Whistles as potential indicators of stress in bottlenose dolphins
By H. Carter Esch, MSc, Laela S. Sayigh, PhD, Randall S. Wells, PhD, and Jim Blum, PhD

Stress has been defined as an environmental effect on an individual that overtaxes its functional abilities. The diversity of stress responses among marine mammals makes it difficult to develop a comprehensive diagnostic protocol to evaluate stress. The development of a relatively non-invasive tool with which to evaluate stress in bottlenose dolphins (Tursiops truncatus) could allow for assessments of animals that may be at risk, and assessment of free-ranging animals without capture-release. The goal of this study was to evaluate whether vocalizations, specifically signature whistles, could serve as possible indicators of acute (or short-term) stress in bottlenose dolphins. Recordings made during brief capture-release events and during focal follows of undisturbed dolphins in Sarasota Bay, Florida, were used to address this question. Although there is no evidence that capture-release events have any long or short-term adverse impacts on members of the Sarasota dolphin community, it is likely that they are a source of short-term stress to the dolphins. We asked the following questions:

Will whistle rates and number of loops (repetitive elements in whistles) be greater:
(1) during capture-release than during undisturbed focal follows?
(2) at the beginning of a capture-release session than at the end of a session?
(3) during an individual’s first capture-release session than during later sessions?
(4) when a mother is caught and released with a dependent calf than without a dependent calf?

We also examined whether the duration of loops and/or inter-loop intervals, and maximum and minimum frequency of whistles change in any of the above contexts. Loop number was significantly higher during capture-release than during focal follows, and decreased significantly from the beginning to the end of an individual’s capture-release session. Loop duration was significantly shorter at the beginning than at the end of an individual session. Whistle rate was also significantly higher during capture-release than during focal follows, and during a dolphin’s first capture-release than during subsequent sessions. Females caught with a dependent calf produced whistles with significantly higher maximum frequencies and shorter inter-loop intervals than when caught and released without a dependent calf.

Whistle rate and loop number emerged from this study as the most promising potential vocal indicators of short-term stress in bottlenose dolphins. The findings of this project indicate that while significant patterns in whistle parameters relative to a dolphin’s involvement in capture-release operations can be detected, there is no indication of any long-term adverse impact. In fact, findings suggest that habituation occurs within a capture-release session, and from one session to the next. Based on the results of this study, further research would be warranted on assessing the utility of signature whistle rate and loop number as behavioral indicators of short-term stress in bottlenose dolphins. If the patterns in whistle parameters detected in this study persist once effects such as age and prior capture-release experience have been quantified, these measures could be utilized in conjunction with physiological indicators to ground-truth their reliability as indicators of various types of stressors in bottlenose dolphins.