Hoelzel, AR, Dahlheim, M, and SJ Stearn. 1998. Low genetic variation among killer whales (Orcinus orca) in the eastern North Pacific and genetic differentiation between foraging specialists. The Journal of Heredity 89:121-128.

(Tone Rawlings)

Hoelzel et al., as well as others, assert that there are two social groupings among killer whales, the transients and residents. There are a host of characteristics that distinguish such groupings. However, there is minimal data on the genetic features of these groups. So, they collected tissue samples from killer whale pods from the coasts of Alaska, British Columbia, and Washington state to establish genetic distinction between the transient and resident types.

They used mtDNA sequencing, microsatellite, single-stranded conformational polymorphism, and phylogenetic analyses to interpret similarities or differences between the two groupings. In all, 73 samples were analyzed by these tests. They found that mtDNA analysis showed that pods either fell within the resident clade or the transient clade. Further, 91% of the variation was accounted for by variation between transients and residents and not by geographic location. They also found little variation within local populations which might suggest founding events. Significant allelic frequency differences in the microsatellite data indicated limited dispersal between the two groups. This was supported by their low estimate of the rate of genetic migration based on Rst. Thus mating occurs primarilty within these groupings.

Hoelzel et al. suggested that the genetic differences between the sympatric resident and transient populations could have evolved by behavioral isolation. Specifically they focused on the prey capture type and behavior as the characteristics that drive the genetic differences between the groupings. The resident types prey on fish, whereas the resident group preys on marine mammals. Further, the resident pods "wrangle" fish prey by dispersing and then converging on the prey. This coordinated behavior may be a factor leading to the stability of pursuing fish prey. Individuals in pods that pursued marine mammals were capable of acquiring sufficient prey on their own. So, hunting group size was smaller then pod size. Hoelzel et al. goes on to point out that the differences in resource exploitation could account for the observed differences in social structure and behavior.