Population density, cooperation, and the evolution of toothed whale societies

Sam Froman Walmsley, Richard Connor

Odontocete societies come in two primary forms: stable matrilineally-based groups, often with bonds between groups, and labile associations among related and unrelated females in a ‘fission-fusion’ grouping pattern. There is no general theory to explain why these differing strategies evolved. Primate models of social bond evolution, based on cooperation in food competition, lack utility for highly mobile odontocetes, who live in a three-dimensional environment where food defense is impractical, and the group is often the only refuge against predators. Here we offer a general model of odontocete social evolution, illustrated by agent-based simulation. In our model, individuals (or subgroups) must disperse to forage, exposing them to danger from predators or rivals; a danger they may mitigate by investing in additional social bonds. This model can explain the evolution of labile versus stable matrilineal societies, as well as social bonds between groups. Variation in local population densities, movement patterns, predation risk, and food distribution will be key for testing the predictions of our model.