ECOLOGICAL AND SOCIAL DETERMINANTS OF GROUP-SIZE IN TRANSIENT KILLER WHALES

Most analyses of the relationship between group size and food intake o f social carnivores have shown a discrepancy between the group size th at maximizes energy intake and that which is most frequently observed. Around southern Vancouver Island, British Columbia, killer whales of the so-called transient form forage in small groups, and appear to pre y exclusively on marine mammals. Between 1986 and 1993, in approximate ly 434 h of observations on transient killer whales, we observed 138 a ttacks on five species of marine mammals. Harbor seals were most frequ ently attacked (130 occasions), and the observed average energy intake rate was more than sufficient for the whale's energetic needs. Energy intake varied with group size, with groups of three having the highes t energy intake rate per individual. While groups of three were most f requently encountered, the group size experienced by an average indivi dual in the population (i.e., typical group size) is larger than three . However, comparisons between observed and expected group sizes shoul d utilize only groups engaged in the behavior of interest. The typical size of groups consisting only of adult and subadult whales that were engaged primarily in foraging activities confirms that these individu als are found in groups that are consistent with the maximization of e nergy intake hypothesis. Larger groups may form for (1) the occasional hunting of prey other than harbor seals, for which the optimal foragi ng group size is probably larger than three; and (2) the protection of calves and other social functions.