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.