ABSTRACT
We tested whether in two-chick broods of the blue-footed booby (Sula nebouxii) elevated circulating corticosterone in the socially subordinate broodmate facilitates submissive behavior and/or enhances food solicitation. Implanting corticosterone in 17 subordinate chicks (experimental broods) produced changes in the behavior of chicks and parents over the first two days, relative to 17 matched families (control broods) where subordinate chicks were implanted with empty capsules. Experimental broods showed increased activity/wakefulness of the dominant broodmate and, consequently, increased simultaneous activity of both broodmates, but there was scant evidence that subordinates submitted more readily when attacked. Implanted subordinates increased their rate of spontaneous submission over the total observation time, but this increase was mostly explained by the additional time when both broodmates were simultaneously active. There was little sign that extra corticosterone induced more begging, except possibly by eliciting increased activity. Experimental broods increased their rate of feeding, and most if not all of the increase was due to the increased activity and increased feeding rate of dominant broodmates. On the third and fourth days after implantation all effects of implanted corticosterone disappeared, except for the elevated activity and feeding rates of dominant chicks. At the end of four days, subordinates implanted with corticosterone showed no increase in circulating corticosterone and experimental broods showed no gain in mass or body size, relative to controls. Extra corticosterone, above the high level that normally circulates in subordinate chicks, apparently does not enhance submission to aggression or food solicitation, but provokes a cascade of changes in the behavior of broodmates and parents.
