Sperm whale social units: Variation and change

Sperm whale (Physeter macrocephalus) photoidentification data spanning 12 years of study around the Galapagos Islands were examined to investigate the size, variability, and stability of social units. Adult females and immature whales of both sexes have two types of associates: 'constant companions,' which are members of an individual's 'stable' social unit, and 'casual acquaintances,' which are temporarily associating members of different units. We analysed long-term association patterns and calculated that individuals have a mean of 11.3 constant companions. Estimated social unit size ranged from 3 to 24 individuals. Evidence of splitting and merging of units and of transfer of individuals between units is presented. The estimated overall frequency of these unit-membership changes is 6.3% per individual per year. These forms of unit dynamics are rare in species with male dispersal and matrilineally related social groups, and cannot be easily explained in this species. There is considerable variation in unit size (perhaps caused by demographic processes), suggesting that the benefits of remaining in a social unit usually outweigh selection for some optimal unit size. However, the occurrence of merging and transfers suggests that the ecological or social cost/benefit of leaving one's matrilineal unit may sometimes outweigh the cost/benefit of staying.