Social organization of mammal-eating killer whales: group stability and dispersal patterns

The social organization of mammal-eating "transient" killer whales (Orcinus orca) was studied off southern Vancouver Island from 1985 through 1996. St rong and long-term associations exist between individual transients, so set s of individuals with consistently high association levels, termed pods, ca n be delineated. Pods consist of individuals of mixed ages and sexes, and t ypically contain an adult female and one or two offspring (averaging 2.4 in dividuals). The mother-offspring bond remains strong into adulthood for som e male (and less often for female) offspring. Other males disperse from the ir maternal pod and appear to become "roving" males, spending some of their time alone, and occasionally associating with groups that contain potentia lly reproductive females. These males appear to have no strong or long-term relationships with any individuals, and adult male - adult male associatio ns occur significantly less often than expected by chance. Females that dis perse from their natal pod appear to be gregarious (having high average ass ociation rates) but socially mobile (having low maximum association rates). Differences in social organization from the sympatric fish-eating "residen t" killer whales (where no dispersal of either sex occurs) likely relate to differences in foraging ecology. Transient killer whales maximize per capi ta energy intake by foraging in groups of three individuals, whereas no suc h relationship has been documented for resident killer whales.