Melanin-based plumage ornaments as sexual and social signals: functions and evolution
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In most animal signalling systems, reliability of the signals is maintained by their costs payed by the signaller. Plumage colours have long been studied in the contexts of sexual and social signalling, yet our understanding of their honesty-maintaining mechanisms is incomplete. Melanin-based coloration had been hypothesized to be cheap to produce, thereby questioning its potential to reliably signal individual quality in sexual or social competition. In a series of inter- and intraspecific studies I investigated whether melanin-based coloration is related to sexual and social selection, and I tested whether melanin signals may be reliable due to the costs of increased predation risk or to the regulatory effects of elevated testosterone levels. First, using phylogenetic comparative methods I showed that interspecific variation in the extent of melanin-based black coloration is related to sexual display behaviour in plovers and allies (Charadriida) and to reproductive investment in cardueline finches (Carduelinae), as predicted by sexual selection theory. Second, using two passerine birds as model species, I demonstrated that melanin ornaments may function in both sexual and social signalling. In penduline tits (Remiz pendulinus) the size of the black eye-stripe of males predicts their success in mating but not in male-male competition, suggesting that females prefer more melanized males. In house sparrows (Passer domesticus) both the size of the black throat patch and the conspicuousness of the white wingbar predict the males’ success in social competition, suggesting that these ornaments act as multiple cues in status signalling. Finally, I studied two possible sources of reliability of melanin ornaments. Using the house sparrow, I experimentally tested whether individual variation in throat patch size and wingbar area and conspicuousness predicts the predator-related risk taken by males and females, and found no support that predation constrains melanization in this species. Using the comparative approach I found that in a wide range of bird species, the extent of black plumage is related to the circulating levels of testosterone in both males and females, supporting that testosterone may regulate the link between melanization and competitiveness. In sum, my research has provided both inter- and intraspecific evidence that melanin-based ornaments may function in sexual selection and status signalling, and may honestly signal competitive ability through a physiological link. Further studies are important to ascertain the costs of producing and maintaining melanin ornaments, with specific respect to the mechanisms of testosterone-regulation.