Stability and group specificity of stereotyped whistles in resident killer whales, Orcinus orca, off British Columbia

Resident killer whales off British Columbia form four acoustically distinct clans, each with a unique dialect of discrete pulsed calls. Three clans belong to the northern and one to the southern community. Resident killer whales also produce tonal whistles, which play an important role in close-range communication within the northern community. However, there has been no comparative analysis of repertoires of whistles across clans. We investigated the structural characteristics, stability and group specificity of whistles in resident killer whales off British Columbia. Acoustic recordings and behavioural observations were made between 1978 and 2003. Whistles were classified spectrographically and additional observers were used to confirm our classification. Whistles were compared across clans using discriminant function analysis. We found 11 types of stereotyped whistles in the northern and four in the southern community with some of the whistle types being stable over at least 13 years. In northern residents, 10 of the 11 whistle types were structurally identical in two of the three acoustic clans, whereas the whistle types of southern residents differed clearly from those of the northern residents. Our study shows that killer whales that have no overlap in their call repertoire use essentially the same set of stereotyped whistles. Shared stereotyped whistles might provide a community-level means of recognition that facilitates association and affiliation of members of different clans, which otherwise use distinct signals. We further suggest that vocal learning between groups plays an important role in the transmission of whistle types.