Patterns of Social Affiliation and Group Composition for Bottlenose Dolphins (Tursiops truncatus) in San Luis Pass, Texas

Abstract

Group sizes, group composition, and association patterns of bottlenose dolphins (Tursiops truncatus) were investigated in the San Luis Pass area (Sep. 1995- Aug. 1996) of the Galveston Bay Estuary to examine differences in community structure of individuals inhabiting different portions of the estuary. Group sizes (n = 83) ranged from 1 to 29 (x̄ = 10.6) and were seasonally variable, with the largest groups occurring in spring (x̄ = 16.3) and the smallest groups during the fall (x̄ = 6.3). Seventy-one individuals were identified using photoidentification, and the sex of six dolphins (three males, three females) was determined. At least 31 (48.4%) groups were of mixed sex. Twenty-nine dolphins that were identified five or more times were used to calculate half-weight coefficients of association (COAs), which ranged from 0.00 to 0.83 (x̄ = 0.46). Coefficients of association for male pairs were higher than COAs for female and mixed-sex pairs. Permutation tests were performed to test for nonrandom associations and presence of preferred or avoided companions. The null hypothesis of random association was rejected, indicating that dolphins preferentially associated with some individuals and avoided others. In all replicates, three known-male pairs had significantly large COAs. These preliminary results suggested that, excluding mother-calf pairs that were not examined, male pairs formed the most stable social bonds.