American Society of Naturalists

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“Sex-dependent expression and fitness consequences of sunlight-derived color phenotypes”

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Juan A. Fargallo, Félix Martínez, Kazumasa Wakamatsu, David Serrano, and Guillermo Blanco (June 2018)

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Color variation in birds related to sunlight-temperature conditions as a potential capacity for acclimatization

Vultures become paler in nests more exposed to sunlight

Adult griffon vultures <i>Gyps fulvus</i>. <br />(Credit: Alberto Carreño)
Adult griffon vultures Gyps fulvus.
(Credit: Alberto Carreño)

There is no clear evidence of whether endothermic animals respond to environmental variations in solar radiation or temperature through changes in external coloration. A new study by researchers from the National Museum of Natural Sciences–CSIC in Madrid, Spain, in collaboration with researchers from the Doñana Biological Station–CSIC, Spain, and Fujita Health University, Japan, shows that griffon vultures Gyps fulvus that have grown up in nests more exposed to sunlight produce paler plumages. The researchers also find that females show paler plumages than males and that lighter males show higher mortality during the first years of life. The interest of the study lies in three main points: (i) the possibility that endothermic animals, such as birds, might respond to variations in sunlight-temperature by changing coloration, (ii) the potential costs of trait plasticity, with males being more sensitive to them, and (iii) the differences in survival associated with coloration, which may be an ecological mechanism promoting sexual dimorphism. The study was conducted in the Hoces del Riaza Natural Park, Segovia, Spain, from 1996 to 2015.

Fledgling griffon vultures <i>Gyps fulvus</i> in nests with different sunlight exposure. (Credit: Guillermo Blanco)
Fledgling griffon vultures Gyps fulvus in nests with different sunlight exposure. (Credit: Guillermo Blanco)


Hoces del Riaza Natural Park. <br />(Credit: Juan A. Fargallo)
Hoces del Riaza Natural Park.
(Credit: Juan A. Fargallo)

To understand whether early phenotypes are adaptive, knowledge of the environmental factors involved in their variation and the derived benefits from their expression is needed. Temperature and sunlight are considered two major selective forces influencing phenotypic coloration of birds at a global scale. However, within-population color adaptations in response to sunlight-temperature variation have been scarcely investigated and their acclimatization capacity is currently unknown. In addition, the sexes differ in their sensitivity to environmental factors during growth. This study evaluates how melanin plumage coloration varies in relation to sex and sunlight exposure in developing griffon vultures Gyps fulvus, and how this correlates with survival. The results show that individuals grown in nests more exposed to sunlight developed paler plumages as predicted by the thermal melanism hypothesis. In addition, paler males, but not females, have lower survival probability. Finally, a significant sexual dichromatism in fledgling plumage was observed, with females being paler than males. These results reveal within-population color variation related to sunlight-temperature conditions, probably as a capacity for local acclimatization through color plasticity, and also provide evidence of sex differences in fitness optima for this trait under ecological pressures.