Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

David A. Mann

Keywords

auditory evoked potential (AEP), beam pattern, bottlenose dolphin, pilot whale, Risso's dolphin, spotted dolphin

Abstract

Odontocetes use echolocation to detect, track, and discriminate their prey, as well as negotiate their environment. Their hearing abilities match the frequency of greatest sensitivity to the higher frequencies used for foraging and navigation. Hearing and echolocation together provide odontocetes with a highly developed biosonar system. This dissertation examines the hearing ability of several odontocete species to understand what signals they can perceive during echolocation. The variability in hearing ranges between species is examined in the context of phylogenetic and ecological differences among taxa. An autonomous hydrophone array is also developed that could be used in an expanded form in field deployments to study echolocation signals in a wider range of species.

Methods for measuring hearing sensitivity include both psychophysical and electrophysiological procedures. Behavioral methods require a large time commitment, for both training and data collection, and can only be performed on captive dolphins. Auditory evoked potential (AEP) methods are non-invasive, rapid measurements of the brain's response to sound stimuli and allow for audiograms to be collected on stranded, high risk dolphins. By determining the hearing abilities of odontocetes either in captivity or during stranding, data can be collected about inter- and intraspecies variability, and the occurrence of hearing impairment. It can also be used as another diagnostic tool to determine the releasability of a stranded animal.

A juvenile male short-finned pilot whale (Globicephala macrorhynchus) that stranded in Curacao had severe hearing impairment at all frequencies tested. Four female short-finned pilot whales tested had the best sensitivity at 40 kHz. The juveniles had greater high frequency sensitivity than the adult pilot whales. Cutoff frequencies were between 80 and 120 kHz.

Hearing sensitivity was determined for the two mother/calf pairs of Risso's dolphins (Grampus griseus) before and after antibiotic treatment in order to measure any potential effects of antibiotic treatment. Greatest sensitivity occurred at 40 kHz and cutoff frequencies were around 120 kHz for all dolphins tested. Changes in hearing sensitivity after antibiotic dosage were 12 dB or less in all cases except one. The adult female Betty showed a threshold shift at 120 kHz of 54 dB from May to June, which partially demonstrates the presence of an ototoxic effect at one frequency. Dosages of antibiotics during drug treatment detailed in this study should be considered safe dosages of antibiotics for Risso's dolphins.

AEP and behavioral methods were used to collect audiograms for three Stenella spp. dolphins. The frequency of best hearing for the Atlantic spotted dolphin and the spinner dolphin was 40 kHz, and their upper cutoff frequencies were above 120 kHz. The pantropical spotted dolphin had the greatest sensitivity at 10 kHz, and had severe high frequency hearing loss with a cutoff frequency between 14 and 20 kHz.

Comparisons of high frequency hearing sensitivities among the species tested show two distinct groups. Short-finned pilot whales and Risso's dolphins have a cutoff frequency below 120 kHz, whereas Stenella spp. dolphins have cutoff frequencies above 120 kHz. Expanding the comparison to include other species, killer whales, pygmy killer whales, false killer whales, and long-finned pilot whales also have cutoff frequencies below 120 kHz. Common bottlenose dolphins, white-beaked dolphins, Indo-Pacific humpback dolphins, rough-toothed dolphins, and common dolphins have cutoff frequencies above 120 kHz. Genetic evidence exists for two subfamilies within Delphinidae (Vilstrup et al., 2011) and those species with cutoff frequencies below 120 kHz belong to the subfamily Globicephalinae and those species with cutoff frequencies above 120 kHz belong to the subfamily Delphininae.

An autonomous, field-deployable hydrophone array was developed to measure free-swimming echolocation. The array contained 25 hydrophones, two cameras, and a synchronization unit on a PVC frame. The distinct click train was used to time-align all 25 channels, and the light was used to synchronize the video and acoustic recordings. Echolocation beam patterns were calculated and preliminary evidence shows a free-swimming dolphin utilizes head movement, beam steering and beam focusing.

Among all areas of cetacean biology more research is necessary to gain a clearer picture of how odontocetes have adapted to function in their acoustic environment. The array system developed can be used to study how dolphins use echolocation in the wild, the impacts of anthropogenic sound on echolocation production, and the potential consequences of high frequency hearing loss.

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