be a poor predictor of parasite load when investigating its relationships with individual parasite species. Variation in parasite intensities at the community level was summarised using principal component analysis to derive synthetic indexes of relative parasite species abundance and absolute parasite load. The relative abundance of parasite species was strongly related to bill colour, plasma carotenoid levels and body mass: birds with relatively more cestodes and chewing lice and relatively less Plasmodium and coccidia had a more colourful bill, circulated more carotenoids and were heavier. These results suggest that bill colour more accurately reflects the relative intensities of parasite infection, rather than one-by-one relationships with parasites or absolute parasite burden. Investigating patterns of multiple parasite infection would thus improve our understanding of the information conveyed by coloured signals on parasite load.

Maternal investment of female mallards is influenced by male carotenoid-based coloration.

The differential allocation hypothesis predicts that females modify their investment in a breeding attempt according to its reproductive value. One prediction of this hypothesis is that females will increase reproductive investment when mated to high-quality males. In birds, it was shown that females can modulate pre-hatch reproductive investment by manipulating egg and clutch sizes and/or the concentrations of egg internal compounds according to paternal attractiveness. However, the differential allocation of immune factors has seldom been considered, particularly with an experimental approach. The carotenoid-based ornaments can function as reliable signals of quality, indicating better immunity or ability to resist parasites. Thus, numerous studies show that females use the expression of carotenoid-based colour when choosing mates; but the influence of this paternal coloration on maternal investment decisions has seldom been considered and has only been experimentally studied with artificial manipulation of male coloration. Here, we used dietary carotenoid provisioning to manipulate male mallard (Anas platyrhynchos) bill coloration, a sexually selected trait, and followed female investment. We show that an increase of male bill coloration positively influenced egg mass and albumen lysozyme concentration. By contrast, yolk carotenoid concentration was not affected by paternal ornamentation. Maternal decisions highlighted in this study may influence chick survival and compel males to maintain carotenoid-based coloration from the mate-choice period until egg-laying has been finished.

Seasonal Changes in Colour: A Comparison of Structural, Melanin- and Carotenoid-Based Plumage Colours

BackgroundPlumage coloration is important for bird communication, most notably in sexual signalling. Colour is often considered a good quality indicator, and the expression of exaggerated colours may depend on individual condition during moult. After moult, plumage coloration has been deemed fixed due to the fact that feathers are dead structures. Still, many plumage colours change after moult, although whether this affects signalling has not been sufficiently assessed.Methodology/Principal FindingsWe studied changes in coloration after moult in four passerine birds (robin, Erithacus rubecula; blackbird, Turdus merula; blue tit, Cyanistes caeruleus; and great tit, Parus major) displaying various coloration types (melanin-, carotenoid-based and structural). Birds were caught regularly during three years to measure plumage reflectance. We used models of avian colour vision to derive two variables, one describing chromatic and the other achromatic variation over the year that can be compared in magnitude among different colour types. All studied plumage patches but one (yellow breast of the blue tit) showed significant chromatic changes over the year, although these were smaller than for a typical dynamic trait (bill colour). Overall, structural colours showed a reduction in relative reflectance at shorter wavelengths, carotenoid-based colours the opposite pattern, while no general pattern was found for melanin-based colours. Achromatic changes were also common, but there were no consistent patterns of change for the different types of colours.Conclusions/SignificanceChanges of plumage coloration independent of moult are probably widespread; they should be perceivable by birds and have the potential to affect colour signalling.

Dissecting Carotenoid from Structural Components of Carotenoid‐Based Coloration: A Field Experiment with Great Tits (Parus major)

Carotenoid-based yellowish to red plumage colors are widespread visual signals used in sexual and social communication. To understand their ultimate signaling functions, it is important to identify the proximate mechanism promoting variation in coloration. Carotenoid-based colors combine structural and pigmentary components, but the importance of the contribution of structural components to variation in pigment-based colors (i.e., carotenoid-based colors) has been undervalued. In a field experiment with great tits (Parus major), we combined a brood size manipulation with a simultaneous carotenoid supplementation in order to disentangle the effects of carotenoid availability and early growth condition on different components of the yellow breast feathers. By defining independent measures of feather carotenoid content (absolute carotenoid chroma) and background structure (background reflectance), we demonstrate that environmental factors experienced during the nestling period, namely, early growth conditions and carotenoid availability, contribute independently to variation in yellow plumage coloration. While early growth conditions affected the background reflectance of the plumage, the availability of carotenoids affected the absolute carotenoid chroma, the peak of maximum ultraviolet reflectance, and the overall shape, that is, chromatic information of the reflectance curves. These findings demonstrate that environment-induced variation in background structure contributes significantly to intraspecific variation in yellow carotenoid-based plumage coloration.

Oxidative stress and information content of black and yellow plumage coloration: an experiment with greenfinches

Carotenoid and melanin pigments in the plumage of birds are hypothesized to be sensitive to oxidative stress. We manipulated oxidative status of captive greenfinches (Carduelis chloris L.) by the administration of buthionine sulfoximine (BSO), a selective inhibitor of the synthesis of glutathione (GSH), an intracellular antioxidant. Half of the birds in the treated group, as well as in the control group, also received dietary carotenoid (lutein) supplementation. BSO treatment reduced erythrocyte GSH levels and caused oxidative damage as indicated by the increased concentration of plasma malondialdehyde (MDA), an end product of lipid peroxidation. BSO treatment also reduced the brightness (i.e. increased blackness) of the tips of tail feathers grown during the experiment. These results show that a low systemic GSH level is required for development of eumelanin plumage coloration and that such a low GSH level is also potentially dangerous for the organism. Carotenoid supplementation increased plasma carotenoid levels and chroma of the yellow parts of the feathers grown during the experiment. However, carotenoid supplementation did not reduce plasma MDA levels. Manipulation of GSH did not affect plasma carotenoids or carotenoid-based plumage coloration. These findings argue against the antioxidant function of lutein in vivo and carotenoid signaling of antioxidant status.

Variation in Plumage Coloration of Northern Cardinals in Urbanizing Landscapes

Abstract Biologists know relatively little of how one of the most important avian phenotypic signals, feather coloration, may be affected by anthropogenic changes resulting from urbanization. We examined the relationship between urbanization and carotenoid-based plumage color of Northern Cardinals (Cardinalis cardinalis) in 13 riparian forests distributed across a rural-to-urban landscape gradient in central Ohio, USA. Feathers and morphometric measurements were collected from breeding territorial males (n = 129) and females (n = 145) during March–August 2006–2008. Plumage brightness of males, but not females, increased with body condition (i.e., size-adjusted mass) and declined with amount of urbanization surrounding forests in which cardinals bred. The extent plumage coloration reflected condition was partially mediated by landscape composition. Specifically, the relationship between brightness and body condition was most pronounced in the most rural landscapes. The interdependency of male coloratio...

Carotenoid-based coloration, oxidative stress and corticosterone in common lizards

Environmental factors including stressors, health status and social context significantly affect carotenoid-based coloration. For instance, stressors may induce the diversion of carotenoids from pigmentation pathways, potentially explaining why stressed animals often exhibit reduced coloration. However, we recently showed that high blood corticosterone concentrations, which are part of the physiological stress response, are associated with increased redness of the belly in the common lizard (Lacerta vivipara). This result clearly contrasts with the findings of many studies of carotenoid-based coloration because corticosterone is believed to increase oxidative stress. Here, we examined whether these positive effects are influenced by differences in food availability. We tested the effect of high corticosterone levels on carotenoid-based coloration, antioxidant enzyme activity and oxidative damage in common lizards subject to low and high food availability. Food restriction abolished the carotenoid-based color enhancement when corticosterone concentrations in animals were high. We discuss how carotenoid-based color can honestly signal individual quality in this species and how the increased redness induced by corticosterone could be a terminal investment in an environment where long-term survival prospects are poor but not when immediate survival is endangered.

Carotenoid Composition of Invertebrates Consumed by Two Insectivorous Bird Species

Dietary carotenoids are important pigments, antioxidants, and immune-stimulants for birds. Despite recent interest in carotenoids in bird ecology, we know surprisingly little about the carotenoid content of invertebrates consumed by birds. We compared carotenoid (lutein, beta-carotene, and total) concentrations in invertebrates brought to nestlings by two insectivorous passerines, the great tit, Parus major and the pied flycatcher, Ficedula hypoleuca. We also compared carotenoid levels between environments that were either polluted by heavy metals or were not polluted, because the carotenoid-based plumage color of P. major nestlings is affected by environmental pollution. Lepidopterans were the most carotenoid-rich food items and contained the largest proportion of lutein. There were no differences in carotenoid concentrations in the food items of the two bird species but P. major nestlings obtained more carotenoids from their invertebrate diet than F. hypoleuca nestlings because the P. major diet had a higher proportion of lepidopteran larvae. In polluted areas, P. major nestlings consumed lower levels of dietary carotenoids than in unpolluted areas because of temporal differences in caterpillar abundance between polluted and unpolluted sites. Our study suggests that pollution-related difference in nestling plumage color in P. major is related to varying dietary proportion of lutein-rich food items rather than pollution-related variation in insect carotenoid levels.

Carotenoid-based coloration predicts resistance to oxidative damage during immune challenge

Many animal ornaments may have evolved as signals advertising the quality of the bearer. The honesty of the information content of these signals would rely on the costs associated with their expression, these being relatively greater for low-quality than for high-quality individuals. Given the physiological functions of carotenoids, carotenoid-based ornaments could indicate individual immunocompetence, and possibly the ability to mount an immune response at a lower cost. We evaluated whether the red carotenoid-based coloration of male red-legged partridges (Alectoris rufa) predicts the capacity of the individual to counteract the oxidative stress generated by a cell-mediated immune response. Individuals were subcutaneously injected with phytohaemagglutinin (PHA) or phosphate buffer solution (PBS) as a control. We found that eye ring pigmentation predicted the change in the amount of peroxidized lipids (TBARS) in blood after the PHA-induced inflammatory challenge. The degree of pigmentation of this carotenoid-based ornament was also negatively related to individual changes in gamma-glutamyl transferase (GGT), another biomarker of oxidative stress involved in antioxidant metabolism (i.e. glutathione recycling). However, changes in circulating carotenoids did not significantly explain changes in lipid peroxidation or GGT levels, suggesting that the higher resistance to oxidative stress of those individuals with more pigmented eye rings was not directly mediated by their greater circulating levels of carotenoids. Our results indicate that carotenoid-based coloration can predict not only immune responsiveness (more coloured males mount greater responses) but also an individual's ability to counter the oxidative stress generated during immune challenge (more coloured males experience less oxidative damage when mounting an immune response).

Oxidative stress and the effect of parasites on a carotenoid-based ornament

It has been drawn to our attention that Fig. 1A,C,D in J. Exp. Biol.213, 400-407 appears to show the same data as fig 1a,d,e in a paper published by (some of) the same authors in Journal of Evolutionary Biology (Mougeot et al., 2010). The authors wish to clarify the relationship between these two papers and rectify the lack of cross-referencing.Both papers focus on red grouse and are outputs from the same large-scale field manipulation undertaken in 2006. They represent two, very different, stand-alone studies addressing different conceptual issues. The central experimental manipulation of both papers is the drug treatment of birds. The authors felt it was necessary to include background data in each paper to give the reader confidence in the efficacy of the experimental treatment and to allow the reader to better understand and interpret the main results of the papers, which explore very different physiological mechanisms that influence ornamental colouration.For this reason, the treatment effects on parasite burden and comb colouration were presented in both papers, although a different index of comb redness was used [adjusted R-values (fig. 1e in J. Evol. Biol.) vs adjusted R/(G+B) values (Fig. 1C in J. Exp. Biol.)]. Both indices are accurate reflections of comb redness, gave similar results and have been used in previous studies in different contexts to characterize comb colour. Both papers also include the same data on levels of circulating carotenoids [Fig. 1D in J. Exp. Biol.; fig. 1d in J. Evol. Biol.]. The authors feel the inclusion of these data in both papers can be justified conceptually. In the J. Exp. Biol. paper, the trade-off between the use of carotenoids for colouring ornaments vs dealing with oxidative stress is central to the key question. In the J. Evol. Biol. paper, these data reinforce the argument that carotenoid levels and ornamental colouration covary with corticosterone levels.The reiteration of background data on parasite removal and carotenoid levels does not compromise the uniqueness of each paper. The papers were submitted to, and published by, the respective journals in parallel, and the absence of cross-referencing was an oversight on the part of the authors, who acknowledge that the figure legends should have cited the corresponding paper and that the different indices used for comb redness should have been clarified.Consequently, the text of Fig. 1 in the J. Exp. Biol. paper should have read:Fig. 1. Mean ± s.e.m. changes over time in (A) T. tenuis abundance (worms male−1) (reproduced from Mougeot et al., 2010); (B) comb area (mm2); (C) comb redness (adjusted R/(G+B)-value; see Materials and methods) (data from Mougeot et al., 2010); (D) plasma carotenoid concentration (mg ml−1) (reproduced from Mougeot et al., 2010); (E) plasma concentration of malondialdehyde (MDA; nmol ml−1) and (F) plasma TAS (total antioxidant status; mmol ml−1). White squares: control males; black squares: treated (dosed) males (purged of parasites). Numbers above error bars refer to sample size (number of males).The following reference should also have been included in the Reference list:Mougeot, F., Martinez-Padilla, J., Bortolotti, G. R., Webster, L. M. I. and Piertney, S. B. (2010). Physiological stress links parasites to carotenoid-based colour signals. J. Evol. Biol.23, 643-650.The authors apologise for any inconvenience caused.

Physiological stress links parasites to carotenoid-based colour signals

Vertebrates commonly use carotenoid-based traits as social signals. These can reliably advertise current nutritional status and health because carotenoids must be acquired through the diet and their allocation to ornaments is traded-off against other self-maintenance needs. We propose that the coloration more generally reveals an individual's ability to cope with stressful conditions. We tested this idea by manipulating the nematode parasite infection in free-living red grouse (Lagopus lagopus scoticus) and examining the effects on body mass, carotenoid-based coloration of a main social signal and the amount of corticosterone deposited in feathers grown during the experiment. We show that parasites increase stress and reduce carotenoid-based coloration, and that the impact of parasites on coloration was associated with changes in corticosterone, more than changes in body mass. Carotenoid-based coloration appears linked to physiological stress and could therefore reveal an individual's ability to cope with stressors.

Testing the influence of local forest canopy clearing on phenotypic variation in Trinidadian guppies

1. Factors contributing to the maintenance of phenotypic variation in nature are often difficult to determine. Secondary sexual traits might be particularly interesting in this regard due to the interaction they experience between multiple selective agents. One way to examine such effects is to monitor populations following environmental change. Human-caused changes can be particularly useful here because they often involve an abrupt and extreme alteration of specific habitat features. This alteration can then precipitate phenotypic plasticity, changes in adaptive landscapes, and modified evolutionary trajectories. The consequences of habitat manipulations on local populations can therefore improve our understanding of phenotypic variation in complex ecological systems. 2. We took advantage of a human-caused environmental disturbance to examine factors influencing phenotypic variation in Trinidadian guppies (Poecilia reticulata). Differences in canopy cover along the stream have been hypothesized to explain some of this variation, but this has been hard to test directly. We here attempt a direct test of this hypothesis by monitoring changes in guppy size and colour following a dramatic decrease in canopy cover due to tree removal for agricultural activity. 3. Although male and female body size increased following canopy clearing, little change was observed in the overall amount of melanin-based colours, carotenoid-based colours, and structural colours on males. We further compared phenotypes before and after canopy clearing at the disturbed site to those from two nearby reference sites that are at extreme ends of canopy cover. Overall, variation in colour was attributed to differences among sites, irrespective of canopy differences. We also found considerable temporal variation in some colour elements at a given site. 4. Our results suggest that differences in light availability do not cause rapid and dramatic changes in guppy colour. The substantial unexplained variation must therefore be due to factors other than canopy (measured here) and predation regime (all sites were ‘low-predation’). Because of the multiple and complex interactions involved in the expression and maintenance of sexually selected traits, our study emphasizes the need for a better understanding of both the genetic and environmental sources of co-variation between sexual ornaments and preferences.

The carotenoid-continuum: carotenoid-based plumage ranges from conspicuous to cryptic and back again

BackgroundCarotenoids are frequently used by birds to colour their plumage with green, yellow, orange or red hues, and carotenoid-based colours are considered honest signals of quality, although they may have other functions, such as crypsis. It is usually assumed that red through yellow colours have a signalling function while green is cryptic. Here we challenge this notion using the yellow and green colouration of blue tits (Cyanistes caeruleus), great tits (Parus major) and greenfinches (Carduelis chloris) as a model.ResultsThe relationship between colouration (chroma, computed using visual sensitivities of conspecifics) and detectability (contrast against natural backgrounds as perceived by conspecifics and avian predators) followed a similar curvilinear pattern for yellow and green plumage with minimum detectability at intermediate levels of carotenoid deposition. Thus, for yellow and green plumage, colours at or close to the point of minimum detectability may aid in crypsis. This may be the case for blue and great tit green and yellow plumage, and greenfinch green plumage, all of which had comparably low levels of detectability, while greenfinch yellow plumage was more chromatic and detectable. As yellow and green blue tit colouration are strongly affected by carotenoid availability during moult, variation in pigment availability between habitats may affect the degree of background-matching or the costliness of producing cryptic plumage.ConclusionsIncreasing carotenoid-deposition in the integument does not always lead to more conspicuous colours. In some cases, such as in blue or great tits, carotenoid deposition may be selected through enhanced background-matching, which in turn suggests that producing cryptic plumage may entail costs. We stress however, that our data do not rule out a signalling function of carotenoid-based plumage in tits. Rather, it shows that alternative functions are plausible and that assuming a signalling function based solely on the deposition of carotenoids in the integument may not be warranted.

Evolution of female carotenoid coloration by sexual constraint in Carduelis finches

BackgroundFemales often express the same ornaments as males to a similar or lesser degree. Female ornaments can be adaptive, but little is known regarding their origins and mode of evolution. Current utility does not imply evolutionary causation, and therefore it is possible that female ornamentation evolved due to selection on females, as a correlated response to selection on males (sexual constraint), or a combination of both. We tested these ideas simulating simple models for the evolution of male and female correlated traits, and compared their predictions against the coloration of finches in the genus Carduelis.ResultsFor carotenoid-based ornamental coloration, a model of sexual constraint on females fits the Carduelis data well. The two alternative models (sexual constraint on males, and mutual constraint) were rejected as causing the similarities in carotenoid coloration between males and females. For melanin coloration, the correlation between the sexes was weaker, indicating that males and females evolved independently to a greater extent.ConclusionsThis indicates that sexual constraint on females was an important mechanism for the evolution of ornamental carotenoid coloration in females, but less so for melanin coloration. This does not mean that female carotenoid coloration is non-adaptive or maladaptive, because sexual dichromatism could evolve if it were maladaptive. It suggests, however, that most evolution of female carotenoid coloration was male-driven and, when adaptive, may not be an adaptation stricto sensu.