Call-type matching in vocal exchanges of free-ranging resident killer whales, Orcinus orca

Previous sound recordings of resident (fish-eating) killer whale groups have revealed matrilineal group-specific call repertoires and a strong tendency for calls of the same type to be produced in series. Vocal interactions between individual free-ranging animals, however, have remained unexplored because it has not been possible to identify signallers reliably with a single hydrophone. Here we link acoustic arrivals of calls on a towed hydrophone array with visual tracking of photo-identified individuals to ascribe calls to a focal animal when it was separated from other members of its matrilineal group by more than 35 m, and thereby out of visual range. We confirm that individual members of a matrilineal group share a repertoire of stereotyped calls, and we statistically examine timing of stereotyped calls produced by one individual relative to calls produced by other members of its group. Analysis of the intervals between stereotyped calls indicated that calls were produced in group bouts with a criterion interval of 19.6 s separating bouts. We were therefore careful to develop randomization tests that preserved call interval structure. Focal whales produced 36% of their calls within 5 s of a call from a nonfocal animal, four times more calls than expected by chance based upon a rotation randomization test. Consecutive calls produced by different individuals during group-calling bouts matched call type more than expected by chance. Vocal exchanges of stereotyped calls with type matching appear to be an important aspect of intragroup calling in killer whales, although the function of this calling behaviour remains to be explored.