Sex Differences in Patterns of Association Among Indian Ocean Bottlenose Dolphins

Patterns of association among bottlenose dolphins resident in Shark Bay, Western Australia were analyzed using party membership data. Parties contained an average of 4.8 individuals, but party size and composition were unstable. While these temporary parties often contained both males and females, long term consistent associations generally were between members of the same sex. The highest association coefficients, resulting from very frequent co-occurrence within parties were between males and between mothers and offspring. Males formed subgroups of two or three individuals who consistently associated with each other, and these were stable over periods of at least seven years in some cases. Male subgroups preferentially associated with particular other male subgroups. Females associated most consistently with other females, although not to the same extent as some males. Female associations were better described as a network rather than discrete subgroups. Male-female associations were generally inconsistent and depended in part on female reproductive state. Mothers and their offspring associated very consistently for at least 4 years.

Affiliations: 1: Department of Psychology, University of Michigan, Ann Arbor, MI. 48109, U.S.A.; 2: Department of Biology, University of Michigan, Ann Arbor, MI. 48109, U.S.A.; 3: Biology Department, Woods Hole Oceanographic Institute, Woods Hole, MA. 02543, U.S.A.