Abstract

Hearing impairment in humans increases with age and is an important problem for older adults. In addition to age-related changes, there are also gender influences, with males being significantly more likely than females to experience hearing impairment, especially at increased ages.^1,3,4 Similar findings have been found among bottlenose dolphins.^2,5 Due to continually improving levels of medical care and husbandry, the number of geriatric dolphins at the Navy Marine Mammal Program is increasing, and there is a desire to better understand the physiology of dolphin aging to meet special needs of older animals.⁶ Hearing assessments were conducted on 73 bottlenose dolphins at the Navy using evoked potential audiometry. Hearing sensitivities were grouped into six categories: 0 = Normal (cutoff > 100 kHz), 1 = Very High Frequency Loss (70 kHz < cutoff < 100 kHz), 2 = High Frequency Loss (40 kHz < cutoff < 70 kHz), 3 = Mid Frequency Loss (cutoff < 40 kHz), 4 = Profound Hearing Loss (no brainstem response at most frequencies), 5 = Notched Hearing Loss (greater than 20 dB difference in sensitivity between sensitive regions of hearing range). Hearing categories were analyzed with age (mean ± SD = 22.1 ± 12.7 y) using a nonparametric Kruskal-Wallis one-way analysis of variance; the prevalence of normal and abnormal hearing was also compared among dolphins 1) less than 30 y old and 2) equal to or greater than 30 y old using a Chi-square test and calculated risk ratio. Significant differences were observed in mean ages among six hearing categories (0, 1, 2, 3, 4, 5) (p = 0.007), four hearing categories (0, 1, 2, 3–5) (p = 0.002), and two hearing categories (0 [normal], 1–5 [abnormal]) (p = 0.0002). Mean age for normal hearing levels was 16.4 y, while mean age for abnormal hearing levels was 28.2 y. Dolphins aged 30 years and older were 3.3 times more likely to have abnormal hearing than dolphins under 30 years of age. Hearing abilities were also analyzed comparing two hearing categories (normal, abnormal) with sex (M, F). Male dolphins were 1.5 times more likely to have abnormal hearing than female dolphins. Bottlenose dolphins are comparable to humans with regards to age- and sex-related hearing impairment. Similar parallels have been reported between dolphins and humans with regards to hematologic and serum chemistry values.⁷ These findings, and others, demonstrate the need for further study of geriatric dolphin health and its correlates with human health.

Acknowledgements

The authors would like to thank Dr. Sam Ridgway and the veterinary team of the National Marine Mammal Foundation, as well as Dr. Eric Jensen and the animal care and training staff at the US Navy Marine Mammal Program.

* Presenting author

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