The size, composition and function of wild bottlenose dolphin (Tursiops sp.) mother-calf groups in Shark Bay, Australia

The fission-fusion society of bottlenose dolphins seems particularly well suited to balancing the costs and benefits of grouping. Here, we examined four nonmutually exclusive hypotheses regarding the causes and/or functions of calf grouping patterns when with and away from the mother: maternal influence, same-sex bond formation, protection and social skill development. We evaluated the role of calf ( calf sex, age and separation from their mothers) and maternal characteristics ( foraging time and sociality) in shaping the size and composition of motherecalf groups using focal observations of 49 mothers and 89 calves. Mother-calf groups were biased towards association with females of all ages and with male calves, but contained relatively few juvenile and adult males. Although there was some support for all of the hypotheses, the social skills hypothesis best explained male calf social patterns, and maternal influence better explained female calf social patterns. While maternal sociality influenced calves of both sexes, daughters mirrored the mother during separations, whereas sons did the opposite, seeking more social contact if their mothers were solitary and seeking less social contact if their mothers were sociable. During motherecalf separations, calves of both sexes preferentially associated with immatures over adults, but male calves preferred juvenile and adult male associates, which female calves, like their mothers, avoided. Calf social patterns illustrate the specific social and ecological challenges that they face in relation to and independent of their mothers. (c) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.