During the last 4 years, we have been tagging dolphins with suction-cup-mounted digital acoustic archival tags (DTAGs) at the end of a health assessment to monitor their movement and use of sounds in the wild. We have been especially interested in tagging pairs of individuals to help us investigate the social dynamics and communication of these animals. This season exceeded our best expectations, with 3 mother-calf pairs and 3 male-male pairs tagged, and individual tags lasting for periods of 1.5-22 hours. During the first part of each tag deployment, behavioral observations were collected to visually classify the behavior of the tagged animals, identify other dolphins in the area, and document social interactions with untagged dolphins.
These data are already providing us with an exciting view on group coordination. Some individuals in the Sarasota community form long-term associations such as male-male alliances (where pairs of males bond for decades) or mother-calf pairs (where the calf remains dependent on its mother for the first 3-5 years), bonds that require these animals to continually coordinate their movement and behavior. Tag sensors monitor the movement of both tagged animals over time, and we can use methods from information theory to quantify the influence that each animal exerts on the movement of the other animal. These measures provide us with a functional measure of leadership within an animal pair. Preliminary results show that bonded dolphins indeed coordinate their movement closely so as to remain as a cohesive social unit. The movement of mother-calf pairs seems to be predominantly governed by the mother, although with occasional brief leadership reversals. By digging into the acoustic information in the tag, we can now test whether changes in individual foraging motivation (inferred by eavesdropping on the echolocation clicks of each tagged animal) influences the inclination to follow another animal.
Another important factor in understanding the social communication of animals is the relative spatial location of individuals within a group. From acoustic tags, we can estimate the separation distance between simultaneously tagged individuals, but this requires that all animals are tagged and that they produce sounds. One of the tools we deployed this year that takes a different approach to this problem is a stereo geolocation system. This system uses a 3D camera system to sample the location of dolphins at every surfacing. Using this system, we collected accurate, GPS-referenced positions of tagged animals and other individuals that they interacted with during the behavioral observations. We can then link these spatial positions of animals with the acoustic information from tags to test how different social calls function to coordinate movement and gain a better functional understanding of bottlenose dolphin communication signals. For example, we plan to examine the movements of animals relative to one another when signature whistle copies are made; these whistle copies are a notable feature of the recordings of many of the pairs of tagged animals.
The DTAG data have also provided unprecedented insights into the use of non-whistle sounds, called pulsed sounds, which have thus far been largely ignored in studies of free-ranging dolphins (except for studies of their use in echolocation). We have noted a variety of different types of burst-pulse sounds on the tags; for example, “quacks” were so named based on their distinctive sound. “Quacks” share similarities with sounds found to be associated with aggressive contexts in several studies of dolphins under human care. When these sounds are interpreted in the context of the concurrent social and movement behaviors of the dolphins, we may gain rare insights into their communicative function. While we have still to analyze these data quantitatively, these sounds were observed primarily in the context of male alliances chasing females, supporting a primarily aggressive function in wild animals.
This article was published on page 9 in the November 2014 issue of Nicks n Notches