Group size and aggression: 'recruitment incentives' in a cooperatively breeding primate

In many cooperatively breeding birds and mammals, group size is positively correlated with reproductive success. In marmoset and tamarin monkeys, species that display cooperative breeding, the presence of helpers appears to be critical for offspring survival, and breeders might be expected to display social strategies that would regulate group size. This study investigated the association between group size and aggression towards strangers in Wied's black tufted-ear marmosets, Callithrix kuhlifrom small groups (with no helpers present) and large groups (with helpers present). Residents were exposed to multiple presentations of male and female strangers. Breeding females from large groups spent more time in close proximity to strangers, showed higher levels of agonistic displays and engaged in higher levels of aggressive behaviour towards intruders than did breeding females from small groups. Breeding male behaviour did not dramatically differ as a function of group size. After the removal of the intruder, female breeders from large groups showed higher levels of scent-marking relative to baseline observations, and time spent in close proximity to the partner increased after exposure to female intruders in large, but not small, groups. The results reveal that breeders from small groups are tolerant of strangers, which may facilitate the recruitment of additional group members, whereas breeders from large groups, particularly females, are intolerant of strangers, which may inhibit immigration. These findings have implications for understanding mechanisms that regulate immigration in cooperatively breeding animals, and suggest of how breeders from small groups might reconcile the serious limitation of having few or no helpers.