Group dynamics and estimated communication range of social sounds used by wild bottlenose dolphins during fission-fusion events
By Ester Quintana-Rizzo, PhD candidate, University of South Florida

Two of the objectives of my dissertation project are to evaluate the definition of “group” for bottlenose dolphins, and to estimate the communication range of social sounds used by dolphins during the formation and division of groups. Bottlenose dolphins exhibit flexible associations in which the composition of temporary groups changes frequently as partners join and leave in a fluid manner. The definition of a group was evaluated using parameters such as distance. This is important because some definitions consider some Delphinid species to be members of a group if they are within 10 m 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 relative to their mothers 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 in a radius of approximately 100 m. However, in this study dolphins that did not join a focal female were also observed in this radius. Those individuals are called satellites. Thus, if only a criterion of distance is used to define a group the presence of satellites would make it difficult to distinguish associates from satellites.

To find distant partners, dolphins use whistles when separated. I quantified the communication range of whistles to determine if they can be heard by dolphins at the distances of natural separations of mothers and their dependent calves in Sarasota Bay, Florida. This information is important to understand the role that acoustic communication may play during the formation and division of group. We conducted sound transmission experiments in two habitats (shallow water areas and channels). Results showed that sound propagation varied with habitat type, bottom type, depth, and sound source level. Sounds were more attenuated in areas with seagrass bottom than any other bottom type. Using data from propagation measurements and background noise measurements, we estimated the distance a hypothetical dolphin whistle could be detected in different habitats. In a seagrass shallow water area with mean water depth equal to 1.57 m, a loud whistle (source level = 165 dB) ranging in frequency from 5-19 kHz can travel and still be heard by a dolphin at 150 m. In a shallow area with mud bottom and mean depth equal to 1.4 m, all whistle frequency components of the same whistle can travel up to 2 km and still be heard by a dolphin. In channels, whistles could be detectable over a much larger range (>10 km). Our findings suggest that in Sarasota Bay, the communication range of social sounds is much greater than the distance of mean separations of mothers and calves. Ecological pressures might play an important role in determining the distance of separation between mothers and calves. This research is part of my doctoral dissertation project at the College of Marine Science, University of South Florida. Field work and my studies at USF have been supported by a several funding agencies: NOAA Fisheries, the USF Acoustic Laboratory, the USF Physiology Laboratory, the USF Jack Lake Fellowship, and the USF Garrels Fellowship.