Sexual Dimorphism in Melanin Pigmentation, Feather Coloration and Its Heritability in the Barn Swallow (Hirundo rustica)

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.

Geographic variation in sexual selection and implications for speciation in the Barn Swallow

“…Interestingly, although birds with darker ventral color have a weaker immune response, individuals with darker throat color have higher survival in a Danish population (Galván and Møller 2013), similar to Japanese H. r. gutturalis (Hasegawa et al 2013). Melanic throat color is heritable in Italian populations (Saino et al 2013c), suggesting that different color patches could advertise different information.…”

A novel locus on chromosome 1 underlies the evolution of a melanic plumage polymorphism in a wild songbird

“…We used microanalytical methods to quantify the eumelanin and pheomelanin content of the feathers from brown and grey morphs (see [49] for a detailed description of the method). Feather samples ( ca 7 mg) from the back of 12 brown and 12 grey individuals were homogenized with a Ten-Broeck homogenizer at a concentration of 10 mg ml −1 and 100 µl aliquots were subjected to Soluene-350 solubilization [50], alkaline hydrogen peroxide oxidation [51] and hydriodic acid hydrolysis [52].…”

Development, Regeneration, and Evolution of Feathers

“…The results showed that the differences in plumage and skin between both sexes remained the same after skin graft exchange or hormone alterations; i.e., the resulting plumage was based on the donor, not the recipient, indicating that a genetic factor may also play a role in sexual dichromatism (106, 107). For example, melanin-based coloration in the barn swallow ( Hirundo rustica rustica ) was found to differ between genders, plumage regions, and even geographical locations (108). These findings implicate pigmentation patterning as an important and complex issue that requires further investigation.…”