Whistles as Potential Indicators of Stress in Bottlenose Dolphins (Tursiops truncatus)

Abstract

We examined the possibility that parameters of bottlenose dolphin signature whistles may serve as indicators of stress. Bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida, were recorded 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 over the 37+-year duration of the research. Whistles recorded during both brief capture–release and undisturbed, free-ranging conditions were examined to determine whether whistle parameters differ during capture–release versus undisturbed conditions; at the beginning of a capture–release session versus at the end of a session; during an individual's 1st capture–release session versus later capture–release sessions; and when a mother is caught and released with a dependent calf versus without a dependent calf (i.e., she has no dependent calf at the time of capture–release). We examined a variety of acoustic parameters, including whistle rate, number of loops (repetitive elements), maximum and minimum frequency, and loop, interloop, and whistle duration. We found that whistle rate and number of loops were greater during brief capture–release events than during undisturbed conditions; 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, for example by correlating physiological indices 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.