We model cooperative prey-sharing behaviour by piscivorous resident killer whales.
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Prey sharing was nonreciprocal and directed towards close maternal kin.
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Adult females were more likely to share fish than adult males or subadults.
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Fitness benefits of kin-directed prey sharing promote philopatry of both sexes.
The vast majority of social animals exhibit sex-biased dispersal as a strategy to reduce kin competition and avoid inbreeding. Piscivorous ‘resident’ killer whales, Orcinus orca, of the eastern North Pacific, however, are unusual in that both sexes remain philopatric throughout life, forming highly stable, multigeneration matrilines that are closed to immigration. We conducted a 12-year study documenting extensive cooperative prey sharing within these matrilines, and hypothesized that extreme natal philopatry in resident killer whales arose due to inclusive fitness benefits gained by provisioning maternal kin. We found that prey sharing was nonreciprocal, and even though whales routinely foraged in mixed associations containing multiple matrilines, prey sharing among individuals belonging to different matrilines was very infrequent. Furthermore, maternal relatedness was a significant predictor of the frequency of prey sharing between individuals, with close maternal kin sharing more often than distant relatives or nonkin. Adult females were much more likely to share prey than adult males or subadults, probably because they mainly provisioned their offspring. However, food sharing was not limited solely to maternal care; all age–sex classes engaged in this behaviour by sharing with close maternal relatives, such as siblings and mothers. We also investigated the frequency of prey sharing between mothers and their offspring as a function of offspring sex and age, and found that maternal food sharing with daughters declined after daughters reached reproductive maturity, which could help to explain matriline fission events. The evolution of kin-directed food sharing requires the ability to reliably discriminate kin, which resident killer whales likely achieve through social familiarity and vocal dialect recognition. We propose that lifetime philopatry of both sexes has been selectively favoured in this population due to the inclusive fitness benefits of kin-directed food sharing, a cooperative behaviour that may also inhibit dispersal by reducing resource competition among kin.
