Data from behavioral observations and acoustic recordings of free-ranging
bottlenose dolphins (Tursiops truncatus) were analyzed to determine whether signature
whistles occur in the wild, and to determine when whistle crystallization occurs in calves.
The study animals were part of a resident community of bottlenose dolphins in the waters
near Sarasota, Florida. Three mother-calf pairs and their associates were recorded for a
total of 141.25 hours between May and August of 1994 and 1995. Each whistle was
conservatively classified into one of four categories: signature, probable signature,
upsweep, or other. Overall whistle production varied significantly with both activity and
group size. Whistles occurred more frequently than expected during socializing and in
groups of over five dolphins, and occurred less frequently than expected during travelling
and in groups of two dolphins. Signature and probable signature whistles made up
approximately 52% of all whistles produced by these free-ranging bottlenose dolphins.
Each of the three calves studied developed its whistle prior to four months of age.
Each mother’s signature whistle comprised approximately 3% of the whistles recorded
during the first year of her calf’s life, regardless of social or auditory environment, which
varied greatly among the calves. One calf developed a signature whistle similar to that of
its mother.
To evaluate whether certain acoustic features of whistles are perceptually
important to dolphins, 44 playback experiments were conducted with temporarily
captured dolphins near Sarasota, Florida in June of 2000 and 2001. Response measures
included number of head turns toward or away from the speaker, echolocation bouts, and
whistles. In a modified habituation-dishabituation design, habituation to the first stimulus did not occur. Additionally, there were low overall response rates to most synthetic
stimuli. There were no significant differences in responses to whistles containing
differing numbers of loops or differing contours, with the exception that head turn rates
were higher in response to modified synthetic upsweeps than to several other synthetic
stimuli. In addition, whistle response rates were significantly higher to known, natural
whistle stimuli than to synthetic upsweeps. These results suggest that natural whistles and
modified upsweeps contain whistle features not adequately conveyed by other synthetic
stimuli
