Social decision-making in a wild parrot relies on both individual recognition and intrinsic markers

Abstract

O_LIDominance hierarchies are generally thought to form over time via memory of repeated interactions. However, dominance hierarchies are also occasionally reported in species with fission-fusion social dynamics, where individuals may encounter large numbers of individuals, leading to incomplete social information. It it has been alternatively proposed that the complex decision-making required in these circumstances may lead to increased selection for social cognition and memory, or to the evolution of mixed strategies that rely on memory for interaction with familiars and status signals for strangers. C_LIO_LIHere, test these competing hypotheses by recording social associations and aggressive interactions in a highly social, large-brained parrot, the sulphur-crested cockatoo ( Cacatua galerita). We followed 411 individuals across three neighbouring roost sites, where individuals exhibit stable dominance hierarchies within roosts, alongside a high degree of fission-fusion dynamics within roosts and regular between-roost movements. C_LIO_LIWe found evidence that sulphur-crested cockatoos use a two-fold social strategy when initiating or reacting to an aggression. For familiar individuals, aggressions were initiated or escalated based on rank difference. When facing less familiar individuals, decisions to interact -- or escalate -- were based on the relative weight, with interactions directed towards, and more likely to escalate between, individuals of similar weight. C_LIO_LIOur results suggest that social knowledge remains an important determinant of aggressive interactions in highly fission-fusion systems, but that individuals can dynamically incorporate other potential cues of competitive ability when knowledge is lacking. C_LI