Corticosterone, reproductive status, and body mass in a cooperative breeder, the Florida scrub-jay (Aphelocoma coerulescens)

In the cooperatively breeding Florida scrub-jay, nonbreeders are subordinate to the breeders with which they share a territory. Corticosterone is secreted in response to a wide range of stressors, including social stress, and suppresses reproductive and territorial behaviors in several taxa; thus, elevated baseline levels of corticosterone might be a causal mechanism of reproductive suppression. To test the hypothesis that nonbreeder Florida scrub-jays are reproductively suppressed through the actions of corticosterone, we compared corticosterone levels of nonbreeders and breeders and found no differences. However, baseline corticosterone levels only provide information about a bird's current hormonal status. Virtually all species exhibit a rapid rise in glucocorticoids in response to an acute stressor. If the adrenocortical response of nonbreeders is greater than that of breeders, this might be a mechanism whereby nonbreeders remain reproductively quiescent. We compared the responses of breeders and nonbreeders to the acute stress of being captured and held for 1 h. Both exhibited significant but equivalent increases in corticosterone titers. Because in some species heavier or fatter individuals have reduced glucocorticoid responses to stressors, we examined whether body mass was correlated with corticosterone titers. Both baseline and maximum corticosterone levels covaried with body mass, and the effect of body mass on corticosterone levels explained the increase due to capture and handling. Our data do not support the hypothesis that Florida scrub-jay nonbreeders are reproductively suppressed through the actions of corticosterone.