Time of Day and Social Change Affect Underwater Sound Production by Bottlenose Dolphins (Tursiops truncatus) at the Brookfield Zoo

Abstract

Abstract This study investigated diel changes in ambient noise levels and the number of whistles produced by bottlenose dolphins ( Tursiops truncatus ) at the Brookfield Zoo in Brookfield, Illinois. Automated, continuous 24-h underwater recordings were made from 1 January to 31 March 2008. The number of whistles, types of whistles, and background noise level were examined for each hour. Nine distinct frequency contours were identified, named, and analyzed for minimum frequency, maximum fre-quency, peak frequency, and duration. Since all pumps and filters at the Seven Seas Exhibit of Brookfield Zoo were housed in a sepa-rate building isolated from the dolphins’ pools, background noise was relatively low and consistent throughout the day (95 to 98 dB re: 1 µPa). However, when the zoo staff used a scrubber to clean the pool walls, the background noise was higher and fluctu-ated (up to 112 dB re: 1 µPa). The dolphins whistled significantly less during these scrubbing periods. The dolphins exhibited a distinct diel pattern in whistle production. Increased whistle production coincided with increased interactions with humans during feeding/training sessions; the number of whistles peaked in the late afternoon (~1600 h) and then quickly tapered off throughout the night. The investigation began with eight dolphins; however, the death of one young male and the transport of two adult males to another facility left five dolphins: two adult females and two juve-nile females along with an unrelated young male. These changes provided an opportunity to explore how social change affected whistle production. After the two adult males were transported out of the facility, two of the distinct whistle types disappeared, suggesting that each of the two dol-phins had a unique whistle type. The results of this investigation highlight the usefulness of passive recording for monitoring ambient noise, as well as for documenting the activity pattern and social interactions of captive bottlenose dolphins.