Abstract
Melanin is the main pigment in animal coloration and considerable variation in the concentrations of the two melanin forms (pheo- and eumlanin) in pigmented tissues exists among populations and individuals. Melanin-based coloration is receiving increasing attention particularly in socio-sexual communication contexts because the melanocortin system has been hypothesized to provide a mechanistic basis for covariation between coloration and fitness traits. However, with few notable exceptions, little detailed information is available on inter-individual and inter-population variation in melanin pigmentation and on its environmental, genetic and ontogenetic components. Here, we investigate melanin-based coloration in an Italian population of a passerine bird, the barn swallow (Hirundo rustica rustica), its sex- and age-related variation, and heritability. The concentrations of eu- and pheomelanin in the throat (brown) and belly (white-to-brownish) feathers differed between sexes but not according to age. The relative concentration of either melanin (Pheo:Eu) differed between sexes in throat but not in belly feathers, and the concentrations in males compared to females were larger in belly than in throat feathers. There were weak correlations between the concentrations of melanins within as well as among plumage regions. Coloration of belly feathers was predicted by the concentration of both melanins whereas coloration of throat feathers was only predicted by pheomelanin in females. In addition, Pheo:Eu predicted coloration of throat feathers in females and that of belly feathers in males. Finally, we found high heritability of color of throat feathers. Melanization was found to differ from that recorded in Hirundo rustica rustica from Scotland or from H. r. erythrogaster from North America. Hence, present results show that pigmentation strategies vary in a complex manner according to sex and plumage region, and also among geographical populations, potentially reflecting adaptation to different natural and sexual selection regimes, and that some coloration components seem to be highly heritable.
Highlights
Organisms vary in coloration of their body surfaces that are exposed to the external environment
The concentration of both eu- and pheomelanins significantly differed between males and females and this was the case at both plumage regions (Table 1)
Linear models with sex and plumage region as main effects showed that sex differences in the concentrations of both melanin forms varied between plumage regions
Summary
Organisms vary in coloration of their body surfaces that are exposed to the external environment. Due to the diverse and potentially contrasting natural and sexual selection pressures that act on coloration, the functional analysis of such variation occurring both at the intra- and inter-specific level is complex [1,2,3,4,5]. Coloration can affect body thermal balance via differential reflection of solar radiation and is crucial to crypsis and mimicry of both prey and predators [6,7,8,9]. The analysis of color variation should rely on the dissection of the mechanisms and constraints that cause it, and on the costs-to-benefits balance of ex novo production or allocation of dietary pigments to the production of traits under natural and sexual selection. Conditiondependent expression of melanin- or carotenoid-based coloration may have to be balanced against costs that may be paid in diverse currencies ([11,12,13,14,15], but see [16,17]), and it has been proposed that such trade-offs underpin the reliability of coloration in signaling individual quality or alternative life-history strategies under frequency-dependent or balancing selection
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