Surviving at sea: ecological and behavioural predictors of calf mortality in Indian Ocean bottlenose dolphins, Tursiops sp.

Abstract

We examined the behavioural ecology of mothers and calves during the first year of life to identify variables predicting calf mortality among bottlenose dolphins. Specifically, we investigated whether the primary cause of calf mortality was poor calf condition or shark predation. Seventy-five per cent of calves that died (N = 12) showed visible signs of poor health compared with 4.7% of the survivors (N = 21). Calves that spent more time swimming in infant position (in contact under the mother) in the first year of life were more likely to die by 3 years of age than calves that spent less time swimming in infant position. No other behavioural or ecological measures tested were associated with calf mortality. We suggest that the mother and/or calf may compensate for poor calf health by increasing the time spent in contact with the mother. In contrast, frequent mother–calf separation and high rates of calf socializing and foraging are proposed to be indicators of vigour. Although most of our results do not support the predation hypothesis, mothers altered one behaviour in relation to predation risk. Mothers rested less in deep water during the warm months, where and when predation risk was high, than during the cool months, when predation risk was low, suggesting that predation risk influenced maternal vigilance. However, calf mortality was not higher in warm months than in cool months. Our results suggest that poor calf condition, not predation, is the primary cause of calf mortality.