Developmental changes in separation and activity of wild bottlenose dolphin calves (Tursiops sp.)

Bottlenose dolphins live in fission-fusion societies, characterized by frequent changes in group size and composition and long-term social relationships. As part of a longitudinal study of mother-calf behavior of wild Indian Ocean bottlenose dolphins (Tursiops, sp.) in Shark Bay, Australia, we studied the context, behavior and characteristics of mother-calf separations at close (>2m<10m), moderate (≥10m<50m) and far (≥50m) distances. While calves are physically precocious at birth, they exhibit a prolonged period of dependency and typically nurse for three to six years. Using focal follows of 77 calves observed between 1989 and 2005, we investigated developmental changes in calf behavior during separations. In the first year (4-11 months), calves spent approximately one-third of their time separated (>2m) from the mother. The overall time calves spent separated did not change from year one to year three (ANOVA: F2,21=1.12, P=0.335, N=23). But, the time spent at far distances tended to increase from year one to year two (Bonferroni: T=-3.571, P=0.160, N=23) while the time spent at close distances decreased over all three years (ANOVA: F2,21=5.819, P=0.006, N=23). Further, over all three years males spent more time at far distances from their mothers than females (ANOVA: F2,21=11.227, P=0.003, N=23). This may be attributed to the fact that males are weaned at earlier ages than females, although both suggest that males are more independent. During far distance separations, males socialized more than females (Mann-Whitney U test: U=272.5, P=0.048, Nmale=27, Nfemale=29) and females foraged more than males (Mann-Whitney U test: U=254, P=0.024, Nmale=27, Nfemale=29). At closer distances, no sex differences were apparent. These differences in activity at far distances might serve as precursors to the sex-specific adult reproductive strategies. While traveling far potentially increases energy expended and risk of shark attack, calves that separate may benefit from increased social and foraging experience and allow mothers to forage more efficiently.