Ecological and social determinants of group size in transient killer whales

Most analyses of the relationship between group size and food intake of social carnivores have shown a discrepancy between the group size that 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 prey exclusively on marine mammals. Between 1986 and 1993, in approximately 434 h of observations on transient killer whales, we observed 138 attacks on five species of marine mammals. Harbor seals were most frequently 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 highest energy intake rate per individual. While groups of three were most frequently encountered, the group size experienced by an average individual in the population (i.e., typical group size) is larger than three. However, comparisons between observed and expected group sizes should 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 individuals are found in groups that are consistent with the maximization of energy intake hypothesis. Larger groups may form for (1) the occasional hunting of prey other than harbor seals, for which the optimal foraging group size is probably larger than three; and (2) the protection of calves and other social functions.