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This study examines factors influencing female reproductive success in wild Indian Ocean bottlenose dolphins (Tursiops sp.) in Shark Bay, Australia. Eighty-three females and their 142 calves were surveyed between 1988 and 1998 (3457 surveys); 42 calves were also observed during focal follows for 1330 h. Calf mortality is 44% by age 3 (n = 110). Reproduction is moderately seasonal (September-January), peaking from October to December. Calf loss between August and December is followed by rapid conception (1-2 months), whereas conception is delayed (2-9 months) if calf loss occurs between January and July. Weaning ages ranged from 2.7 to 8.0 years, but 66.7% (42 calves) were weaned by their fourth birthday. Females tended to wean mid-pregnancy. Accordingly, median interbirth interval was 4.1 years. Female reproductive success was classified as 0, 1, 2, or 3 according to the number of calves who survived to age 3 over a 10-year period (n = 38 females with complete histories). We examined whether factors affecting predation or food availability, water depth, and group size, were related to female reproductive success. Group size was unrelated to water depth or female reproductive success, but reproductive success was predicted by water depth (p <.002). Shallow water may allow mothers and calves to detect and avoid predatory sharks. Alternatively, or additionally, prey density may be higher in shallow water compared to deep water.
Janet Mann (Wed,) studied this question.