Revised taxonomy of eastern North Pacific killer whales (Orcinus orca): Bigg's and resident ecotypes deserve species status

Abstract

Killer whales (Orcinus orca) are currently recognized as a single ecologically and morphologically diverse, globally distributed species. Multiple morphotypes or ecotypes have been described, often associated with feeding specialization, and several studies have suggested taxonomic revision to include multiple subspecies or species in the genus. We review the ecological, morphological and genetic data for the well-studied 'resident' and Bigg's (aka 'transient') ecotypes in the eastern North Pacific and use quantitative taxonomic guidelines and standards to determine whether the taxonomic status of these killer whale ecotypes should be revised. Our review and new analyses indicate that species-level status is justified in both cases, and we conclude that eastern North Pacific Bigg's killer whales should be recognized as Orcinus rectipinnus (Cope in Scammon, 1869) and resident killer whales should be recognized as Orcinus ater (Cope in Scammon, 1869).

Keywords: Cetacea; odontocete; speciation.

Figure 1.

Expected range maps for (a) resident and (b) Bigg’s killer whales, including locations of samples used for mitogenome analysis (figure 5a, resident n = 106, Bigg’s n = 93) [15]. Distribution ranges have been inferred based on published identifications of individuals that are identified by ecotype [–53]. Sample distributions cover the known ranges of both ecotypes, with the exception of residents of Oregon and northern California, and both ecotypes off northern Japan (Hokkaido) in the western Pacific [48,54]. Sample maps for microsatellite data are in electronic supplementary material, figure S2.

Figure 2.

Canonical variate 1 and 2 plots for cranial shape features that distinguish among ecotypes for (a) skull morphology (resident (n = 17), Bigg’s (n = 13) and offshore (n = 6)) and (b) dentary bone morphology (resident (n = 21), Bigg’s (n = 12) and offshore (n = 8) specimens) (reprinted from [103]).

Figure 3.

PCA plot of first two principal components based on (a) 88 SNPs: offshore (n = 3), resident (n = 11), Bigg’s (n = 30) from data in Morin et al. [15]; (b) 26 microsatellites: offshore (n = 5), resident (n = 250), Bigg’s (n = 116) (samples genotyped at ≥20 loci) [56]; unpublished); (c) 3678 RADseq SNPs: offshore (n = 7), resident (n = 52) and Bigg’s (n = 37) populations [57,62]; (d) 1 00 000 (subset from 6 371 282) SNPs from 147 high-coverage genomes of offshore (n = 7), Bigg’s (n = 14) and resident (n = 126) samples from multiple geographically and behaviourally defined subpopulations (Alaska, northern and southern resident populations) (based on subset of SNP genotype data from [113]. See Supplementary Materials for methods and data set information.

Figure 4.

Structure assignment probability plots for K = 3 groups from (a) 26 microsatellites: offshore (n = 5), resident (n = 250), Bigg’s (n = 116) samples genotyped at ≥ 20 loci) (; unpublished); (b) 3340 RADseq SNPs (polymorphic in sample set): offshore (n = 7), resident (n = 52) and Bigg’s (n = 37) populations [57,62]. Vertical bars represent the individual assignment probability for each group inferred by Structure (groups identified by shading), with samples sorted by a priori ecotype assignment. See electronic supplementary material for methods and data set information.

Figure 5.

Global phylogenetic trees of killer whales based on (a) haplotypes from 452 mitogenomes and (b) 49 nuclear genome sequences. Reprinted with permission from Morin et al. [15] (figure 2; by permission from John Wiley & Sons, licence 5458310335802) and [9] (electronic supplementary material, figure S3b, by permission from Andrew D. Foote). Black branches in (a) lead to haplotypes that are from animals that have not been identified to ecotype (see electronic supplementary material, table S1 from [15]).

Figure 6.

Illustrations of (a) O. ater and (b) O. rectipinnus from Scammon [138,140]. These illustrations were likely made by Scammon, or made under his guidance from his field notes and sketches. Whether they represent renderings of specific specimens, or composite sketches, is unknown.

Figure 7.

Photographs of neotype skulls for (a) Orcinus rectipinnus (USNM 594671) and (b) Orcinus ater (USNM 594672).

Figure 8.

Vertical images of (a) an adult male Bigg’s killer whale (BKW) from the West Coast Transient population of Bigg’s killer whales and (b) an adult male resident killer whale (RKW) from the sympatric Southern Resident population of resident killer whales. Images are scaled to the estimated asymptotic lengths of 7.3 m [20] and 6.9 m [145], respectively. Vertical images were collected using an octocopter drone using methods described by Durban et al. [146], provided by John Durban and Holly Fearnbach.