Sex and testosterone effects on growth, immunity and melanin coloration of nestling Eurasian kestrels
Article first published online: 11 DEC 2006
Journal of Animal Ecology
Volume 76, Issue 1, pages 201–209, January 2007
How to Cite
FARGALLO, J. A., MARTÍNEZ-PADILLA, J., TOLEDANO-DÍAZ, A., SANTIAGO-MORENO, J. and DÁVILA, J. A. (2007), Sex and testosterone effects on growth, immunity and melanin coloration of nestling Eurasian kestrels. Journal of Animal Ecology, 76: 201–209. doi: 10.1111/j.1365-2656.2006.01193.x
- Issue published online: 11 DEC 2006
- Article first published online: 11 DEC 2006
- Received 31 May 2006; accepted 20 October 2006
- sex allocation
- 1Sex differences in testosterone levels and sex-biased sensitivity to testosterone are the basis of some ideas postulated to account for sex-linked environmental vulnerability during early life. However, sex variation in circulating testosterone levels has been scarcely explored and never manipulated at post-natal stages of birds in the wild.
- 2We measured and experimentally increased circulating testosterone levels in nestling Eurasian kestrels Falco tinnunculus. We investigated, possible sexual differences in testosterone levels and the effect of this hormone on growth (body mass and tarsus length) and cell-mediated immunity in males and females. We also explored testosterone effects on rump coloration, a highly variable melanin-based trait in male nestlings. We analysed data on circulating testosterone levels of nestlings in 15 additional bird species.
- 3Increased levels of testosterone tended to negatively affect body condition, reduced cell-mediated immune responses in male and female nestlings and also diminished the expression of grey rump coloration in male nestlings. No sex differences were observed in testosterone levels in either control or increased testosterone group nestlings, and no interactions were found between sex and treatment. However, male nestlings showed a lower cell-mediated immune response than females in both groups.
- 4Our results indicate first, that a high level of testosterone in all nestlings in a brood entails costs, at least in terms of immunity, coloration and probably growth. Secondly, sex differences in post-natal cell-mediated immunity, and consequently in the capacity to prevent diseases, cannot be explained by sex differences in circulating testosterone levels. Finally, by comparing published data at an interspecific level, contradictory sex patterns in circulating testosterone levels have been found, supporting the idea that circulating testosterone might not be a proximate factor causing sex-dependent vulnerability in bird species.