The mixed blessing of echolocation: differences in sonar use by fish-eating and mammal-eating killer whales

Abstract

Despite well-documented experimental evidence of echolocation in toothed whales, virtually nothing is known about the use and functional significance of cetacean sonar in the wild. Here, the patterns of echolocation sounds produced by killer whales, Orcinus orca, off British Columbia and Alaska are described. Two sympatric populations with divergent 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 more often than marine mammal-eating ‘transient’ killer whales. The click trains of residents averaged 7 s, more than twice as long as the trains of transients. Click repetition rates within resident's trains were constant or changed gradually; within transient's trains they often fluctuated abruptly. Transients produced isolated single or paired clicks at an average rate of 12/h, four times as often as residents. In general, 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 marine mammals have more acute hearing than fish in the frequency range of sonar clicks. In both populations, echolocation use per individual decreased with increasing group size, suggesting the sharing of information between group members. No relationships were found between echolocation activity and water clarity for whales of either population. Transient whales often travelled or foraged without discernibly echolocating, suggesting that passive listening provides cues for prey detection and orientation.