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Eat yourself sexy: how selective macronutrient intake influences the expression of a visual signal in common mynas
Abstract
Producing colored signals often requires consuming dietary carotenoid pigments. Evidence that food deprivation can reduce coloration, however, raises the question of whether other dietary nutrients contribute to signal coloration, and furthermore, whether individuals can voluntarily select food combinations to achieve optimal coloration. We created a 2-way factorial design to manipulate macronutrient and carotenoid access in common mynas ( Acridotheres tristis ) and measured eye patch coloration as a function of the food combinations individuals selected. Mynas had access to either water or carotenoid-supplemented water and could eat either a standard captive diet or choose freely between three nutritionally defined pellets (protein, lipid, carbohydrate). Mynas supplemented with both carotenoids and macronutrient pellets had higher color scores than control birds. Male coloration tended to respond more to nutritional manipulation than females, with color scores improving in macronutrient- and carotenoid-supplemented individuals compared to controls. All mynas consuming carotenoids had higher levels of plasma carotenoids, but only males showed a significant increase by the end of the experiment. Dietary carotenoids and macronutrient intake consumed in combination tended to increase plasma carotenoid concentrations the most. These results demonstrate for the first time that consuming specific combinations of macronutrients along with carotenoids contribute to optimizing a colorful signal and point to sex-specific nutritional strategies. Our findings improve our knowledge of how diet choices affect signal expression and, by extension, how nutritionally impoverished diets, such as those consumed by birds in cities, might affect sexual selection processes and ultimately population dynamics.
... To date, field-based research has shown that protein is a limiting factor for this species in Australian cities [72,73]. In addition, experimental nutrition choice studies have confirmed its ability to select novel foods based on protein content and to select nutrient blends that maximize the coloration of a sexual signal [66,58]. This has led to the hypothesis that nutrient deprivation might be driving the species' range expansion from highly urbanized eastern seaboard to more rural townships in the west [73] . ...
Humans are translocating species beyond their native ranges increasingly fast. These translocations create a natural experiment to explore the role of cognition in invasiveness. Alien vertebrate species face many behavioral challenges upon introduction to novel environments. But here, we focus on how alien species might use cognition to find and adopt new foods. Cognitive processes are particularly well suited to this challenge, a prediction supported by large-scale comparative analyses of alien species’ historical introductions. Here, we parse the steps involved in approaching, handling, tasting, and evaluating novel food sources and, for each one, describe which cognitive abilities are the most relevant. In bringing attention to the functional importance of innovative feeding both conceptually and empirically, synthesizing the cognitive processes involved, highlighting the current void of knowledge, and arguing that alien species are particularly well suited to controlled experimental cognitive studies, this piece scaffolds future experimental cognitive research.
... Indeed, mynas displayed a more saturated, redder eye patch with a low UV component in areas with low build-up cover. Peneaux et al. (2020Peneaux et al. ( , 2021b showed that eye patch coloration of mynas is formed by a combination of carotenoid pigments and microstructures and that coloration is severely reduced in response to dietary deficiencies in macronutrients and carotenoids, creating a possible link between poor nutrition and changes in coloration found here. This conclusion is consistent with previous work attributing decreased carotenoid-based coloration in urban birds to decreased quality, rather than quantity, of urban food sources (Hõrak et al., 2000;Isaksson, 2009;Isaksson and Andersson, 2007; for a review see Peneaux et al., 2021a). ...
Theory suggests that overcrowding and increased competition in urban environments might be detrimental to individual condition in avian populations. Unfavourable conditions could be compounded by changes in dietary niche with additional consequences for individual quality of urban birds. We analysed the isotopic signatures, signal coloration, body condition, parasitic loads (feather mites and coccidia), and immune responsiveness of 191 adult common (Indian) mynas (Acridotheres tristis) captured in 19 localities with differing levels of urbanization. The isotopic signature of myna feathers differed across low and high urbanized habitats, with a reduced isotopic niche breadth found in highly urbanized birds. This suggests that birds in high urban environments may occupy a smaller foraging niche to the one of less urbanized birds. In addition, higher degrees of urbanization were associated with a decrease in carotenoid-based coloration, higher ectoparasite loads and higher immune responsiveness. This pattern of results suggests that the health status of mynas from more urbanized environments was poorer than mynas from less modified habitats. Our findings are consistent with the theory that large proportions of individual birds that would otherwise die under natural conditions survive due to prevailing top-down and bottom-up ecological processes in cities. Detrimental urban ecological conditions and search for more favourable, less crowded habitats offers the first reasonable explanation for why an ecological invader like the common myna continues to spread within its global invasive range.
Conspicuous coloured displays from ultraviolet to bright red have been documented in many species throughout the animal kingdom. These colours often occur as sexual signals and can be incorporated into different types of integuments (e.g. scales, feathers, skin). Two main mechanisms are known to produce coloured integuments: pigmentation and tissue structure. Although pigmental and structural coloration are separate mechanisms and can occur independently, some coloured displays might emerge from a combination of both. Here, we demonstrate, using biochemical, optical and morphological methodologies, that the yellow coloration of the skin located around the eye of Common (Indian) Mynas ( Acridotheres tristis) is produced by both light-reflecting nanostructures and light-absorbing carotenoid pigments. Our analysis confirms that nanostructured collagen in the avian dermis work in combination with carotenoid pigments to produce vivid integumentary colours. Identifying the mechanisms behind the production of a coloured signal provides a basis for predicting how a signal’s function might be influenced by environmental factors such as fledgling nutrition.
Ornaments can evolve to reveal individual quality when their production/maintenance costs make them reliable as “signals” or if their expression level is intrinsically linked to condition by some unfalsifiable mechanism (“indices”). The latter has been mostly associated with traits constrained by body size. In red ketocarotenoid-based colourations, that link could, instead, be established with cell respiration at the inner mitochondrial membrane (IMM). The production mechanism could be independent of resource (yellow carotenoids) availability, thus discarding costs linked to allocation trade-offs. A gene coding for a ketolase enzyme (CYP2J19) responsible for converting dietary yellow carotenoids to red ketocarotenoids has recently been described. We treated male zebra finches with an antioxidant designed to penetrate the IMM (mitoTEMPO) and a thyroid hormone (triiodothyronine) with known hypermetabolic effects. Among hormone controls, MitoTEMPO downregulated CYP2J19 in the bill (a red ketocarotenoid-based ornament), supporting the mitochondrial involvement in ketolase function. Both treatments interacted when increasing hormone dosage, indicating that mitochondria and thyroid metabolisms could simultaneously regulate colouration. Moreover, CYP2J19 expression was positively correlated to redness but also to yellow carotenoid levels in the blood. However, treatment effects were not annulated when controlling for blood carotenoid variability, which suggests that costs linked to resource availability could be minor.
The mechanisms involved in the production of red carotenoid‐based ornaments of vertebrates are still poorly understood. These colorations often depend on enzymatic transformations (ketolation) of dietary yellow carotenoids, which could occur in the inner mitochondrial membrane (IMM). Thus, carotenoid ketolation and cell respiration could share biochemical pathways, favoring the evolution of ketocarotenoid‐based ornaments as reliable indices of individual quality under sexual selection. Captive male red crossbills (Loxia curvirostra Linnaeus) were exposed to redox‐active compounds designed to penetrate and act in the IMM: an ubiquinone (mitoQ) or a superoxide dismutase mimetic (mitoTEMPO). MitoQ can act as an antioxidant but also distort the IMM structure, increasing mitochondrial free radical production. MitoQ decreased yellow carotenoids and tocopherol levels in blood, perhaps by being consumed as antioxidants. Contrarily, mitoTEMPO‐treated birds rose circulating levels of the second most abundant ketocarotenoid in crossbills (i.e. canthaxanthin). It also increased feather total red ketocarotenoid concentration and redness, but only among those birds exhibiting a redder plumage at the start of the study, that is, supposedly high‐quality individuals. The fact that mitoTEMPO effects depended on original plumage color suggests that the red‐ketocarotenoid‐based ornaments indicate individual quality as mitochondrial function efficiency. The findings would thus support the shared pathway hypothesis.
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In the past thirty years, carotenoid‐based animal signals have been an intense focus of research because they can potentially broadcast an honest reflection of individual reproductive potential. Our understanding of the underpinning physiological functions of carotenoid compounds is still emerging, however. Here, we argue that wildlife researchers and managers interested in assessing the impact of environmental quality on animal populations should be taking advantage of the signalling function of carotenoid‐based morphological traits. Using birds as a model taxonomic group, we build our argument by first reviewing the strong evidence that the expression of avian carotenoid displays provides an integrated measure of a multitude of diet‐ and health‐related parameters. We then present evidence that human‐induced rapid environmental change (hirec) impacts the expression of carotenoid signals across different critical periods of a bird’s lifetime.
Carotenoid coloration is widely recognized as a signal of individual condition in various animals, but despite decades of study, the mechanisms that link carotenoid coloration to condition remain unresolved. Most birds with red feathers convert yellow dietary carotenoids to red carotenoids in an oxidation process requiring the gene encoding the putative cytochrome P450 enzyme CYP2J19. Here, we tested the hypothesis that the process of carotenoid oxidation and feather pigmentation is functionally linked to mitochondrial performance. Consistent with this hypothesis, we observed high levels of red ketolated carotenoids associated with the hepatic mitochondria of moulting wild house finches (Haemorhous mexicanus), and upon fractionation, we found the highest concentration of ketolated carotenoids in the inner mitochondrial membrane. We further found that the redness of growing feathers was positively related to the performance of liver mitochondria. Structural modelling of CYP2J19 supports a direct role of this protein in carotenoid ketolation that may be functionally linked to cellular respiration. These observations suggest that feather coloration serves as a signal of core functionality through inexorable links to cellular respiration in the mitochondria.
There is growing interest in the question of how urbanization affects the ecology of birds, across timescales from relatively short‐term physiological responses to long‐term evolutionary adaptation. The ability to gain the required nutrients in urban habitats is a key trait of successful urban birds. Foraging behavior, in itself, increasingly is recognized as a complex nutritional phenomenon, where the ratios, proportions, and amounts of macronutrients (protein, carbohydrate, and lipid) in foods, meals, and diets have been shown to exert a driving influence. Yet, despite the rising trend of urbanization, the importance of food quality and quantity in urban ecology, and the growing evidence demonstrating the pervasive and sometimes complex role of macronutrients in foraging behavior, the nutritional ecology of urban birds remains poorly understood. Here, we review the foraging behavior and role of macronutrients in the ecology of urban birds and demonstrate how incorporating a multidimensional approach to nutrition can provide new insights into their urban ecology. To that end, we demonstrate how a macronutrient‐based view can aid in understanding the relationships between natural, anthropogenic, and supplementary foods. We then provide an overview of multidimensional nutritional niche concepts that can be used to generate explanatory and predictive models for urban bird ecology. We conclude that multidimensional nutritional ecology provides an appropriate framework for understanding the roles that nutrition plays in the relationships between urban birds and their environments.
Thirty years of research has made carotenoid coloration a textbook example of an honest signal of individual quality, but tests of this idea are surprisingly inconsistent. Here, to investigate sources of this heterogeneity, we perform meta-analyses of published studies on the relationship between carotenoid-based feather coloration and measures of individual quality. To create color displays, animals use either carotenoids unchanged from dietary components or carotenoids that they biochemically convert before deposition. We hypothesize that converted carotenoids better reflect individual quality because of the physiological links between cellular function and carotenoid metabolism. We show that feather coloration is an honest signal of some, but not all, measures of quality. Where these relationships exist, we show that converted, but not dietary, carotenoid coloration drives the relationship. Our results have broad implications for understanding the evolutionary role of carotenoid coloration and the physiological mechanisms that maintain signal honesty of animal ornamental traits.
Invasion success is dependent on the ability of a species to discover and exploit novel food resources. Within this context, individuals must be willing to taste novel foods. They must also be capable of evaluating the nutritional content of new foods, and selecting their relative intake in order to fulfil their nutritional needs. Whereas the former capacity is well studied, little is known about the latter capacity. First, using the common myna as a model avian invader species, we quantified the willingness of mynas to taste novel foods relative to familiar ones. Mynas readily tasted high protein (HP) novel foods and consumed them in higher quantities compared to a familiar food. Data showed that at three different levels – mixes, ingredients and macronutrients – intake could not be explained by a random model. In experiment 2, we confirmed that mynas were making their selection based on protein (P) content rather than a selection for novelty per se. When given the choice of three equally unfamiliar foods, mynas again ate disproportionately from the high protein relative to high lipid and high carbohydrate foods. Analysis revealed that mynas consumed amounts of protein that were closer to the ones in their natural diet. Finally, in experiment 3, we measured inter-individual variation in innovation and exploration propensities, and examined associations with inter-individual variation in consumption of specific macronutrients. This analysis revealed that individuals that selected HP pellets were more exploratory and individuals that selected HC pellets were quicker to solve the innovative foraging task. These findings indicate that not only the willingness to taste novel foods, but also the capacity to evaluate their nutritional content, might be central to the myna's substantial ecological success.
Significance
The yellow, orange, and red colors of birds are produced through the deposition of carotenoid pigments into feathers and skin, and often function as signals in aggressive interactions and mate choice. These colors are hypothesized to communicate information about individual quality because their expression is linked to vital cellular processes through the mechanisms of carotenoid metabolism. To elucidate these mechanisms, we carried out genomic and biochemical analyses of the white recessive canary breed, which carries a heritable defect in carotenoid uptake. We identified a mutation in the SCARB1 gene in this breed that disrupts carotenoid transport function. Our study implicates SCARB1 as a key mediator of carotenoid-based coloration and suggests a link between carotenoid coloration and lipid metabolism.
Colorful ornaments have been the focus of sexual selection studies since the work of Darwin. Yellow to red coloration is often produced by carotenoid pigments. Different hypotheses have been formulated to explain the evolution of these traits as signals of individual quality. Many of these hypotheses involve the existence of a signal production cost. The carotenoids necessary for signaling can only be obtained from food. In this line, carotenoid-based signals could reveal an individual's capacity to find sufficient dietary pigments. However, the ingested carotenoids are often yellow and became transformed by the organism to produce pigments of more intense color (red ketocarotenoids). Biotransformation should involve oxidation reactions, although the exact mechanism is poorly known. We tested the hypothesis that carotenoid biotransformation could be costly because a certain level of oxidative stress is required to correctly perform the conversion. The carotenoid-based signals could thus reveal the efficiency of the owner in successfully managing this challenge. In a bird with ketocarotenoid-based ornaments (the red-legged partridge; Alectoris rufa), the availability of different carotenoids in the diet (i.e. astaxanthin, zeaxanthin and lutein) and oxidative stress were manipulated. The carotenoid composition was analyzed and quantified in the ornaments, blood, liver and fat. A number of oxidative stress biomarkers were also measured in the same tissues. First, we found that color and pigment levels in the ornaments depended on food levels of those carotenoids used as substrates in biotransformation. Second, we found that birds exposed to mild levels of a free radical generator (diquat) developed redder bills and deposited higher amounts of ketocarotenoids (astaxanthin) in ornaments. Moreover, the same diquat-exposed birds also showed a weaker resistance to hemolysis when their erythrocytes were exposed to free radicals, with females also enduring higher oxidative damage in plasma lipids. Thus, higher color production would be linked to higher oxidative stress, supporting the biotransformation hypothesis. The recent discovery of an avian oxygenase enzyme involved in converting yellow to red carotenoids may support our results. Nonetheless, the effect could also depend on the abundance of specific substrate carotenoids in the diet. Birds fed with proportionally higher levels of zeaxanthin showed the reddest ornaments with the highest astaxanthin concentrations. Moreover, these birds tended to show the strongest diquat-mediated effect. Therefore, in the evolution of carotenoid-based sexual signals, a biotransformation cost derived from maintaining a well-adjusted redox machinery could coexist with a cost linked to carotenoid acquisition and allocation (i.e. a resource allocation trade-off).
The foraging challenge for predators is to find and capture food with adequate levels of energy and nutrients. Marine predators require particularly sophisticated foraging strategies that enable them to balance self- and offspring-feeding, and also in many circumstances simultaneously consider the nutritional constraints of their partners. Here we combined the use of dietary analysis, proximate composition and nutritional geometry (right-angled mixture triangle nutritional models) to examine the macronutrient preferences of Australasian gannets (Morus serrator) at Farewell Spit gannetry in New Zealand. Our results showed intra- and inter-specific variation in the protein, lipid and water composition of prey captured by our sample of 111 Australasian gannets. In addition, we observed significant differences in the Australasian gannets’ nutritional niche between seasons. We provide evidence of sex-specific macronutrient foraging strategies in a successful marine predator in the wild. We have shown that in spite of fluctuations in the nutritional composition of foods available to Australasian gannets, males consistently capture prey with higher protein-to-lipid ratios and lower lipid-to-water ratios than females. These results aid to better understand the evolutionary relationship between macronutrient selection and sex-specific traits in wild animals. They also suggest an incentive for these predators to combine individually imbalanced but nutritionally complementary foods to achieve dietary balance, further highlighting the likelihood that prey selection is guided by the balance of macronutrients, rather than energy alone.
Carotenoid coloration is the one of the most frequently studied ornamental traits in animals. Many studies of carotenoid coloration test the associations between carotenoid coloration and measures of performance, such as immunocompetence and oxidative state, proceeding from the premise that carotenoids are limited resources. Such studies commonly involve supplementing the diets of captive birds with carotenoids. In many cases, however, the amount of carotenoid administered is poorly justified, and even supposedly carotenoid-limited diets may saturate birds' systems. To quantify the relationships among the amount of carotenoids administered in experiments, levels of circulating carotenoids, and quantities of carotenoids deposited into colored ornaments, we performed a meta-analysis of 15 published studies that supplemented carotenoids to one of seven songbird species. We used allometric scaling equations to estimate the per-gram carotenoid consumption of each study's subjects, and we used meta-regression to evaluate the effects of this carotenoid dose on differences in coloration and plasma carotenoid levels between supplemented and control groups of birds. After accounting for supplementation duration and species, we observed a significant positive correlation between carotenoid intake and response of plasma carotenoid level to supplementation. The presence of supplemental carotenoids also tended to increase the expression of ornamental coloration, but the magnitude of the carotenoid dose did not significantly affect how strongly coloration changed with supplementation. Further, coloration effect sizes had no significant relationship with plasma carotenoid effect sizes. We also found significant heterogeneity in responses among studies and species, and the parameters used to measure color significantly affected response to supplementation. Our results emphasize the importance of performing dosage trials to determine what supplementation levels provide limited versus surplus carotenoids and of measuring the natural level of carotenoid intake by the study species to validate the appropriateness of supplementation levels for a particular study species and experimental design.
Supplementation of a basal casein-dextrose diet (40 μg added lutein/g) with 2% coconut or olive oil increased serum lutein
significantly (P<.05) when fed to broiler chicks depleted of their oxycarotenoid stores. The influence of chain length, saturation,
and concentration of dietary lipids on the absorption of lutein was investigated.
Coconut oil, which contains primarily short chain, saturated fatty acids, gave higher (P<.05) serum lutein values than olive
oil, which contains primarily long chain, unsaturated fatty acids. Diets supplemented with 5% capric, lauric, myristic, palmitic,
stearic, oleic, or linoleic acids or tristearin, triolein, or trilaurin could be divided into three groups based on the ability
of the lipids to promote lutein absorption into serum when they were fed. The long chain, saturated fatty acids (myristic,
palmitic, and stearic) and triglyceride, tristearin, promoted minimal absorption, whereas the short chain, saturated lauric
acid promoted the highest absorption. The short chain, saturated triglycerides and fatty acids and long chain, unsaturated
fatty acids and triglycerides promoted intermediate absorption of lutein. Lutein absorption from a white corn-soybean-starch
diet depended on concentration of dietary lipid (0, 2, 4, 6, 8, and 10%) when cottonseed oil was substituted isocalorically
for starch. Serum and toeweb lutein increased linearly up to four-fold and three-fold, respectively, as the dietary fat increased
up to 6%, where a break-point occurred. Additional fat caused smaller increases of lutein. Graded levels of dietary lauric
or oleic acid gave similar results, whereas stearic acid (up to 10%) did not support a significant (P<.05) absorption. The
absorption of lutein promoted by mixtures of oleic and stearic acids correlated positively with the amount of oleic acid but
not stearic acid. The results can be explained on the basis of the micellar theory of digestion and on the basis of oxycarotenoids
being nonpolar, nonamphiphilic lipids.
Anthropogenic environments can offer rich sources of energy to urban wildlife, but little is known about how they impact on nutritional balance and food selection. Common mynas (Sturnus tristis) provide a powerful model system for testing the nutritional constraints and priorities of an invasive species that has successfully adapted to urban environments. Here, we use behavioral observations, field-based feeding trials, videography, and the right-angled mixture triangle model (RMT) to examine the macronutrient preferences of these invasive birds. Our behavioral observations showed that mynas consumed insects (40.6%), worms (33.2%), human discards (17.6%), and plants (8.6%). Our feeding trials using nutritionally defined foods showed that mynas had a clear preference for food dishes containing only high-protein (HP) pellets over high-lipid (HL) or high-carbohydrate (HC) pellets. In addition, mixed feeders were also presented in 2 combinations: 1) contained equal proportions of HP and HC pellets and 2) equal proportions of HP and HL pellets. HP pellets were selectively consumed from both mixed feeders, this involving an increase in feeding time. Overall, the RMT showed that mynas consumed a higher proportion of protein from the feeders than in their natural diet. Furthermore, the majority of our observations of birds feeding at the dishes containing HP foods ended in intraspecific aggression, suggesting that protein is a contestable
resource. These results suggest that mynas at our urban study site are deficient in protein relative to fats and carbohydrates.
Vertebrate nutritional ecology is the study of the interrelationships between food resources in the environment and the consumptive use of these food resources by vertebrate animals. Consumptive use includes how vertebrate animals procure, digest, absorb and metabolise available foods in the environment to satisfy their requirements for health, growth and reproduction. The challenge for wild vertebrates is to satisfy their nutritional requirements in an environment that often varies in food availability and quality. Vertebrate animals have evolved a diverse and interesting suite of solutions to the problem(s) of acquiring necessary nutrients and energy in a world where food often does not want to be eaten.
Carotenoid-based ornaments may have evolved as a consequence of their costs of production, which would assure the reliability of the traits as signals of individual quality. Different costs due to carotenoid allocation to the signal have been proposed, considering the scarcity of these pigments at the environment (ecological cost) and their physiological properties that would trade against the maintenance of the organism. Carotenoids of many red ornaments (ketocarotenoids) are often the result of biotransformation of those pigments abundant in the diet (usually lutein and zeaxanthin). Some authors have suggested that such a conversion implies a cost relevant for signaling because it requires high levels of antioxidant vitamins in the tissues where biotransformation takes place. We explore this hypothesis in red-legged partridges (Alectoris rufa) by analyzing ketocarotenoids in the ornaments (bare parts) and carotenoids, vitamin A in different forms (free and esterified) and vitamin E in blood, liver and fat. Ketocarotenoids in ornaments (astaxanthin and papilioerythrinone) were not found in internal tissues, suggesting that they were directly transformed in the bare parts. However, ketocarotenoid levels where positively correlated with the levels of their precursors (zeaxanthin and lutein, respectively) in internal tissues. Interestingly, ketocarotenoid levels in bare parts negatively and positively correlated with vitamin A and E in the liver, respectively, the same links only being positive in blood. Moreover, retinyl and zeaxanthin levels in liver were negatively related. We hypothesize that storing substrate carotenoids in the main storage site (the liver) implies a cost in terms of regulating the level of vitamin A.
Knowledge of the genetic basis of sexual ornaments is essential to understand their evolution through sexual selection. Although carotenoid-based ornaments have been instrumental in the study of sexual selection, given the inability of animals to synthesize carotenoids de novo, they are generally assumed to be influenced solely by environmental variation. However, very few studies have directly estimated the role of genes and the environment in shaping variation in carotenoid-based traits. Using long-term individual-based data, we here explore the evolutionary potential of a dynamic, carotenoid-based ornament (namely skin colouration), in male and female common kestrels. We first estimate the amount of genetic variation underlying variation in hue, chroma and brightness. After correcting for sex differences, the chroma of the orange-yellow eye ring colouration was significantly heritable (h2 ± se = 0.40 ± 0.17), whereas neither hue (h2=0) nor brightness (h2=0.02) was heritable. Second, we estimate the strength and shape of selection acting upon chromatic (hue and chroma) and achromatic (brightness) variation, and show positive and negative directional selection on female but not male chroma and hue, respectively, whereas brightness was unrelated to fitness in both sexes. This suggests that different components of carotenoid-based signals traits may show different evolutionary dynamics. Overall, we show that carotenoid-based coloration is a complex and multifaceted trait. If we are to gain a better understanding of the processes responsible for the generation and maintenance of variation in carotenoid-based coloration, these complexities need to be taken into account.This article is protected by copyright. All rights reserved.
Dietary access to carotenoids is expected to determine the strength of carotenoid-based signal expression and potentially
to maintain signal honesty. Species that display carotenoid-based yellow, orange, or red plumage are therefore expected to
forage selectively for carotenoid-rich foods when they are depositing these pigments during molt, but whether they actually
do so is unknown. We set out to address this in the hihi (Notiomystis cincta), a New Zealand passerine where males, but not females, display yellow carotenoid-based plumage. We measured circulating
carotenoid concentrations in male and female hihi during breeding and molt, determined the nutritional content of common foods
in the hihi diet, and conducted feeding observations of male and female hihi during molt. We found that although male and
female hihi do not differ significantly in plasma carotenoid concentration, male hihi have a greater proportion of carotenoid-rich
foods in their diet than do females. This is a consequence of a greater fruit and lower invertebrate intake than females and
an avoidance of low-carotenoid content fruit. By combining behavioral observations with quantification of circulating carotenoids,
we present evidence that colorful birds forage to maximize carotenoid intake, a conclusion we would not have drawn had we
examined plasma carotenoids alone.
Carotenoid pigments are involved in different physiological processes (e.g., immunoenhancement, antioxidant activity) in addition to coloring plumage and integuments. As animals cannot synthesize these pigments de novo, it has been proposed that carotenoids constitute a limiting resource that birds may specifically seek in their food. Confirming this hypothesis, it was recently found that birds can discriminate between carotenoid-enriched diets and control diets, even if both have the same color, suggesting that there may be underlying non-visual (e.g., olfactory, taste) mechanisms for detecting carotenoid presence or enrichment in foods. In this study, we performed two experiments with male House Finches (Carpodacus mexicanus) to test if this species is able to discriminate between (1) carotenoid-enriched and plain sunflower seeds (while controlling for food coloration), and (2) plain seeds scented with β-ionone, which is a carotenoid-degradation product that is common in many fruits and is one of the most powerful flavor-active organic compounds, or a sham odorant. We found that finches did not show significant food preferences in either experiment, indicating that they did not use odor or flavor cues associated with carotenoids to discriminate between foods. However, our results do not rule out the possibilities that other flavors or odors can be used in discrimination or that finches may learn to discriminate flavors and odors over longer periods of time or at other times of year through post-ingestive feedback mechanisms.
Despite the importance of proximate mechanisms for un- derstanding costs and functions of signals, relatively little is known about physiological processes underlying colorful plumage production. We investigated yellow carotenoid-based breast plumage and ultraviolet (UV)/blue structural color of tail feathers in nestling blue tits. At peak plumage production and at fledging, we examined plasma concentra- tion of protein, the substance of feathers, and levels of carotenoids, which are deposited in yellow plumage and also required for support of growth and immune development. Males showed more UV-shifted hue and higher chroma and UV chroma of the tail feathers. In males only, tail chroma and UV chroma were strongly but negatively related to circulating plasma protein at the period of peak feather production. Breast plumage was more chromatic in males, and yellow coloration was strongly condition dependent in males only. Production of more chromatic plumage appeared to deplete carotenoids from plasma, since male fledglings had less circulating carotenoids than females. This had no obvious consequences for these males, since coloration was unre- lated to cell-mediated immune responses, humoral immune status, stress, and parasitism. Nonetheless, the pronounced sex specificity of condition dependence of structural and carotenoid-based coloration is suggestive of a signaling function of the fledgling plumage.
In the wild, male House Finches (Carpodacus mexicanus) vary in plumage color from pale yellow to bright red. I investigated the proximate basis of this variation in plumage brightness, as well as the basis for variation in the extent of ventral carotenoid pigmentation. Regardless of their age or coloration in the wild, captive males converged on a similar appearance after completing prebasic molt on a standardized diet, with significantly less variance in coloration than is found among wild males. Captive males that were fed a diet deficient in carotenoid pigments grew pale yellow feathers; males fed a diet supplemented with β-carotene grew pale orange feathers; and males fed a diet supplemented with canthaxanthin grew bright red feathers. Stored carotenoids did not appear to be an important source of feather pigments. Red males captured from the wild just prior to fall molt and fed a carotenoid-deficient diet did not grow more colorful feathers than males that had been held in captivity on a carotenoid-deficient diet for six to nine months prior to fall molt. In a wild House Finch population in southeastern Michigan, the mean plumage coloration of yearling males was significantly drabber than the mean coloration of older males, although both groups displayed approximately the same range of coloration. Wild males tended to become brighter between their first and second springs, but thereafter, males tended to display a similar plumage coloration between years. The extent of ventral carotenoid pigmentation (color-patch size) also was partly dependent on carotenoid intake. Captive males whose diet was supplemented with canthaxanthin produced significantly larger patches after captive molt than before captive molt, and canthaxanthin-supplemented males also expressed significantly larger patches than males in the carotenoid-deficient or β-carotene-supplemented groups. Among wild males, there was a significant positive correlation between patch size and plumage brightness.
Carotenoids are interesting in avian ecology due their dietary origin and the resulting environmental dependence of certain functions and traits such as plumage pigmentation and the immune system. Carotenoids are frequently assumed to be limited for birds, but data on availability is rarely collected. Thus, to understand the role of the carotenoids as mediators of trade-offs, additional information on the whole chemical pathway of carotenoids is needed. In conjunction with previous studies on carotenoid ecology and physiology of the Great Tit Parus major living in Swedish urban vs. rural environments, I here present data on 1) carotenoid concentration in two primary carotenoid producers, birch Betula verrucosa and oak Quercus robur and 2) vegetation cover on Great Tit territories. Previous studies found lower carotenoid concentrations in caterpillars and plumage pigmentation in urban compared to rural areas. Correspondingly, urban trees had significantly lower total carotenoid concentration in comparison to rural trees, but there was no environmental difference in the relative amount of lutein and zeaxanthin (Lut:Zx ratio) in sun-exposed leaves. Second, urban Great Tits had significantly more shade in their territories than rural birds, which may affect the leaves Lut:Zx composition. The results, taken together, show that the environmental difference in carotenoid concentration of caterpillars and Great Tit yellow breast coloration, may occur because urban trees synthesise less or have used up more carotenoids in response to environmental stress compared to rural trees. Thus, tracking carotenoid content in different steps of the food chain raises questions about how fine-scale environmental heterogeneity can affect optimal foraging behaviour and individual variation in carotenoid-dependent traits.
Some of the most spectacular visual signals found in the animal kingdom are based on dietarily derived carotenoid pigments (which cannot be produced de novo), with a general assumption that carotenoids are limited resources for wild organisms, causing trade-offs in allocation of carotenoids to different physiological functions and ornamentation. This resource trade-off view has been recently questioned, since the efficiency of carotenoid processing may relax the trade-off between allocation toward condition or ornamentation. This hypothesis has so far received little exploratory support, since studies of digestive efficiency of wild animals are limited due to methodological difficulties. Recently, a method for quantifying the percentage of fat in fecal samples to measure digestive efficiency has been developed in birds. Here, we use this method to test if the intensity of the carotenoid-based coloration predicts digestive efficiency in a wild bird, the house finch (Haemorhous mexicanus). The redness of carotenoid feather coloration (hue) positively predicted digestion efficiency, with redder birds being more efficient at absorbing fats from seeds. We show for the first time in a wild species that digestive efficiency predicts ornamental coloration. Though not conclusive due to the correlative nature of our study, these results strongly suggest that fat extraction might be a crucial but overlooked process behind many ornamental traits.
The carotenoids are not only amongst the most widespread of the naturally occurring groups of pigments, but probably also have the most varied functions; witness their known roles in photokinetic responses of plants, in phototropic responses of fish and as vitamin A precursors in mammals and birds. Pigments with such wide distribution and such diverse functions are obviously of great interest to biological scientists with very different specializa tions, especially as it is unlikely that the study of the functions of carotenoids is anywhere near complete. The primary aim of the present work is to discuss the distribution, bio genesis and function of the carotenoids throughout the plant and animal kingdoms in such a way that, because of, rather than in spite of its bio chemical bias, it will be of value to workers interested in all the biological aspects of these pigments. The biochemical approach is considered the most effective because, generally speaking, most progress in the study of carotenoids in living material has been achieved using biochemical techniques, be they applied by zoologists, botanists, entomologists, microbiologists or other specialists; what is even more important is that a consideration of the present position makes it certain that further fundamental progress will also be made along biochemical lines.
The first edition of this book has established itself as one of the leading references on generalized additive models (GAMs), and the only book on the topic to be introductory in nature with a wealth of practical examples and software implementation. It is self-contained, providing the necessary background in linear models, linear mixed models, and generalized linear models (GLMs), before presenting a balanced treatment of the theory and applications of GAMs and related models. The author bases his approach on a framework of penalized regression splines, and while firmly focused on the practical aspects of GAMs, discussions include fairly full explanations of the theory underlying the methods. Use of R software helps explain the theory and illustrates the practical application of the methodology. Each chapter contains an extensive set of exercises, with solutions in an appendix or in the book’s R data package gamair, to enable use as a course text or for self-study.
I studied female mate preference in relation to male plumage coloration in the House Finch (Carpodacus mexicanus). Males from an introduced eastern U.S. population averaged significantly brighter in plumage coloration, and males from an introduced Hawaiian population significantly drabber, than males from the parent population in California. Males from Guerrero, Mexico displayed a mean coloration similar to that of males from the eastern U.S., but with a much reduced color-patch size. In controlled feeding experiments, I found that male house finches from all populations possess the same potential to display bright or drab plumage and that variation in plumage coloration (but not patch size) reflects the type and quantity of carotenoid pigments ingested by individuals during molt. In the wild, male plumage coloration was correlated with parental investment and viability. In laboratory and field experiments, females from all populations chose the most brightly colored, largest patched males available, regardless of the appearance of males in their population. A phylogenetic analysis indicated that the small patch size displayed by Mexican House Finches is derived from a larger patch size, and indirect evidence suggests that small-patched males may have access to smaller quantities of carotenoids than large-patched males. These results support the hypothesis that plumage coloration evolved via intersexual selection as an indicator of male quality. In addition, the lack of conformity between male appearance and female preference among populations provides evidence against species isolation models and runaway sexual selection models for the evolution of male display traits.
The House Finch is among the most mundane birds, so ubiquitous and familiar across the U.S. and Canada that it does not rate a glance from most bird enthusiasts. But males have carotenoid-based plumage coloration that varies markedly among individuals, making the House Finch a model species for studies of the function and evolution of colorful plumage. In more depth and detail than has been attempted for any species of bird, this book takes a tour of the hows and whys of ornamental plumage coloration. The book begins by reviewing the history of the study of colorful plumage, which began in earnest with the debates of Darwin and Wallace but which was largely forgotten by the middle of the 20th century. Documenting the extensive plumage variation among males both within and between populations of House Finches, the book explores the mechanisms behind plumage variation and looks at the fitness consequences of condition-dependent ornament display for both males and females. The book concludes by examining the processes by which carotenoid-based ornamental coloration may have evolved.
We present a graphical approach, which we believe can help to integrate nutrition into the broader biological sciences, and introduce generality into the applied nutritional sciences. This 'Geometric Framework' takes account of the fact that animals need multiple nutrients in changing amounts and balance, and that nutrients come packaged in foods that are often hard to find, dangerous to subdue and costly to process. We then show how the Geometric Framework has been used to understand the links between nutrition and relevant aspects of the biology of individual animals. These aspects include the physiological mechanisms that direct the nutritional interactions of the animal with its environment, and the fitness consequences of these interactions. Having considered the implications of diet for individuals, we show that these effects can translate into the collective behaviour of groups and societies, and in turn ramify throughout food webs to influence the structure of ecosystems.
To establish a framework for detailed investigations of the roles of protein nutrition and metabolism as proximate factors limiting avian egg production, I analyzed the amino acid compositions of the mixed proteins in the albumen and yolk of eggs of Zebra Finches Taeniopygia guttata and Pigeons Columba livia and compared these data with analyses of eggs from three other avian Orders. I also analyzed the amino acid compositions of muscles and carcasses of a representative passerine, the White-crowned Sparrow Zonotrichia leucophrys gambelii and compared these analyses with analyses available for other species. The amino acid compositions of the mixed proteins of albumen and yolk were very similar, as were the amino acid compositions of eggs or tissues among the species examined. Using the aforementioned measures, I compared the apparent essential amino acid (EAA) requirements for egg production (i.e. the EAA deposited) with (1) the estimated daily EAA requirements for maintenance in nonproductive birds, (2) the amino acid composition of the mixed proteins of muscle (a source of amino acids during fasting or periods of trophic shortages) and (3) the average amino acid concentrations in several food types. For all species examined, during egg production, the daily requirements for Phe + Tyr increased most among the EAA, followed by Lys or Val. The sulfur amino acids (SAA) and Phe + Tyr, or Trp were the first and second limiting amino acids, respectively, for synthesizing egg proteins from amino acids derived from muscle proteins. The SAA and Lys were the most likely to be limiting for synthesizing egg proteins from dietary proteins. Even the most prolific passerines can probably meet their SAA demands for egg production without highly selective foraging, but some selective foraging may be needed. Many ducks, however, may need to forage intensively for SAA-rich foods to avoid severely depleting their body protein reserves and (or) very inefficient use of EAA and energy.
Foraging innovations are increasingly recognized as an important source of phenotypic plasticity, evolutionary change and adaptation to environmental challenges. One line of research has successfully demonstrated that innovation can represent a stable individual trait, but by the same token has shown strong contextual effects on innovation. We examined the effects of social context on innovative foraging behaviour. Across two separate experiments, we measured the individual propensity of Indian mynahs, Acridotheres tristis, to innovate when alone, in pairs, or in groups of five birds. Although innovators remained consistent in their relative innovation performance ranking (high, medium, low), the presence of one or more conspecifics reduced the likelihood of innovating, and increased innovation latencies, significantly relative to when individuals were tested alone. A neophobia test in which latency to forage was compared in both the absence and the presence of a novel object, in each of two social contexts (solitary versus social), showed that the presence of conspecifics caused mynahs to forage significantly faster in a safe situation (object absent) relative to when alone, but to delay foraging in a risky situation (object present). Together, these findings suggest that sociality can have contrasting effects on foraging in safe and risky situations, and, in some species at least, effects of sociality on innovative foraging may hence be more akin to those observed in the presence of risk. Negotiation over engaging with risks inherent to innovative foraging offered the most likely explanation for socially inhibited innovation behaviour, and may act to constrain the diffusion of innovations under some conditions.
Research indicates that invasion is a multi-step process, where each stage is contingent on the stage that precedes it. Numerous hypotheses addressing the factors that influence each stage of the invasion process have been formulated, but how well does this theory match what occurs in the natural world? We created a general conceptual model for the invasion process based on invasion theory. Using a composite 41-year data set, we then reconstructed the invasion sequence of the common myna (Acridotheres tristis) to investigate the similarities between invasion theory and this observed invasion. We observed a lag period before population growth of 2.7 (±0.3) years, a maximum rate of population growth of 24.1 (±6.4) birds per km(2) per year, a lag period before spreading of six years and an average spreading rate of 0.4 km per year. The length and duration of these stages correspond closely with what invasion process theory would anticipate. We suggest that a conceptual model, coupled with basic species, environment and event information, could be a useful tool to enhance the understanding and management of invasions.
Behavioural innovation is believed to be an important way in which animals devise solutions to new problems, yet the factors underpinning individual differences in innovation remain unclear. Here, we asked how motivational states and emotional responses to novelty shape the innovation process with a series of experiments in common mynas, Sturnus tristis. To this aim, we measured experimentally the willingness of adult individuals to eat a new food (consumer innovation) and to develop a new foraging technique (motor innovation), as well as their degree of neophobia, exploration, shyness, motivation and activity levels. Common mynas showed some propensity for consumer and motor innovations, with 55% and 22% of individuals solving the respective tasks. Moreover, individuals that solved the task significantly decreased their latency to solve it subsequently, indicating that learning had occurred. Differences in problem-solving performance were not related to either population or sex, and individuals that solved the consumer task did not solve the motor innovation task faster. The likelihood of solving the motor task increased with propensity of the individual to explore the test apparatus, suggesting that the task was solved by trial and error. Exploration increased with the motivation to feed and decreased with the degree of neophobia. Thus, while differences in innovation propensity between individuals may result from cognitive differences, our results highlight that they may also reflect particular motivational states or emotional responses of individuals to novel situations.
Traditional methods for colour quantification are complicated by the fact that colours change depending on illumination, and that different observers often perceive colours differently. Here we describe a new affordable method, which improves methods relying on human observers, to quantify patterns and colour variations. The procedure combines customized software with the use of digital cameras and commercial photofinishing software. The computer routines correct unavoidable illumination changes during image capturing, making all images comparable. Colours are quantified in a continuous scale of the conventional colour models developed for the human vision system, such as HSB, RGB, CMYK, or Lab, amenable for statistical analyses. We illustrate the use of this technique showing a previously unknown sexual dimorphism in the red-legged partridge, Alectoris rufa, undetectable with the unaided human eye. We also demonstrate that the digital system provides a finer discrimination than human observers for scoring the plumage of partridges belonging to two different subspecies. This method has potential applications in behavioural ecology, physiology, genetics, evolutionary biology, and taxonomy.
In the current resurgence of interest in the biological basis of animal behavior and social organization, the ideas and questions pursued by Charles Darwin remain fresh and insightful. This is especially true of The Descent of Man and Selection in Relation to Sex, Darwin's second most important work. This edition is a facsimile reprint of the first printing of the first edition (1871), not previously available in paperback. The work is divided into two parts. Part One marshals behavioral and morphological evidence to argue that humans evolved from other animals. Darwin shoes that human mental and emotional capacities, far from making human beings unique, are evidence of an animal origin and evolutionary development. Part Two is an extended discussion of the differences between the sexes of many species and how they arose as a result of selection. Here Darwin lays the foundation for much contemporary research by arguing that many characteristics of animals have evolved not in response to the selective pressures exerted by their physical and biological environment, but rather to confer an advantage in sexual competition. These two themes are drawn together in two final chapters on the role of sexual selection in humans. In their Introduction, Professors Bonner and May discuss the place of The Descent in its own time and relation to current work in biology and other disciplines.
Carotenoids are among the most prevalent pigments used in animal signals and are also important for a range of physiological functions. These concomitant roles have made carotenoidbased signals a popular topic in behavioural ecology while also causing confusion and controversy. After a thorough background, we review the many pitfalls, caveats and seemingly contradictory conclusions that can result from not fully appreciating the complex nature of carotenoid function. Current controversies may be resolved through a more careful regard of this complexity, and of the immense taxonomic variability of carotenoid metabolism. Studies investigating the physiological trade-offs between ornamental and physiological uses of carotenoids have yielded inconsistent results. However, in many studies, homeostatic regulation of immune and antioxidant systems may have obscured the effects of carotenoid supplementation. We highlight how carefully designed experiments can overcome such complications. There is also a need to investigate factors other than physiological trade-offs (such as predation risk and social interactions) as these, too, may shape the expression of carotenoidbased signals. Moreover, the processes limiting signal expression individuals are likely different from those operating over evolutionary time-scales. Future research should give greater attention to carotenoid pigmentation outside the area of sexual selection, and to taxa other than fishes and birds.
1. Two feeding experiments were conducted to determine the meta‐bolisable energy (ME) requirement of laying Japanese quail. Birds were fed to provide 5.5 g protein and 167, 209, 251 or 293 kJ ME/d.2. As ME intake increased from 167 to 293 kJ egg production increased.3. Quadratic relationships between ME intake and egg production, ME intake and egg weight, and ME intake and egg mass (g egg/ bird d) were derived and used to determine the energy required for an acceptable production rate.4. To maintain a production of 8.3 g egg/bird d (90 eggs/100 bird d with a mean weight of 9.3 g) required 260 kJ ME/bird d.
After a brief discussion of the essential chemical properties of carotenoids, the identification, occurrence and distribution of the pigments in the gonads and related organs of plants and animals have been reviewed. The information available concerning the metabolism of carotenoids in reproductive tissues and their possible function in reproduction has also been reviewed systematically.In the higher plants, as maturation proceeds the carotenoid content of the tissues decreases, owing probably to the redirection of carotenoid synthesis into the reproductive organs. Further, the synthetic mechanism itself must be altered in some way because new carotenoids, never encountered in leaves, are produced in the reproductive organs. There is no evidence that carotenoids are involved in fertilization. In fact, many pollens contain no carotenoids.Differential distribution of carotenoids in male and female gametes is encountered in some species of both algae and fungi, but whereas nothing is known about carotenoid metabolism and function in relation to reproduction in fungi, it is claimed that appropriate mixtures of stable and labile crocetin dimethyl esters stimulate both male and female gametogenesis in the alga Chlamydomonas.In aquatic animals carotenoids are actively mobilized into the gonads during sexual reproductive activity, and in some cases are rendered water-soluble by attachment to proteins. Carotenoids do not appear to be necessary for normal embryonic development, but one report indicates their possible importance as fertilization hormones. Carotenoids may be present in the gonads to ensure that the larvae are adequately equipped for chromatophore responses. Changes in general coloration associated with sexual activity are also often mediated through carotenoids; in the case of the fish Cyclopterus lumpus, for example, astaxanthin appears in the skin in large amounts during the summer spawning period. Similar changes can be observed in some insects, for instance, the bright yellow colour of sexually mature locusts is due to accumulation of β-carotene in the cuticle.In mammals which store carotenoids, these pigments undergo metabolic changes associated with parturition, but as not all mammals store carotenoids, and since many similar changes occur with vitamin A, it is doubtful whether the carotenoids are of significance per se.Although carotenoids appear to be unnecessary for the normal development of birds in both the pre- and post-embryonic stages, they are actively mobilized into birds' eggs. They may, however, be necessary for optimum conditions of fertilization by increasing the sperm competitive ability of cocks, i.e. when hens are inseminated with an equal mixture of spermatozoa from cocks on an adequate diet and those on a lutein-deficient diet, a larger proportion of offspring are produced from the normal group than from the lutein-deficient group.
Many social-learning opportunities expose animals to the behaviour of conspecifics, but also to causes and consequences of those behaviours. Attending to information over and above social behaviour per se may provide a strategy by which the reliability of social information is ensured. Earlier work in Indian mynahs, Sturnus tristis, has demonstrated that observers become more wary in a location in which they are accustomed to foraging after they have viewed a conspecific undergo a ‘predator’ attack at that site. We determined whether observation of both an alarmed demonstrator and the cause of the conspecific’s alarm (capture by a human) were critical to such observational learning. Experimental observers watched a demonstrator mynah display high levels of alarm in response to pursuit and capture by a human, while control mynahs watched a demonstrator express a similar level of alarm to a threatening nearby human, but visual access to the human by observers was blocked. Analysis of observer behaviour at the feeding site both before and after observational training revealed that experimental observers remained wary at the feeding site after training relative to before, relative to control observers that became far less wary, strongly suggesting that both social and causal information were important for observational learning. This result contributes to the growing body of empirical evidence that use of social learning is modulated by a rich variety of contextual information that may help ensure that its use is adaptive.
Summary 1. Male House Finches (Carpodacus mexicanus) colour their sexually selected plumage with carotenoid pigments, and there has been much interest in the factors that affect their ability to become bright red rather than drab yellow. 2. There is good support for the notions that health, nutritional condition and total carotenoid intake influence colour expression, but there are also suggestions that acquiring particular types of carotenoids from the diet may be important for developing red plumage. 3. We used high-performance liquid chromatography (HPLC) to analyse the types and amounts of endogenous (in plasma and liver) and integumentary (in newly grown feathers) carotenoids in a wild, native population of moulting male and female House Finches from the south-western United States to determine the carotenoid-accumulation strategies for becoming optimally colourful. 4. Four plant carotenoids - lutein, zeaxanthin, β-cryptoxanthin and β-carotene - were detected in plasma and liver. However, as was found previously, 11 carotenoids were observed in colourful plumage, with xanthophylls (e.g. lutein, dehydrolutein) predom- inant in yellow feathers and ketocarotenoids (e.g. adonirubin, 3-hydroxy-echinenone) in red feathers. This indicates endogenous modification of ingested carotenoids. 5. Birds that accumulated more of one type of carotenoid in plasma and liver did not necessarily accumulate more of all other types, suggesting that individuals are not employing a simple 'more is better' strategy for coloration. Instead, when forward stepwise regression was used to examine the ability of individual types of carotenoids in plasma and liver to explain variation in red plumage pigments and plumage redness, we found that the lone variable remaining in all models was β-cryptoxanthin concentration. 6. This supports the idea that, unlike some other songbirds (e.g. yellow Carduelis finches), there is a specialized biochemical strategy that male House Finches follow to become red and most sexually attractive - to accumulate as much β-cryptoxanthin in the body as possible. β-Cryptoxanthin is a less common dietary carotenoid than the typical xanthophylls and carotenes in grains and fruits and may be limited enough in the diet that, to become colourful, House Finches might adopt selective foraging strategies for the most β-cryptoxanthin-rich foods.