Masked underwater audiogram for a Pacific white-sided dolphin

Abstract

Little is known about abilities of odontocetes to hear in a noisy environment. Using a descending staircase method and go/no-go response, underwater hearing of a female, Pacific white-sided dolphin masked by two levels of white noise (30 dB apart) was measured. The same dolphin had an unmasked audiogram [Tremel et al. (1998)]. Frequencies were tested in octaves between 500 Hz and 64 kHz, with 15 to 30 trials/frequency. Masked thresholds at maximum noise were similar to those on a bottlenose dolphin [Moore and Au (1982)], a beluga [Johnson et al. (1989)], and a false killer whale [Thomas et al. (1990)]. At 8 kHz, masked thresholds were more sensitive than unmasked thresholds, perhaps because masking and pool noise were similar levels. The highest critical ratio (43) was at 500 Hz and the lowest critical ratio (14) at 8 kHz. Critical ratios at 8 kHz or higher were comparable to other species. However, critical ratios at 500 Hz (43) and at 1 kHz (40) were higher than in the beluga (20 and 18, respectively). [The authors are grateful for support by the marine mammal staff at the Shedd Aquarium and financial support by ONR.]