First evidence for widespread sharing of stereotyped non-signature whistle types by wild dolphins

Abstract

Bottlenose dolphins (Tursiops truncatus) have long fascinated animal communication researchers due to their large brains and varied communicative signals. However, studying dolphin communication is challenging due to difficulties in identifying which individual is making a sound. A long-term study of a resident population of free-ranging dolphins in waters near Sarasota, Florida, employs brief catch-and-release as a research tool for health assessments, and this has enabled us to obtain high-quality recordings of known individual dolphins for 4 decades. We have a catalog of the individually distinctive signature whistles (SW) of most animals in the community; these referential signals function much like human names. Whistles other than the signature, known as “non-signature whistles” (NSW) are very poorly understood. We are building a catalog of stereotyped NSWs that are shared by more than one individual, and have so far identified more than 20 shared NSW types, two of which were produced by at least 25 different individuals. These whistles were recorded both during health assessments and from free-swimming dolphins wearing non-invasive, suction-cup attached digital acoustic tags (DTAGs), which we are able deploy on dolphins prior to release. We have embarked on playback studies to study the functions of both SW and shared NSW, in which we film movement responses with drones and record vocal responses with hydrophones and DTAGs. Control trials (n=21), in which we played the target dolphin’s own SW, elicited approach in 76% of trials in which we were able to observe the response. These data support the affiliative nature of SW copies, which appear to function as a way to initiate contact with another individual. Playbacks of a widely shared NSW, NSWA (n=8 trials), consistently elicited avoidance responses (100% of 6 observed trials) , suggesting an alarm or warning type function. Another widely shared NSW, NSWB, has elicited varying responses in 8 trials, depending on the age and sex of the target dolphin. NSWB was also produced by the target dolphin in response to 2 playback trials. Our suggested function of NSWB is a “query”- type whistle, produced when something unexpected or unfamiliar is heard. In this case, we would expect different responses depending on age and sex class, for example with males likely being more interested in exploring unfamiliar stimuli than females with calves. Overall, this research presents the first evidence for shared, stereotyped, context-specific whistles in bottlenose dolphins, and suggests that NSWs play an important role in the dolphin communication system.