Bottlenose dolphins are acoustic animals: they navigate and find food successfully, throughout day and night, by listening for the faint echoes generated by their strong, directional echolocation clicks. They find and recognize dolphins using individually specific signature whistles, and they employ a variety of other acoustic signals to interact with others of their species.
For the past 5 years, we have been deploying acoustic recording tags, known as DTAGs, on bottlenose dolphins at the end of their health assessment and prior to release to listen in on these acoustically active animals. We attach these tags to dolphins using four small suction cups, and they stay on for periods of up to 24 hours while recording the movement, depth, and sounds produced by tagged individuals, in addition to acoustic signals from nearby dolphins. We have focused our tagging efforts primarily on closely bonded pairs of dolphins who are constantly close together, and to date we have recorded data from more than 19 mother-calf pairs and 7 male alliances. In combination with vessel-based observations of behavior and social interactions, these accumulating datasets help us understand the social lives and acoustic communication of wild bottlenose dolphins.
At present, these data feed into a variety of research projects pertaining to foraging, energetics, and communication. Two student projects are looking into how pairs of bottlenose dolphins use whistles to maintain or re-establish acoustic contact during separations, how loud whistles are, and how far away functionally different whistle types can be detected. This is especially important to evaluate how vessel noise from recreational vessels affects communication range and information flow between dolphins in a heavily urbanized area such as Sarasota Bay, where a boat passes by dolphins on average once every 6 minutes.
Another ongoing avenue of research involves investigating how bottlenose dolphins use different types of signals during aggressive interactions, mainly between pairs of allied males and females or mother-calf pairs. As interactions become more aggressive, often involving brief chases, males may use high-amplitude resonant pops or jaw claps in addition to very fast sequences of echolocation clicks, called burst pulses. Surprisingly, males also seem to exchange repeated, low-frequency calls (called “quacks” due to their distinctive sound) as they are moving around females. Quacks are easy to separate from other signals, and likely help allied males coordinate activity and synchrony as they are trying to consort with a female. Many of the same sound types have been observed with dolphins under human care as well as in more extreme cases of aggression, including attempted infanticide involving a pair of bottlenose dolphin males and a newborn calf in Savannah, Georgia, confirming their general importance for aggressive interactions with others of the same species. Similar signals have been observed on acoustic monitors in other places around the world, but only thanks to the SDRP field site can we start to understand what the function of such signals may be.
This article appeared on page 11 of the 2017 SDRP Annual Report, Nicks n Notches.