Hormonal correlates of siblicide in Nazca boobies: support for the Challenge Hypothesis

The androgen hormone testosterone (T) mediates vertebrate aggression in many contexts and according to the Challenge Hypothesis is up-regulated during social challenges. While originally applied to challenges experienced by breeding adults, we show for the first time that T is similarly up-regulated during deadly sibling aggression in young birds. When two nestling Nazca boobies hatch, one--usually the older chick--virtually always kills the other chick by pushing it from the nest. We compared concentrations of T, dehydroepiandrosterone (DHEA; a precursor of T), and corticosterone (Cort; a stress hormone) of chicks at various stages. T was elevated during fights in both chicks in two-chick broods, but not before and after fights, and not in chicks lacking a nest mate. DHEA was elevated 1 day after hatching and declined with age but appeared not to vary in concert with aggression. Cort did not vary across fighting and nonfighting periods. In conjunction with an earlier study [Tarlow, E.M., Wikelski, M., Anderson, D.J., 2001. Hormonal correlates of siblicide in Galapagos Nazca boobies. Horm. Behav. 40:14-20], these results indicate that T is temporarily up-regulated around the time of fights, as predicted by the Challenge Hypothesis. Our data suggest a general role for T during challenges at any time in life, not just during breeding.