Within-male melanin-based plumage and bill elaboration in male house sparrows

If there is a cost to producing a dark color patch, the size of a patch may not correspond with its pigment concentration. The plumage of male house sparrows represents a case of dark, melanin-based ornamentation, but also a case of neglecting the composite nature of dark signals in birds. Here, I investigated what kind of associations exist between the brightness, chroma, and hue of dark integumentary patches and the size of a secondary sexual trait, the bib, in male house sparrows. I found that males with a larger bib also had a darker bib and bill, and a more saturated bib, bill, epaulets, head crown, and breast than small-bibbed males. Male bib coloration in terms of brightness and chroma was more strongly related to bib size than the coloration of other integumentary patches. However, with respect to hue, only the hue of the bill and cheeks was related to bib size. My results indicate that size, brightness, and chroma of the bib, but also chroma of other deeply colored patches, convey redundant information about the signaler's quality in male house sparrows.

Steroid correlates of multiple color traits in the spiny lizard, Sceloporus pyrocephalus

Conspicuous coloration in females is less well studied compared to that in males. Adult female Mexican boulder spiny lizards (Sceloporus pyrocephalus) have conspicuously colored throat, or gular, regions, ranging from red to yellow, while adult males only weakly express such color in their gular region. Both sexes have dark blue-black gular stripes and venter stripes. Understanding proximate mechanisms underlying trait expression can aid in understanding trait function. To characterize the proximate mechanisms potentially influencing color variation among field-captured male and female S. pyrocephalus, we quantified three aspects of color (hue, saturation, brightness) for three body locations (gular region, gular stripes, venter stripes) and then assessed how color was related to reproductive state and concentrations of the plasma steroid hormones testosterone (T) and corticosterone (CORT) in males and T, CORT, and 17-beta estradiol (E(2)) in females. Testes volume was not related to variation in color or in hormones, perhaps because most males were in peak reproductive condition. Large vitellogenic follicles as opposed to oviductal eggs were associated with higher E(2) in females. Males with more dull gular stripes and females with dull venter stripes had significantly higher CORT. Females with red gular regions and pale grey gular stripes had higher T and E(2) concentrations compared to females with a more yellow gular region and darker gular stripes. Thus, gular region color in females could communicate reproductive state; dull gular stripes in males and dull venter stripes in females could communicate stress status.

Feathers at nests are potential female signals in the spotless starling

Although the presence of feathers in the nest is widespread among birds, it has not been previously suggested that feathers can be used as sexual signals. Females of the spotless starling (Sturnus unicolor) regularly carry feathers to their nest, mostly during laying and incubation. We show that the arrangement of these feathers was non-random with respect to the side (obverse or reverse) placed upwards (which can be viewed from the nest entrance). Feathers of the wood pigeon (Columba palumbus) and the spotless starling, which exhibit higher ultraviolet and visible reflectance on their reverse side, were predominantly placed with this side upwards. On the contrary, feathers of the jay (Garrulus glandarius) were predominantly found exhibiting the obverse side, which possesses higher reflectance in this species. Feathers of the azure-winged magpie (Cyanopica cyana), with similar reflectance values on either side, were placed indiscriminately in obverse and reverse positions. The results suggest that feathers are arranged to maximize their conspicuousness within the nest and hence that they might be potentially used as intraspecific signals.

A link between eumelanism and calcium physiology in the barn owl

In many animals, melanin-based coloration is strongly heritable and is largely insensitive to the environment and body condition. According to the handicap principle, such a trait may not reveal individual quality because the production of different melanin-based colorations often entails similar costs. However, a recent study showed that the production of eumelanin pigments requires relatively large amounts of calcium, potentially implying that melanin-based coloration is associated with physiological processes requiring calcium. If this is the case, eumelanism may be traded-off against other metabolic processes that require the same elements. We used a correlative approach to examine, for the first time, this proposition in the barn owl, a species in which individuals vary in the amount, size, and blackness of eumelanic spots. For this purpose, we measured calcium concentration in the left humerus of 85 dead owls. Results showed that the humeri of heavily spotted individuals had a higher concentration of calcium. This suggests either that plumage spottiness signals the ability to absorb calcium from the diet for both eumelanin production and storage in bones, or that lightly spotted individuals use more calcium for metabolic processes at the expense of calcium storage in bones. Our study supports the idea that eumelanin-based coloration is associated with a number of physiological processes requiring calcium.

Nest ornamentation by female spotless starlings in response to a male display: an experimental study

1. The use of behavioural traits by females in signalling condition has been practically ignored in evolutionary theory. However, females may also exhibit ornaments and behavioural displays, although less elaborated than those of males. 2. In this study we suggest that the carrying of feathers by spotless starlings Sturnus unicolor Temminck females to decorate the nest represents an elaborated and costly behaviour that is displayed in response to a courtship male behaviour: the carrying of nest green plants. 3. By experimentally increasing the amount of green plants in the nests, to give the appearance that highly attractive males defended them, we induced females to increase their feather carrying rates. 4. The amount of feathers carried to the nest was correlated to female reproductive experience and laying date, two variables correlated with female body condition. These results suggests that this behaviour may work as an honest indicator of female quality. 5. We conclude that male carrying plants and female carrying feathers can be viewed as two sex-specific functionally related signalling behaviours involved in mutual courtship or status signalling.

Juvenile immune status affects the expression of a sexually selected trait in field crickets

Parasite-mediated sexual selection theory presumes that variation in sexual traits reliably reflects variation in parasite resistance among available mates. One mechanism that may warrant signal honesty involves costs of immune system activation in the case of a parasitic infection. We investigated this hypothesis in male field crickets Gryllus campestris, whose attractiveness to females depends on characteristics of the sound-producing harp that are essentially fixed following adult eclosion. During the nymphal stage, males subjected to one of two feeding regimes were challenged with bacterial lipopolysaccharides (LPS) to investigate condition-dependent effects on harp development as compared to other adult traits. Nymphal nutritional status positively affected adult body size, condition, and harp size. However, nymphal immune status affected harp size only, with LPS-males having smaller harps than control-injected males. In addition, the harps of LPS-males showed a lesser degree of melanization, indicating an enhanced substrate use by the melanin-producing enzyme cascade of the immune system. Thus, past immune status is specifically mirrored in sexual traits, suggesting a key role for deployment costs of immunity in parasite-mediated sexual selection.

The evolution, maintenance and adaptive function of genetic colour polymorphism in birds

The hypothesis that ornaments can honestly signal quality only if their expression is condition-dependent has dominated the study of the evolution and function of colour traits. Much less interest has been devoted to the adaptive function of colour traits for which the expression is not, or is to a low extent, sensitive to body condition and the environment in which individuals live. The aim of the present paper is to review the current theoretical and empirical knowledge of the evolution, maintenance and adaptive function of colour plumage traits for which the expression is mainly under genetic control. The finding that in many bird species the inheritance of colour morphs follows the laws of Mendel indicates that genetic colour polymorphism is frequent. Polymorphism may have evolved or be maintained because each colour morph facilitates the exploitation of alternative ecological niches as suggested by the observation that individuals are not randomly distributed among habitats with respect to coloration. Consistent with the hypothesis that different colour morphs are linked to alternative strategies is the finding that in a majority of species polymorphism is associated with reproductive parameters, and behavioural, life-history and physiological traits. Experimental studies showed that such covariations can have a genetic basis. These observations suggest that colour polymorphism has an adaptive function. Aviary and field experiments demonstrated that colour polymorphism is used as a criterion in mate-choice decisions and dominance interactions confirming the claim that conspecifics assess each other's colour morphs. The factors favouring the evolution and maintenance of genetic variation in coloration are reviewed, but empirical data are virtually lacking to assess their importance. Although current theory predicts that only condition-dependent traits can signal quality, the present review shows that genetically inherited morphs can reveal the same qualities. The study of genetic colour polymorphism will provide important and original insights on the adaptive function of conspicuous traits.

Proximate basis of the covariation between a melanin-based female ornament and offspring quality

In contradiction to sexual selection theory, several studies showed that although the expression of melanin-based ornaments is usually under strong genetic control and weakly sensitive to the environment and body condition, they can signal individual quality. Covariation between a melanin-based ornament and phenotypic quality may result from pleiotropic effects of genes involved in the production of melanin pigments. Two categories of genes responsible for variation in melanin production may be relevant, namely those that trigger melanin production (yes or no response) and those that determine the amount of pigments produced. To investigate which of these two hypotheses is the most likely, I reanalysed data collected from barn owls ( Tyto alba). The underparts of this bird vary from immaculate to heavily marked with black spots of varying size. Published cross-fostering experiments have shown that the proportion of the plumage surface covered with black spots, a eumelanin composite trait so-called "plumage spottiness", in females positively covaries with offspring humoral immunocompetence, and negatively with offspring parasite resistance (i.show $132#e. the ability to reduce fecundity of ectoparasites) and fluctuating asymmetry of wing feathers. However, it is unclear which component of plumage spottiness causes these relationships, namely genes responsible for variation in number of spots or in spot diameter. Number of spots reflects variation in the expression of genes triggering the switch from no eumelanin production to production, whereas spot diameter reflects variation in the expression of genes determining the amount of eumelanin produced per spot. In the present study, multiple regression analyses, performed on the same data sets, showed that humoral immunocompetence, parasite resistance and wing fluctuating asymmetry of cross-fostered offspring covary with spot diameter measured in their genetic mother, but not with number of spots. This suggests that genes responsible for variation in the quantity of eumelanin produced per spot are responsible for covariation between a melanin ornament and individual attributes. In contrast, genes responsible for variation in number of black spots may not play a significant role. Covariation between a eumelanin female trait and offspring quality may therefore be due to an indirect effect of melanin production.

Genetic and environmental components of variation in eumelanin and phaeomelanin sex-traits in the barn owl

Knowledge of the mechanism underlying the expression of melanin-based sex-traits may help us to understand their signalling function. Potential sources of inter-individual variation are the total amount of melanins produced but also how biochemical precursors are allocated into the eumelanin and phaeomelanin pigments responsible for black and reddish-brown colours, respectively. In the barn owl (Tyto alba), a eumelanin trait (referred to as 'plumage spottiness') signals immunocompetence towards an artificially administrated antigen and parasite resistance in females, whereas a phaeomelanin trait ('plumage coloration') signals investment in reproduction in males. This raises the question whether plumage coloration and spottiness are expressed independent of each other. To investigate this question, we have studied the genetics of these two plumage traits. Crossfostering experiments showed that, for each trait, phenotypic variation has a strong genetic component, whereas no environmental component could be detected. Plumage coloration is autosomally inherited, as suggested by the similar paternal-to-maternal contribution to offspring coloration. In contrast, plumage spottiness may be sex-linked inherited (in birds, females are heterogametic). That proposition arises from the observation that sons resembled their mother more than their father and that daughters resembled only their father. Despite plumage coloration and spottiness signalling different qualities, these two traits are not inherited independent of each other, darker birds being spottier. This suggests that the extent to which coloration and spottiness are expressed depends on the total amount of melanin produced (with more melanin leading to a both darker and spottier plumage) rather than on differential allocation of melanin into plumage coloration and spottiness (in such a case, darker birds should have been less spotted). A gene controlling the production of melanin pigments may be located on sex-chromosomes, since the phenotypic correlation between coloration and spottiness was stronger in males than in females.

A female melanin ornament signals offspring fluctuating asymmetry in the barn owl

Sexual selection theory predicts that males advertise quality by displaying extravagant ornaments. By contrast, whether phenotypic variation in females has a signalling function remains an open question. Here, to our knowledge, we provide the first evidence that a female plumage trait can signal fluctuating asymmetry in the offspring. We experimentally demonstrate in wild barn owls (Tyto alba) that the extent to which females display black spots on their plumage does not only signal offspring parasite resistance as shown in a previous study but also developmental homeostasis in the offspring. A greater number of spotted females produced offspring that had more symmetrical feathers during the period of growth. Males, that pair non-randomly with respect to female plumage spottiness therefore appear to gain substantial benefits by mating with heavily spotted females. Genetic variation in plumage spottiness is nevertheless maintained as the covariation between offspring body mass and mother plumage spottiness varies annually depending on environmental conditions.