A revaluation of the role of food in broodmate aggression

Food is the ecological factor assumed to drive the evolution of broodmate aggression and siblicide, but how does food actually influence agonism? Two hypotheses specify a proximate role of food in controlling aggression among avian broodmates. According to the food amount hypothesis, aggression increases with food deprivation; according to the prey size hypothesis, aggression increases when small food parcels are passed directly to chicks' mouths. The food amount hypothesis is confirmed by descriptive information and experiments on four ecologically and taxonomically diverse species (a sulid, a larid, an accipitrid and an alcid), but studies of ardeids supposedly contradict it. It is argued that the descriptive and experimental studies of ardeids are inconclusive, and actually contain evidence supporting the food amount hypothesis. Food-sensitive aggression may be universal in facultatively siblicidal species and appears to constitute an exception to the general rule that aggressiveness of birds and other vertebrates does not increase with food deprivation. There is no convincing evidence that the prey size hypothesis can explain variation among species or during development, and studies showing that heron chicks are more aggressive when prey are small are inconclusive because they did not exclude food deprivation as an alternative explanation. New formulations of the two hypotheses and new research directions are proposed. (C) 2001 The Association for the Study of Animal Behaviour.