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Population structure and mating patterns of killer whales (Orcinus orca) as revealed by DNA analysis

Population structure and mating patterns of killer whales (Orcinus orca) as revealed by DNA analysis
Lance Godfrey Barrett-Lennard


Department of Zoology, University of British Columbia, Vancouver, BC V6T 1Z4, CANADA.


This thesis reports a genetic investigation of population segregation, social organization, and mating patterns in killer whales (Orcinus orca) of the northeastern Pacific Ocean. Previous studies identified two sympatric, non-associating populations, fish-eating residents and mammal-eating transients, and described many aspects of their demography, ecology, and social behaviour. Less is known about a third offshore population. Here, I focused on two aspects of killer whale social organization that are unusual among well- studied mammals: maintenance of complete segregation between residents and transients in sympatry, and lack of dispersal in individual residents of either sex.

I began by developing and testing lightweight pressure-propelled biopsy darts. They were an efficient way of acquiring skin samples from free-ranging whales and caused only minor behavioural responses in sampled animals. Using these darts and sampling stranded carcasses, colleagues and I collected biopsies from 269 individually-identified killer whales in British Columbia and Alaska. I used DNA from the biopsies to sequence the mitochondrial D-loop of 111 matrilines, and genotyped all individuals at 11 polymorphic microsatellite loci.

I found that residents and transients are strongly differentiated genetically and that there is little or no gene flow between them. Both are divided into three genetically- differentiated regional subpopulations. Each resident subpopulation is more closely related to other resident subpopulations than to any transient subpopulation and vice versa, implying that the differences between residents and transients stem from a single divergence. The offshore population is not closely related to either of the other populations. The propensity of killer whales to live in fixed groups of a few hundred individuals apparently allows sympatric or parapatric populations to diverge genetically and could eventually result in speciation.

I examined mating patterns in residents by conducting paternity tests and analysing fixation indices based on microsatellite genotypes. I found that residents rarely mate within their pods. Further, in the most thoroughly-sampled resident subpopulation, most matings were between rather than within acoustic clans (groups of pods with similar acoustic repertoires). Because pods within clans proved to be closely related, inter-clan mating appears to be an inbreeding avoidance mechanism. Most matings were between individuals from the same subpopulation. This pattern of population segregation coupled with inbreeding avoidance closely resembles marriage patterns in many human societies.