Early social development in wild bottlenose dolphins: sex differences, individual variation and maternal influence

Abstract

Bottlenose dolphin (Tursiops sp.) calves are unusual among social mammals in that they have a have prolonged nursing period (3–8 years), but precocious motor ability, enabling them to separate from their mothers and form distinct social bonds at an early age. We examined two measures of calf sociality from birth to 4 years of age: (1) the number of unique associates and (2) the proportion of time that calves spent in groups with nonmothers. Calves (N = 89) associated with a mean ± SE of 27.56 ± 2.24 individuals (range 0–77) and spent 46.84 ± 3.00% of their time in groups (range 0–100%). Mothers (N = 49) averaged 39.39 ± 5.32 associates (range 0–139), when data were combined across all years (and often multiple offspring). Both calf characteristics (sex, age and separation time) and maternal characteristics (sociality and foraging time) contributed to this variation. Although calf associate number and time in groups were positively correlated, sex and age-specific patterns differed depending on the measure used. As separation time increased, both sexes increased associate number, but females decreased and males increased time in groups, indicating that males sought more social contact. Maternal socioecological strategy largely contributed to calf social development, particularly for daughters. As maternal foraging time increased, maternal and calf sociality decreased. Second, the number of associates that calves had when with their mothers predicted associate number during separations, but time spent in groups was not similarly correlated. These early patterns probably influence subsequent social development, including the structure of female networks and male alliances.