Robin W. Baird
Chapter 5 in Cetacean Societies: Field Studies of Dolphins and Whales. Edited by J. Mann, R.C. Connor, P.L. Tyack and H. Whitehead. University of Chicago Press. 2000. 432 p.
Introduction: Among the cetaceans, killer whales (Orcinus orca - Figure 6.1) exhibit several unusual features related to social organization, ecology and behavior. Perhaps the most striking are dispersal patterns. For two so-called "resident" populations in the eastern North Pacific (numbering about 200 and 89 individuals, respectively, as of 1998), neither sex has been recorded dispersing (neither locational nor social dispersal - cf. Isbell and van Vuren 1996) from their natal groups over a 21 year period, nor has immigration into a group been recorded (Bigg et al. 1990a). Natal philopatry by both sexes has not been positively documented for any other population of cetacean, or for that matter, for any other species of mammal. Individuals from "resident" populations feed on fish, and individuals from another, sympatric population, termed "transients," specialize on marine mammal prey. These two forms were termed resident and transient based on research in the 1970s (Bigg et al. 1976; Bigg 1982). These names have been subsequently shown to not be particularly descriptive of the movement patterns and site fidelity of the two forms (Guinet 1990; Baird et al. 1992), but they have been retained as the common names. One apparent consequence of the differences in diet are differences in dispersal patterns. Resident killer whales travel in long-term stable groups comprised of several maternal lineages (Bigg et al. 1990a). However, among transients, all female offspring and all but one male offspring seem to disperse from their maternal groups (social dispersal), but dispersing offspring continue to use their natal range (locational philopatry) (Baird 1994). Besides the difference in diet, resident and transient killer whales also differ in behavior, acoustics, morphology, pigmentation patterns, and genetics (Table 6.1, Figure 6.2). Foraging specializations appear to occur in killer whale populations elsewhere, though research efforts have been generally insufficient to determine whether, similar to the N. Pacific, sympatric forms specialize on different prey types. Individuals of some southern ocean populations feed almost exclusively on marine mammals (Hoelzel 1991; Guinet 1991a; Baird et al. 1992). Predation on marine mammals makes the study of foraging behavior easier than perhaps for any other species of cetacean, because the prey are large, breathe at the surface, and are often captured close to, or even on shore. Several interesting findings have come from these studies, including apparent teaching of hunting skills to offspring (Lopez and Lopez 1985; Guinet 1991b; Hoelzel 1991), and also a strong relationship between group size and foraging success in one population (Baird and Dill 1996). Other studies have demonstrated features for killer whales which appear to be unusual among mammals in general, including the presence of some females who live 20 or more years beyond the birth of their last known offspring (Olesiuk et al. 1990), and the occurrence of group-specific vocal dialects within killer whale populations (Ford and Fisher 1983; Strager 1995). In this paper I review the general biology of killer whales, focusing on several longitudinal studies on free ranging animals. Information on feeding habits, ranging patterns, and social organization and behavior are emphasized.