Group dynamics of humpback dolphins (Sousa chinensis) in the Algoa Bay region, South Africa

Abstract

Group dynamics of humpback dolphins Sousa chinensis inhabiting the Algoa Bay region on the south Eastern Cape coast of South Africa, were investigated by means of boat-based photo-identification surveys undertaken between May 1991 and May 1994. Groups of humpback dolphins varied in size from three to 24 animals (x̄ = seven), with adults representing almost two-thirds of the group members. Births occurred predominantly in summer. Some females, however, may also cycle outside of the apparent summer breeding season, perhaps indicating a secondary winter season. Circumstantial evidence suggests a minimum of a 3-year calving interval. Maternal care lasts at least 3-4 years, but female-calf separation is seemingly not related to the female's next pregnancy. Humpback dolphins displayed varying degrees of residence/fidelity to Algoa Bay. Although a few individuals may possibly be classified as 'resident', most dolphins were infrequent visitors in the Bay and seem to be transient. The social system of humpback dolphins appears to be fluid with only casual and short-lasting affiliations. Strong bonds between individuals other than mothers and calves are uncommon. Lack of consistency in the group membership appears to be the general pattern. The weak site fidelity and possibly extensive long-range movement of the majority of dolphins may contribute to the dynamic nature of humpback dolphin groups. There is probably some form of segregation between sex and/or age classes among humpback dolphins in Eastern Cape waters. The nature and extent of this segregation, however, is not yet sufficiently understood. It is likely that the degree of site fidelity displayed by female humpback dolphins is related to their reproductive stage and increases during the nursing period. Mate-searching behaviour of male humpback dolphins is the most likely reproductive strategy of the species.