THE MIXED BLESSING OF ECHOLOCATION - DIFFERENCES IN SONAR USE BY FISH-EATING AND MAMMAL-EATING KILLER WHALES

Despite well-documented experimental evidence of echolocation in tooth ed whales, virtually nothing is known about the use and functional sig nificance of cetacean sonar in the wild. Here, the patterns of echoloc ation sounds produced by killer whales, Orcinus orca, off British Colu mbia and Alaska are described. Two sympatric populations with divergen t food habits differed markedly in sonar sound production. Individuals belonging to the fish-eating 'resident' population produced trains of characteristic sonar clicks, on average, 4% of the time, 27 times mor e often than marine mammal-eating 'transient' killer whales. The click trains of residents averaged 7 s, more than twice as long as the trai ns of transients. Click repetition rates within resident's trains were constant or changed gradually; within transient's trains they often f luctuated abruptly. Transients produced isolated single or paired clic ks at an average rate of 12/h, four times as often as residents. In ge neral, the isolated clicks and infrequent, short and irregular trains of transients were less conspicuous against background noise than the sonar of residents. This difference in acoustic crypticity may reflect a flexible response to the probability of alerting prey, because mari ne mammals have more acute hearing than fish in the frequency range of sonar clicks. In both populations, echolocation use per individual de creased with increasing group size, suggesting the sharing of informat ion between group members. No relationships were found between echoloc ation activity and water clarity for whales of either population. Tran sient whales often travelled or foraged without discernibly echolocati ng, suggesting that passive listening provides cues for prey detection and orientation. (C) 1996 The Association for the Study of Animal Beh aviour.