Killer whales rely extensively on sound for communication and orientation, and some populations have developed complex repertoires of calls that are transmitted between individuals through vocal learning The ability to modify sounds through learning and experience is rare among mammals and complex vocal learning skills have thus far only been documented in humans, bats, pinnipeds, and cetaceans.Our knowledge about the precise function of killer whale vocal communication, as well as its intended recipients, is currently extremely limited. Call repertoires convey information about population identity and in some cases kinship, but whether free-ranging killer whales actually use this information in behavioural decisions remains unknown.This currently makes it extremely difficult to determine why killer whales have evolved complex vocal repertoires and abilities for vocal learning and ultimately what the reasons for the evolution of vocal learning are. I propose to conduct a 2 year study of killer whales in European waters (Scotland and Norway). I plan to measure the loudness of killer whale calls to determine the distance over which they can be detected by potential recipients.If calls function as contact calls, they should be audible over substantial distances whereas if they primarily function in communication within a foraging group, they may only be audible over a few hundred metres.In addition I plan to use playbacks to test whether killer whales respond differently to the calls of familiar and unfamiliar social groups. Finally I plan to test whether food-associated calls attract other killer whales to a foraging site by conducting playbacks of such sounds to free-ranging killer whales.The proposed study will shed light on the function of vocal behaviour i n populations of free-ranging killer whales and ultimately help us understand the mechanisms responsible for the evolution of complex learned vocal repertoires in cetaceans and other mammals.