Abstract
Segregation by sex is evident at a variety of levels in many birds, fishes, and mammals. Segregation has been observed in marine mammals to varying degrees, but it was previously undocumented in Hector's dolphin (Cephalorhynchus hectori). Forty-three groups (of group size ≤ 5) were sexed using an underwater pole-camera; 91% of groups consisting of two to five individuals (n = 32) were either all male or all female. Sexes were obtained from an additional seven groups containing calves. All of the adults associating with mothers and their young were female. This research suggests that Hector's dolphin groups are highly segregated by sex. Sex segregation might have implications for reproduction in Hector's dolphins, including difficulty in finding a mate as local populations decline. [PUBLICATION ABSTRACT]