No reduction in aggression after loss of a broodmate: a test of the brood size hypothesis

In some vertebrate species, parents create a large brood or litter then, in the event of unfavourable ecological conditions, apparently allow the number of offspring to be adaptively reduced through siblicide. But how is sibling aggression regulated so that deaths occur only in unfavourable conditions? One proposed mechanism is brood size-dependent aggression. Two experiments tested for this mechanism by reducing three-chick broods of blue-footed boobies either during or after the period of dominance hierarchy establishment. In neither experiment did aggression of the two eldest and highest ranking chicks decline after removal of the youngest broodmate, in comparison with controls. These results suggest that dominant booby chicks do not become less aggressive to each other after disappearance of their youngest broodmate and that this species does not show brood size dependent aggression. Elder blue-footed booby chicks increase their attacks on broodmates when they receive less food, and this mechanism may be sufficient to tailor brood size to food availability.