Melanin coloration in New World orioles I: carotenoid masking and pigment dichromatism in the orchard oriole complex

Carotenoids produce the brilliant red, orange, and yellow colors of many animals. However, melanin pigments can also confer some of these same hues. Because carotenoid and melanin colors are produced in different ways and may serve different signaling functions, either within or between species, it is important to establish whether one or both types of pigment are responsible for coloration. We have discovered what appears to be an evolutionary switch from carotenoid- to melanin-based color in two sexually dichromatic New World orioles. Using a combination of reflectance spectrometry and chromatographic analyses of plumage pigments, we found that the chestnut plumage of adult male orchard orioles Icterus spurius is produced predominantly by phaeomelanins. Orchard oriole feathers also contain carotenoids, which appear to be masked by the high concentration of phaeomelanins. In contrast, both carotenoids and phaeomelanins appear to contribute to color in adult male Fuertes's orioles I. fuertesi . Moreover, yellow yearling male and female plumage in both species is produced by carotenoids alone. The masking of carotenoids with phaeomelanins in orchard orioles is interesting in light of the signaling roles that carotenoids are thought to play. In addition, these plumage differences produce a unique case of age and sexual pigment dimorphism in orchard and Fuertes's orioles.     

鸟类羽毛色素的合成机理与调控机制

鸟类作为色彩最丰富的陆生脊椎动物,其体表覆盖着颜色多样的羽毛,在伪装、择偶、信号识别等多方面具有重要功能,因此羽毛颜色引起了研究者的极大兴趣。羽毛颜色总体分为由化学物质产生的色素色和由物理结构产生的结构色,其中常见色素有两大类。根据近年来对羽毛色素的研究进展,本文总结了黑色素和类胡萝卜素的类型、合成途径、获取途径以及相关基因,为深入研究羽毛色素合成、代谢的分子调控机制提供科学依据。     

Contributions of pterin and carotenoid pigments to dewlap coloration in two anole species

Animals can acquire bright coloration using a variety of pigmentary and microstructural mechanisms. Reptiles and amphibians are known to use two types of pigments - pterins and carotenoids - to generate their spectrum of colorful red, orange, and yellow hues. Because both pigment classes can confer all of these hues, the relative importance of pterins versus carotenoids in creating these different colors is not always apparent. We studied the carotenoid and pterin content of red and yellow dewlap regions in two neotropical anole species - the brown anole (Norops sagrei) and the ground anole (N. humilis). Pterins (likely drosopterins) and carotenoids (likely xanthophylls) were present in all tissues from all individuals. Pterins were more enriched in the lateral (red) region, and carotenoids more enriched in the midline (yellow) region in N. humilis, but pterins and carotenoids were found in similar concentrations among lateral and midline regions in N. sagrei. These patterns indicate that both carotenoid and pterin pigments are responsible for producing color in the dichromatic dewlaps of these two species, and that in these two species the two pigments interact differently to produce the observed colors.     

Red-winged blackbirds Agelaius phoeniceus use carotenoid and melanin pigments to color their epaulets

Over the past three decades, the red‐winged blackbird Agelaius phoeniceus has served as a model species for studies of sexual selection and the evolution of ornamental traits. Particular attention has been paid to the role of the colorful red‐and‐yellow epaulets that are striking in males but reduced in females and juveniles. It has been assumed that carotenoid pigments bestow the brilliant red and yellow colors on epaulet feathers, but this has never been tested biochemically. Here, we use high‐performance liquid chromatography (HPLC) to describe the pigments present in these colorful feathers. Two red ketocarotenoids (astaxanthin and canthaxanthin) are responsible for the bright red hue of epaulets. Two yellow dietary precursors pigments (lutein and zeaxanthin) are also present in moderately high concentrations in red feathers. After extracting carotenoids, however, red feathers remained deep brown in color. HPLC tests show that melanin pigments (primarily eumelanin) are also found in the red‐pigmented barbules of epaulet feathers, at an approximately equal concentration to carotenoids. This appears to be an uncommon feature of carotenoid‐based ornamental plumage in birds, as was shown by comparable analyses of melanin in the yellow feathers of male American goldfinches Carduelis tristis and the red feathers of northern cardinals Cardinalis cardinalis, in which we detected virtually no melanins. Furthermore, the yellow bordering feathers of male epaulets are devoid of carotenoids (except when tinged with a carotenoid‐derived pink coloration on occasion) and instead are comprised of a high concentration of primarily phaeomelanin pigments. The dual pigment composition of red epaulet feathers and the melanin‐only basis for yellow coloration may have important implications for the honesty‐reinforcing mechanisms underlying ornamental epaulets in red‐winged blackbirds, and shed light on the difficulties researchers have had to date in characterizing the signaling function of this trait. As in several other birds, the melanic nature of feathers may explain why epaulets are used largely to settle aggressive contests rather than to attract mates.     

Multiple ways to become red: pigment identification in red feathers using spectrometry

Red hues are a challenge in studies on the evolution of bird coloration, as multiple pigments such as carotenoids, pheomelanin, psittacofulvins, porphyrins, turacin, haemoglobin and even exogenous iron-oxides, may confer red colors. Determining the pigment type is paramount and here we investigate the differences in spectrum reflectance for six pigments resulting in red colorations in feathers of different species, with a focus on discriminating among melanins and carotenoids. Pigment chemical identification was obtained from the literature or using High Performance Liquid Chromatography (HPLC) in our laboratory. We have also derived discriminant formulas for identification of the major known types of pigments based on parameters of the reflectance curves obtained with a portable spectrometer. Our results indicate that the reflectance patterns of coloration perceived as red patches widely differ. The distinction between carotenoid- and melanin-based reflectance curves is relatively straightforward: sigmoid versus straight slope. The spectral reflectance curves of feathers containing red psittacofulvins are sigmoid, whereas iron oxide and porphyrin-containing feathers recall pheomelanins in rendering a straight slope. In the case of turacin-based coloration, the spectral shape is unique. For the pigments with enough number of species sampled (i.e., carotenoids, melanins and psittacofulvins) the differences in reflectance shape are important enough to allow separation of carotenoid and melanin derived colorations based on reflectance curves alone.     

Carotenoids, immunocompetence, and the information content of sexual colors: an experimental test

Many male birds use carotenoid pigments to acquire brilliant colors that advertise their health and condition to prospective mates. The direct means by which the most colorful males achieve superior health has been debated, however. One hypothesis, based on studies of carotenoids as antioxidants in humans and other animals, is that carotenoids directly boost the immune system of colorful birds. We studied the relationship between carotenoid pigments, immune function, and sexual coloration in zebra finches (Taeniopygia guttata), a species in which males incorporate carotenoid pigments into their beak to attract mates. We tested the hypotheses that increased dietary carotenoid intake enhances immunocompetence in male zebra finches and that levels of carotenoids circulating in blood, which also determine beak coloration, directly predict the immune response of individuals. We experimentally supplemented captive finches with two common dietary carotenoid pigments (lutein and zeaxanthin) and measured cell-mediated and humoral immunity a month later. Supplemented males showed elevated blood-carotenoid levels, brighter beak coloration, and increased cell-mediated and humoral immune responses than did controls. Cell-mediated responses were predicted directly by changes in beak color and plasma carotenoid concentration of individual birds. These experimental findings suggest that carotenoid-based color signals in birds may directly signal male health via the immunostimulatory action of ingested and circulated carotenoid pigments.     

UV plumage color is an honest signal of quality in male budgerigars

Elaborate and colorful feathers are important traits in female mate choice in birds. Plumage coloration can result from pigments deposited in feathers such as carotenoids and melanins, or can be caused by nano-scale reflective tissues (structurally based coloration), usually producing ultraviolet (UV) coloration. Structural colorations remain the least studied of the three most important feather colorations. Previous studies have found a female preference for UV color in the budgerigar, Melopsittacus undulatus, but it is not clear what information this ornament conveys, nor what is the possible cost associated with its production. We investigated possible correlations between immune response and plumage color of wild-type (green) male budgerigars. In particular we measured the wing web swelling resulting from injection of phytohaemagglutinin (PHA). We did not detect any correlation between the sedimentation rate and morphological and color variables. We found that UV chroma is the best predictor for the cutaneous immune activity. Indeed, male budgerigars with high UV reflectance in the breast feathers showed stronger immune responses. These results are consistent with the idea that UV colors are special signals conveying information about a bird’s condition.     

A test of an antipredatory function of conspicuous plastron coloration in hatchling turtles

Bright colorations in animals are sometimes an antipredatory signal meant to startle, warn, or deter a predator from consuming a prey organism. Freshwater turtle hatchlings of many species have bright ventral coloration with high internal contrast that may have an antipredator function. We used visual modeling and field experiments to test whether the plastron coloration of Chrysemys picta hatchlings deters predators. We found that bird predators can easily distinguish hatchling turtles from their backgrounds and can easily see color contrast within the plastron. Raccoons cannot easily discriminate within-plastron color contrast but can see hatchlings against common backgrounds. Despite this, we found that brightly-colored, high contrast, replica turtles were not attacked less than low contrast replica turtles, suggesting that the bright coloration is not likely to serve an antipredatory function in this context. We discuss the apparent lack of innate avoidance of orange coloration in freshwater turtles by predators and suggest that preference and avoidance of colors are context-dependent. Since the bright colors are likely not a signal, we hypothesize that the colors may be caused by pigments deposited in tissue from maternal reserves during development. In most species, these pigments fade ontogenetically but they may have important physiological functions in species that maintain the bright coloration throughout adulthood.     

The influence of carotenoid acquisition and utilization on the maintenance of species-typical plumage pigmentation in male American goldfinches (Carduelis tristis) and northern cardinals (Cardinalis cardinalis).

Birds display a tremendous variety of carotenoid-based colors in their plumage, but the mechanisms underlying interspecific variability in carotenoid pigmentation remain poorly understood. Because vertebrates cannot synthesize carotenoids de novo, access to pigments in the diet is one proximate factor that may shape species differences in carotenoid-based plumage coloration. However, some birds metabolize ingested carotenoids and deposit pigments that differ in color from their dietary precursors, indicating that metabolic capabilities may also contribute to the diversity of plumage colors we see in nature. In this study, we investigated how the acquisition and utilization of carotenoids influence the maintenance of species-typical plumage pigmentation in male American goldfinches (Carduelis tristis) and northern cardinals (Cardinalis cardinalis). We supplemented the diet of captive goldfinches with red carotenoids to determine whether males, which are typically yellow in color, were capable of growing red plumage. We also deprived cardinals of red dietary pigments to determine whether they could manufacture red carotenoids from yellow precursors to grow species-typical red plumage. We found that American goldfinches were able to deposit novel pigments in their plumage and develop a striking orange appearance. Thus, dietary access to pigments plays a role in determining the degree to which goldfinches express carotenoid-based plumage coloration. We also found that northern cardinals grew pale red feathers in the absence of red dietary pigments, indicating that their ability to metabolize yellow carotenoids in the diet contributes to the bright red plumage that they display.     

Interspecific variation in dietary carotenoid assimilation in birds: links to phylogeny and color ornamentation

Many birds use carotenoid pigments to acquire rich red, orange, and yellow coloration in feathers and bare parts that is used as a signal of mate quality. Because carotenoids are derived from foods, much attention has been paid to the role of diet in generating color variation both within and among avian species. Less consideration has been given to physiological underpinnings of color variability, especially among species. Here, I surveyed published literature (e.g. captive feeding studies) on carotenoid assimilation in six bird species and completed additional controlled carotenoid-supplementation experiments in two others to consider the ability of different taxa to extract carotenoids from the diet in relation to phylogeny and coloration. I found that, for a given level of carotenoids in the diet, passerine birds (zebra finch, Taeniopygia guttata; house finch, Carpodacus mexicanus; American goldfinch, Carduelis tristis; society finch, Lonchura domestica) exhibit higher levels of carotenoids in circulation than non-passerines like gamebirds (domestic chicken, Gallus domesticus; red junglefowl, Gallus gallus; Japanese quail, Coturnix coturnix; red-legged partridge, Alectoris rufa). This difference in carotenoid accumulation is likely due to interspecific variation in micelle, chylomicron, or lipoprotein concentrations or affinities for xanthophyll carotenoids. Passerine birds more commonly develop carotenoid-based colors than do birds from ancient avian lineages such as Galliformes, and the physiological differences I uncover may explain why songbirds especially capitalize on carotenoid pigments for color production. Ultimately, because we can deconstruct color traits into component biochemical, physical, and physiological parts, avian color signals may serve as a valuable model for illuminating the proximate mechanisms behind interspecific variation in signal use in animals.     

Differential accumulation and pigmenting ability of dietary carotenoids in colorful finches

Many animals develop bright red, orange, or yellow carotenoid pigmentation that they use to attract mates. Colorful carotenoid pigments are acquired from the diet and are either directly incorporated as integumentary colorants or metabolized into other forms before deposition. Because animals often obtain several different carotenoids from plant and animal food sources, it is possible that these pigments are accumulated at different levels in the body and may play unique roles in shaping the ultimate color expression of individuals. We studied patterns of carotenoid accumulation and integumentary pigmentation in two colorful finch species--the American goldfinch (Carduelis tristis) and the zebra finch (Taeniopygia guttata). Both species acquire two main hydroxycarotenoids, lutein and zeaxanthin, from their seed diet but transform these into a series of metabolites that are used as colorful pigments in the plumage (goldfinches only) and beak (both species). We conducted a series of carotenoid-supplementation experiments to investigate the relative extent to which lutein and zeaxanthin are accumulated in blood and increase carotenoid coloration in feathers and bare parts. First, we supplemented the diets of both species with either lutein or zeaxanthin and measured plasma pigment status, feather carotenoid concentration (goldfinches only), and integumentary color. Zeaxanthin-supplemented males grew more colorful feathers and beaks than lutein-supplemented males, and in goldfinches incorporated a different ratio of carotenoids in feathers (favoring the accumulation of canary xanthophyll B). We also fed goldfinches different concentrations of a standard lutein-zeaxanthin mix and found that at physiologically normal and high concentrations, birds circulated proportionally more zeaxanthin over lutein than occurred in the diet. Collectively, these results demonstrate that zeaxanthin is preferentially accumulated in the body and serves as a more potent substrate for pigmentation than lutein in these finches.     

Melanin-based plumage coloration in the house finch is unaffected by coccidial infection

For most species of birds, ornamental plumage coloration may result from two types of pigments: carotenoids and melanins. Despite the fact that melanin pigments can be synthesized by birds from basic, amino acid precursors, while carotenoids cannot be synthesized by birds and must be ingested, melanin-based plumage coloration and carotenoid-based plumage coloration have often been treated as a single trait in investigations of the function and evolution of plumage coloration. Expression of carotenoid-based coloration is known to be dependent on condition, while the effects of individual condition have not been well-tested for expression of melanin-based coloration. In this study, we experimentally tested the effect of coccidial infection of the intestinal tract of male house finches during moult on expression of melanin-based plumage coloration. Coccidial infection had a significant negative effect on carotenoid-based coloration, but it had no significant effect on melanin-based feather coloration. Unlike carotenoid-based coloration, melanin-based coloration may be cheap to produce, and honesty of melanin-based coloration my require social mediation.     

The evolution of carotenoid coloration in estrildid finches: a biochemical analysis

The estrildid finches (Aves: Passeriformes: Estrildidae) of Africa, Asia, and Australia have been the focus of several recent tests of sexual selection theory. Many estrildids display bright red, orange, or yellow colors in the beak or plumage, which typically are generated by the presence of carotenoid pigments. In this study, we used high-performance liquid chromatography to investigate the carotenoid content of feathers and other colorful tissues in seven species of estrildids. Star finches (Neochmia ruficauda) and diamond firetails (Stagonopleura guttata) circulated two main dietary carotenoids (lutein and zeaxanthin) through the blood and liver and used both to acquire a yellow plumage color. However, five other estrildids (common waxbill, Estrilda astrild; black-rumped waxbill, Estrilda troglodytes; zebra waxbill, Amandava subflava; red avadavat, Amandava amandava; and zebra finch, Taeniopygia guttata) circulated these same dietary carotenoids along with two metabolites (dehydrolutein and anhydrolutein) through the blood and/or liver and used all four as yellow plumage colorants. We subsequently tracked the distribution of these pigments using a published phylogeny of estrildid finches to determine the evolutionary pattern of carotenoid metabolism in these birds. We found that finches from the most ancient tribe of estrildids (Estrildini) possessed the ability to metabolize dietary carotenoids. Although carotenoids from the most ancestral extant estrildid species have yet to be analyzed, we hypothesize (based on their relationships with other songbirds known to have such metabolic capabilities) that these finches inherited from their ancestors the capability to metabolize carotenoids. Interestingly, later in estrildid evolution, certain taxa lost the ability to metabolize dietary carotenoids (e.g., in the Poephilini), suggesting that the occurrence of carotenoid metabolism can be labile and is likely shaped by the relative costs and benefits of color signaling across different species.     

Evolution of plumage coloration in the crow family (Corvidae) with a focus on the color-producing microstructures in the feathers: a comparison of eight species

Plumage coloration has been the subject for a variety of questions that comprise the center of modern evolutionary biology. Unlike carotenoids that the concentration directly influences the intensity of the color, melanin, in addition to produce brown or black colors, is often involved in producing the structural coloration such as glossiness or iridescence. As the melanin granules can be located in the barbs or the barbules, we aim to (i) discern if the colors observed at macro scale comes from the barbs, the barbules or both in a series of related species and (ii) estimate the evolutionary history of the color-producing mechanisms in the family Corvidae that are known to have melanin-based coloration. From a preliminary comparative analysis on eight representative species, we found three coloration schemes in Corvidae; (1) matte colors of brown or black that were produced in barbs and barbules; (2) non-iridescent structural colors such as blue, bluish gray and white, that were produced in the barbs and (3) iridescent structural colors that were produced only in distal barbules. Comparative character analysis of these coloration schemes suggests that the ancestral state among these species were the colors produced in the barbs and that the color produced in the distal barbules is a derived character. The evolution of iridescence seems tightly linked to the evolution of the colors produced in the distal barbules. Data from more species should be incorporated in order to grasp a full picture on the evolutionary history of plumage coloration in this group of birds.     

Pteridine, not carotenoid, pigments underlie the female-specific orange ornament of striped plateau lizards (Sceloporus virgatus)

Indicator models of sexual selection suggest that signal honesty is maintained via costs of ornament expression. Carotenoid-based visual signals are a well-studied example, as carotenoids may be environmentally limited and impact signaler health. However, not all bright yellow, orange and red ornaments found in vertebrates are carotenoid-based; pteridine pigments may also produce these colors. We examine the contribution of carotenoid and pteridine pigments to the orange reproductive color of female striped plateau lizards (Sceloporus virgatus). This color ornament reliably indicates female mate quality, yet costs maintaining signal honesty are currently unknown. Dietary carotenoid manipulations did not affect orange color, and orange skin differed from surrounding white skin in drosopterin, not carotenoid, content. Further, orange color positively correlated with drosopterin, not carotenoid, concentration. Drosopterin-based female ornaments avoid the direct trade-offs of using carotenoids for ornament production vs egg production, thus may relax counter-selection against color ornament exaggeration in females. Direct experimentation is needed to determine the actual costs of pteridine-based ornaments. Like carotenoids, pteridines influence important biological processes, including immune and antioxidant function; predation and social costs may also be relevant.     

The color of greater flamingo feathers fades when no cosmetics are applied

Greater flamingos use cosmetic coloration by spreading uropygial secretions pigmented with carotenoids over their feathers, which makes the plumage redder. Because flamingos inhabit open environments that receive direct solar radiation during daytime, and carotenoids bleach when exposed to solar radiation, we expected that the plumage color would fade if there is no maintenance for cosmetic purposes. Here, we show that the concentrations of pigments inside feathers and on the surface of feathers were correlated, as well as that there was a correlation between the concentrations of pigments in the uropygial secretions and on the surface of feathers. There was fading in color (becoming less red) in feathers that received direct solar radiation when there was no plumage maintenance, but not so in others maintained in darkness. When we controlled for the initial color of feathers, the feathers of those individuals with higher concentration of pigments on the feather surfaces were those that lost less coloration after experimental exposure of feathers to sunny conditions. These results indicate that exposure to sunlight is correlated with the fading of feather color, which suggests that individuals need to regularly apply makeup to be more colorful. These results also reinforce the view that these birds use cosmetic coloration as a signal amplifier of plumage color. This may be important in species using highly variable habitats, such as wetlands, since the conditions experienced when molting may differ from those when the signal should be functional, usually months after molting.     

Nature’s Palette: Characterization of Shared Pigments in Colorful Avian and Mollusk Shells

Pigment-based coloration is a common trait found in a variety of organisms across the tree of life. For example, calcareous avian eggs are natural structures that vary greatly in color, yet just a handful of tetrapyrrole pigment compounds are responsible for generating this myriad of colors. To fully understand the diversity and constraints shaping nature’s palette, it is imperative to characterize the similarities and differences in the types of compounds involved in color production across diverse lineages. Pigment composition was investigated in eggshells of eleven paleognath bird taxa, covering several extinct and extant lineages, and shells of four extant species of mollusks. Birds and mollusks are two distantly related, calcareous shell-building groups, thus characterization of pigments in their calcareous structures would provide insights to whether similar compounds are found in different phyla (Chordata and Mollusca). An ethylenediaminetetraacetic acid (EDTA) extraction protocol was used to analyze the presence and concentration of biliverdin and protoporphyrin, two known and ubiquitous tetrapyrrole avian eggshell pigments, in all avian and molluscan samples. Biliverdin was solely detected in birds, including the colorful eggshells of four tinamou species. In contrast, protoporphyrin was detected in both the eggshells of several avian species and in the shells of all mollusks. These findings support previous hypotheses about the ubiquitous deposition of tetrapyrroles in the eggshells of various bird lineages and provide evidence for its presence also across distantly related animal taxa.     

Ornamental non-carotenoid red feathers of wild burrowing parrots

Bird plumage colors have the potential to indicate individual quality, condition, health, immunocompetence, or the extend of parental care. Color intensity of feathers has been found to correlate with parameters of individual quality, condition, parental care and breeding success. Psittaciformes are well known for their colorful plumage but the significance of parrot coloration is still poorly understood. Red colors are very common in many parrot species. They are produced by at least four non-carotenoid-based pigments (linear polyenal structure). In the present study, we investigated a collection of red abdominal feathers of a marked population of wild Burrowing Parrots Cyanoliseus patagonus in Patagonia, Argentina. The aims of this study were to investigate the ecological significance of the recently described non-carotenoid-based red pigments of Psittaciformes, and the relationships between objectively assessed plumage color and body size, body condition, breeding success and nestling growth in wild Psittaciformes. We found that sexes differed in plumage coloration (sexual dichromatism), that plumage color was a good predictor of female body condition and male size, and we identified the red coloration of the abdominal patch as a signal of individual quality and parental investment.     

Effect of Dietary Carotenoid Supplementation on Food Intake and Immune Function in a Songbird with no Carotenoid Coloration

Studies of ornamental carotenoid coloration suggest that animals may have evolved specialized mechanisms for maximizing color expression and advertising their potential worth as a mate. For example, when given a choice of foods, many carotenoid‐pigmented fishes and birds select the more colorful, presumably carotenoid‐rich foods, and then accumulate these pigments at high levels in both the integument and systemically, in order to boost their immune system and hence directly advertise their health state with their colors. The majority of animals, however, do not exhibit sexually selected carotenoid coloration, which raises the question of whether they still optimize pigment intake and allocation in ways that boost endogenous accumulation and health. We tested the effect of carotenoid supplementation on food intake, carotenoid accumulation in blood, and both innate and adaptive immunity in male society finches (Lonchura domestica) – a non‐carotenoid‐colored estrildid finch relative of the zebra finch (Taeniopygia guttata; a species in which males do display sexually attractive and health‐revealing carotenoid color). Males fed a carotenoid‐rich diet for 2 wk did not consume more food than control males. Still, consumption of the carotenoid‐rich diet for 2 wk significantly elevated circulating levels of carotenoids in blood in male society finches, yielding the potential for immune enhancement. In fact, carotenoid‐enriched finches performed significantly better than control birds in our assay of constitutive innate immunity (bacterial‐killing activity of whole blood), although not in our test of inducible adaptive immunity (response to a mitogenic challenge with phytohemagglutinin). These results suggest that affinities for carotenoid‐rich foods may be particular to species with sexually selected carotenoid pigmentation, but that, as in humans and other mammals (e.g. mice, rats) without carotenoid color, the immune‐boosting action of carotenoids is conserved regardless of the strength of sexual selection on pigment use.