Helen (Carter) Esch, 2006 Joint Program Student
The welfare of an animal, or its state in relation to how it copes with its environment, is often related to the stress that it experiences. I have been examining the possibility that signature whistles of bottlenose dolphins may serve as indicators of stress.
Bottlenose dolphins produce an individually distinctive whistle which is called a signature whistle. The overall pattern of frequency changes of signature whistles tends to be highly stable. This stability makes signature whistles, as opposed to highly variable nonsignature whistles, ideal signals for the comparison of whistle features in these animals.
I recorded bottlenose dolphins in Sarasota Bay, Florida during brief capture–release events, which are potentially a source of short-term stress to these dolphins, although no effects of chronic or long-term stress have been observed. I analyzed whistles recorded during both brief capture–release and undisturbed, free-ranging conditions to determine whether whistle parameters differed during several scenarios, including capture–release versus undisturbed conditions; at the beginning of a capture–release session versus at the end of a session; during an individual’s first capture–release session versus later capture–release sessions; and when a mother with a dependent calf was caught and released versus an individual without a dependent calf (i.e., no dependent calf at the time of capture–release). I examined a variety of acoustic parameters, including whistle rate, number of loops (repetitive elements), maximum and minimum frequency, and loop, interloop, and whistle duration. I found that the whistle rate and number of loops were greater during brief capture–release events than during undisturbed conditions; the number of loops decreased and loop duration increased over the duration of a capture–release session; whistle rates decreased with number of capture–release sessions; and females caught and released with dependent calves produced whistles with higher maximum frequencies and shorter interloop intervals than when they did not have dependent calves.
Thus, whistles appear to have potential as noninvasive indicators of stress in bottlenose dolphins. Further research is warranted in this area, such as correlating physiological indicators to whistle rates under varying levels of stress. Reliable, noninvasive correlates of stress could be used to monitor dolphins in a variety of circumstances, such as during exposure to anthropogenic noise, or before and after a predicted weather event, such as a hurricane.