Social and reproductive benefits of group fission in wild geladas

Living in large, crowded groups can result in fitness costs due to increased competition and conflict. When dispersing from large groups isn't possible, fissioning into two or more smaller 'daughter' groups may provide an important mechanism for mitigating these costs. Here, we leveraged 15 years of behavioral and demographic data to identify both (i) the social factors that predict group membership decisions and (ii) the adaptive consequences of group fission in the gelada ( Theropithecus gelada ), a grass-eating primate in which the risk of male infanticide — the largest fitness cost to females — is highest in larger groups. After fissioning, females tended to join the same daughter groups as their close kin, closely ranked females, and other strong grooming partners, both female and male. However, females did not join the likely fathers of their dependent infants, even though doing so is expected to prevent infanticide. Group fissions were not preceded by increases in female-female aggression or salient changes in activity budgets but were preceded by increases in infant mortality. In particular, infanticide was elevated prior to fission and plummeted after groups split. Taken together, these data illustrate how animals navigate competing social priorities and highlight the role of fission in ameliorating the costs of group-living.