Sexual segregation when foraging in an extremely social killer whale population

Resident, fish-eating killer whales in the northeastern Pacific Ocean live in multi-generational matrilines containing both sexes. The degree of maternal fidelity and natal philopatry in this killer whale society is extreme even by the standards of lions, elephants or any highly social mammal. Benefits of group living include cooperative foraging and alloparental care, but few studies have explored how killer whales avoid within-group competition for prey. This study measured focal animal behaviour from one population in its legally designated critical habitat. Adult males and females overlapped spatially whilst resting, travelling and socialising, but during feeding bouts, females foraged nearshore in shallower waters, whilst adult males distributed foraging effort throughout the study area, with no statistically significant depth preference. We postulate that sex-biased dispersal in foraging ecology reflects physiological capacity for deeper diving in males than females, which may be either a driver or consequence of extreme sexual dimorphism in the species; alternative interpretations exist. Killer whales appear to be a cosmopolitan species complex including populations that range widely in body size and diet. Our physiological limitation theory could be tested with other ecotypes. For the northern resident killer whale population we studied, we postulate that our finding may indicate a mechanism to avoid or reduce competition for food within the family unit whilst ensuring group cohesion. Investigating sex differences in foraging habitat informs area-based management and conservation of this threatened population, but studies on other ecotypes are needed to improve our understanding of the evolution of sociality in this species.