Dynamics of Group Fission-Fusion: What is a Temporary Group in the Bottlenose Dolphin
By Ester Quintana-Rizzo, MSc, PhD student
University of South Florida

Group of closely spaced moms and calves in Sarasota Bay

    One of the objectives of my dissertation project is to evaluate the definition of a group for the bottlenose dolphin and to examine how dolphins communicate with each other during the formation and division of groups. I am currently working on the data analysis of 107 focal animal behavioral follows and 10 sound transmission experiments conducted in 2003. In particular, I am evaluating the definition of a group based on the traditional parameters of distance and activity, and of new parameters such as duration of an association and of communication range. The evaluation of distance as a parameter to define a group is important because some definitions consider some dolphin species to be members of a group if they are within 10 m (about 30 ft) of each other. However, my analysis shows that the mean distance of separation of associates from focal females is significantly greater than 10 m. The mean distance of separation of dependent calves of focal females was 82 m, whereas the mean distance of separation of other associates was 61 m. Other definitions consider dolphins to be members of a group if they are within a radius of approximately 100 m. However, dolphins that did not join a focal female were also observed in this radius. I called such individuals satellites and their mean distance from focal females was 100 m. This distance was significantly greater than the distance of separation of calves and other associates from focal females.

    Nevertheless, the range of distances in which satellites were observed overlapped with the distance of separations of associates (range = 40-200 m). Thus, if only the criterion of distance is used to define a group the presence of satellites would make it difficult to distinguish associates from satellites.

    I am also evaluating the use of activity to define a dolphin group because some definitions consider dolphins to be a member of a group if they are involved in similar activities. Preliminary analyses suggest that activity alone is not a good predictor of group membership. Coordinated activities between focal females and dependent calves were frequently observed when the distance of temporary separations was equal to or less than 100 m. This may be related to the observer’s limited capability to record activities at greater distances. At distances equal or less than 100 m, focal females were observed traveling and probably feeding. At greater distances, focal females were probably feeding and milling whereas their dependent calves were traveling. It is possible that calves were also feeding at greater distances but that such activity was difficult to record when the calf was far away from the research vessel. In the case of non-calf associates, their activity was similar to that of focal females when the mean distance of separation was 60 m. However, contrary to the activity of dependent calves, other associates temporarily separated from focal females were traveling. The results from the distance and activity to evaluate the definition of a dolphin group were presented at the 41^st Annual Animal Behavior Meeting, June 12-16, 2004, Oaxaca, Mexico. Field work and my studies at USF have been supported by a several funding agencies: NOAA Fisheries, The Chicago Zoological Society, the USF Acoustic Laboratory, the USF Physiology Laboratory, the USF Jack Lake Fellowship, and the USF Garrels Fellowship.