Abstract:

Vocal patterns from two populations of common dolphin (Delphinus delphis) were compared. Field observations took place in the Channel Islands, CA and La Paz, MX. Whistles were recorded with a Sony HTI hydrophone (range=10--32 000 kHz) and schools were observed for group number and distance to the hydrophone by onboard observers. Sounds and spectrograms were analyzed for mean intensities in the (a) 15- to 20-kHz bandwidth, and (b) the ratio of the intensities from the 15- to 20-kHz band compared to the whole spectrum, 0--22 kHz. Significantly different (p<0.001) sound intensity levels were found between the La Paz and Channels Islands populations. When the 15- to 20-kHz band was examined, the values were 104.70(plus or minus)1.73 dB (mean (plus or minus)S.E.) and 86.80 (plus or minus)0.90 dB for the La Paz and Channel Islands, respectively. When the 15- to 20-kHz band was looked at as a function of the total energy in the 0 to 22-kHz band, significantly different energy levels were present as well (La Paz=76.2%, Channel Islands=67.9%). Therefore, the two populations were shown to be significantly different (p=0.001). This suggests that acoustic geographic variation is a useful tool for distinguishing populations within a species, and raises the possibility of external influences determining vocal patterns.