Bottlenose dolphins are unusual among non-human mammals in their ability to learn new sounds. This study investigates the importance of vocal learning in the development of dolphin signature whistles and the influence of social interactions on that process. We used focal animal behavioral follows to observe six calves in Sarasota Bay, Fla., recording their social associations during their first summer, and their signature whistles during their second. The signature whistles of five calves were determined. Using dynamic time warping (DTW) of frequency contours, the calves' signature whistles were compared to the signature whistles of several sets of dolphins: their own associates, the other calves' associates, Tampa Bay dolphins, and captive dolphins. Whistles were considered similar if their DTW similarity score was greater than those of 95% of the whistle comparisons. Association was defined primarily in terms of time within 50 m of the mother/calf pair. On average, there were six dolphins with signature whistles similar to the signature whistles of each of the calves. These were significantly more likely to be Sarasota Bay resident dolphins than non-Sarasota dolphins, and (though not significantly) more likely to be dolphins that were within 50 m of the mother and calf less than 5% of the time. These results suggest that calves may model their signature whistles on the signature whistles of members of their community, possibly community members with whom they associate only rarely.

Little is known about the social organization of bottlenose dolphins (Tursiops truncatus) inhabiting open estuarine systems. Our purpose was to identify and quantify the resightings of bottlenose dolphins and associations formed among them to provide insights into the social organization of the species in the open estuarine system of the Cedar Keys, Florida. We conducted monthly photographic-identification surveys from June 1996 through May 1997. We identified a total of 233 dolphins of which 217 (93%) were classified into one of four categories based on the number of months in which they were sighted. "Rare" dolphins (sighted during 1–2 months) constituted more than half of these individuals (61%), followed by "occasional" (3–5 months; 20%), "common" (>8 months; 12%), and "frequent" (5–6 months; 7%) dolphins. We quantified the associations for dolphins sighted 10 times (26 common, 12 frequent, and 8 occasional) and found that individuals with different resighting patterns associate with each other but the association is mostly at a low level. The level of association among common dolphins, however, was higher within than between the sexes. Association patterns of common dolphins in the Cedar Keys resemble the associations of resident bottlenose dolphins inhabiting closed estuarine bays, suggesting that the selective pressures which induce such associations are the same in these aquatic systems.

Bottlenose dolphins, Tursiops truncatus, exhibit flexible associations in which the compositions of groups change frequently. We investigated the potential distances over which female dolphins and their dependent calves could remain in acoustic contact. We quantified the propagation of sounds in the frequency range of typical dolphin whistles in shallow water areas and channels of Sarasota Bay, Florida. Our results indicated that detection range was noise limited as opposed to being limited by hearing sensitivity. Sounds were attenuated to a greater extent in areas with seagrass than any other habitat. Estimates of active space of whistles showed that in seagrass shallow water areas, low-frequency whistles (7-13 kHz) with a 165 dB source level could be heard by dolphins at 487 m. In shallow areas with a mud bottom, all whistle frequency components of the same whistle could be heard by dolphins travel up to 2 km. In channels, high-frequency whistles (13-19 kHz) could be detectable potentially over a much longer distance (> 20 km). Our findings indicate that the communication range of social sounds likely exceeds the mean separation distances between females and their calves. Ecological pressures might play an important role in determining the separation distances within communication range.