Supplementary feeding compensates for testosterone-induced costs of aggression in male mountain spiny lizards, Sceloporus jarrovi

Abstract

Alternative hypotheses for the costs of aggression in male mountain spiny lizards, Sceloporus jarrovi, were examined using testosterone manipulations and food supplementation. In the first experiment, feeding testosterone-implanted males increased both their energy stores and survivorship relative to those of males given empty implants. This evidence supports the hypothesis that lower survivorship of testosterone-implanted males is a result of the energetic costs of increased territorial aggression. This result, by itself, does not eliminate the hypothesis that testosterone-implanted males have lower survivorship as a result of increased conspicuousness to predators, because feeding may have altered conspicuousness. Contrary to this possibility, it was further demonstrated that giving supplementary food to testosterone-implanted males results in a slight increase in conspicuousness. These fed testosterone-implanted males displayed a slight increase in activity after male-male encounters compared with testosterone-implanted males that were not given supplementary food, but the two groups did not differ with respect to daily activity period, territorial displays or escalation of aggressive encounters. Therefore, because fed testosterone-implanted males were slightly more conspicuous, but still survived better, the data from these two experiments strongly support the hypothesis that energetic costs, rather than costs related to increased conspicuousness to predators, constrain levels of aggression in male S. jarrovi during the summer non-breeding season.