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Cranial shape correlates with diet specialization in northeast Pacific killer whale (Orcinus orca) ecotypes.

University of British Columbia

Resident, transient (Bigg’s), and offshore killer whales (Orcinus orca) live in sympatric and parapatric ranges in the northeast Pacific Ocean. These ecotypes have different vocal repertoires (Ford and Fisher, 1982; Ford, 1991; Yurk, 2002), echolocation use (Barrett-Lennard et al., 1996), foraging strategies (Bigg et al., 1987; Ford et al., 1998; Baird et al., 1992; Deecke et al., 2002; Ford et al., 2011), and sociobiology (Ford and Fisher, 1982; Bigg et al., 1987; Deecke et al., 2000; Baird and Whitehead, 2000; Riesch et al., 2012). Genetic studies corroborate the behavioural evidence that the resident and transient (Bigg’s) populations are reproductively isolated despite the absence of any geographic or temporal barrier (Stevens et al., 1989; Barrett-Lennard, 2000; Hoelzel and Dover, 1991; Morin et al., 2010). The behavioural segregation between the sympatric ecotypes is apparently maintained by cultural mechanisms alone, which is extremely unusual among non-human mammalian species (Barrett-Lennard, 2000; Riesch et al., 2012). These ecotypes also exhibit dramatic resource polymorphisms: resident killer whales feed exclusively on fish, transient (Bigg’s) killer whales primarily hunt marine mammals (Bigg, 1982; Baird et al., 1992, Ford et al., 1998) and offshore killer whales are thought to feed on fishes including Pacific sleeper shark (Somniosus pacificus) (Ford et al., 2011). We do not know if cranial features related to capturing and processing prey have evolved to reflect the dramatic dietary specializations observed in these three ecotypes. The goal of this research was to determine whether there has been divergence of cranial morphology among the three ecotypes. To this end, I measured and compared cranial shape using traditional and geometric morphometrics techniques. I found that transient (Bigg’s) killer whales that bite and tear apart large mammals have more robust cranial skeletons than the piscivorous resident and offshore killer whales that handle smaller prey items. I found that resident and transient (Bigg’s) killer whales are distinguishable based on skull width, rostral width, and mandibular shape, and that offshore killer whales have a more variable morphology that precludes identification based on cranial shape alone.

Text

2016-09-07

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/59077

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/