Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8431
Title: Morphological and ecological evidence for two sympatric forms of Type B killer whale around the Antarctic Peninsula
Author: J. W. Durban, Holly Fearnbach, D. G. Burrows, G. M. Ylitalo, R. Pitman
Publication Year: 2017
Journal: Polar Biology
Volume: 40
Pages: 231-236
Keywords: predator,cetacean,stable isotope,photogrammetry
Abstract:

Killer whales (Orcinus orca) are apex marine predators in Antarctica, but uncertainty over their taxonomic and ecological diversity constrains evaluations of their trophic interactions. We describe two distinct, sympatric forms sharing the characteristic pigmentation of Type B, the most common around the Antarctic Peninsula.  Laser photogrammetry revealed nonoverlapping size differences among adults: Based on a body length index (BLI: blowhole to dorsal fin) adult females of the larger form (‘‘B1’’) were 20 % longer than the smaller form (‘‘B2’’), and adult males were 24 % longer on average. Dorsal fins of B1 adult females were 19 % taller than B2 females, and adult males 32 % taller. Both types were strongly sexually dimorphic, but B1 more so, including for BLI (B1 males = 1.079  females; B2 =  1.059 ) and especially for dorsal fin height (B1 male fins =  2.339  female; B2 =  2.109 ). The characteristically large Type B eye patch was more extensive for B1 than B2, comprising 41 and 37 % of BLI, respectively. Average group size was also significantly different, with B1s in smaller groups (mean 7, range 1–14) and B2s more gregarious (mean 36, range 8–75). Stable isotope analysis of skin biopsies indicated dietary differences: a significantly lower nitrogen 15N/14N ratio in B2s supported observations of feeding primarily on krill consumers (e.g., pygoscelid penguins), while B1s prey mainly on predators of krill consumers (e.g., Weddell seals Leptonychotes weddellii). These differences likely represent adaptations to distinct foraging niches, which has led to genetic divergence; their ecology now needs further study.

Description:

Killer whales (Orcinus orca) are apex marine predators in Antarctica, but uncertainty over their taxonomic and ecological diversity constrains evaluations of their trophic interactions. We describe two distinct, sympatric forms sharing the characteristic pigmentation of Type B, the most common around the Antarctic Peninsula.  Laser photogrammetry revealed nonoverlapping size differences among adults: Based on a body length index (BLI: blowhole to dorsal fin) adult females of the larger form (‘‘B1’’) were 20 % longer than the smaller form (‘‘B2’’), and adult males were 24 % longer on average. Dorsal fins of B1 adult females were 19 % taller than B2 females, and adult males 32 % taller. Both types were strongly sexually dimorphic, but B1 more so, including for BLI (B1 males = 1.079  females; B2 =  1.059 ) and especially for dorsal fin height (B1 male fins =  2.339  female; B2 =  2.109 ). The characteristically large Type B eye patch was more extensive for B1 than B2, comprising 41 and 37 % of BLI, respectively. Average group size was also significantly different, with B1s in smaller groups (mean 7, range 1–14) and B2s more gregarious (mean 36, range 8–75). Stable isotope analysis of skin biopsies indicated dietary differences: a significantly lower nitrogen 15N/14N ratio in B2s supported observations of feeding primarily on krill consumers (e.g., pygoscelid penguins), while B1s prey mainly on predators of krill consumers (e.g., Weddell seals Leptonychotes weddellii). These differences likely represent adaptations to distinct foraging niches, which has led to genetic divergence; their ecology now needs further study.

Theme: Ecosystem approach to improve management of marine resources
Foci: Characterize ecological interactions (e.g. predation, competition, parasitism, disease, etc.) within and among species.