Conditioned hearing sensitivity reduction in a bottlenose dolphin (Tursiops truncatus)

Abstract

The conditioned change in hearing sensitivity during a warning sound preceding a loud sound was investigated in the bottlenose dolphin. Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. When the test/warning stimulus with a frequency of 22.5 or 32 kHz preceded the loud sound with a frequency of 22.5 kHz and a sound pressure level of 165 dB re. 1 μPa rms, hearing thresholds before the loud sound increased relative to the baseline. The threshold increased up to 15 dB. In order to further investigate whether the observed threshold increase was due to conditioning, the dependence of the effect on warning duration and inter-trial interval was investigated. The duration of the warning substantially influenced the effect. Shorter warnings resulted in deeper suppression of responses and higher threshold increases than longer warnings. In contrast, the effect was nearly independent of the duration of the inter-trial interval, i.e. it was independent of the delay from the loud sound to the test/warning sound in the subsequent trial. These data are considered as evidence that the observed hearing threshold increases were not a result of the unconditioned effect of the loud sound and were instead a manifestation of a conditioned dampening of hearing when the bottlenose dolphin anticipated the quick appearance of a loud sound in the same way as previously demonstrated in the false killer whale.

Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus)

“…Arguments in favor of the conditioned, rather than nonconditioned, nature of the observed hearing dampening were presented when the effect was previously described in a false killer whale (Nachtigall and Supin, 2013). Moreover, recently published data obtained in the same subject and under the same experimental conditions as in the present study have shown that the dependence of the effect on temporal inter-relations between the test/warning and loud sound indicates the conditioning nature of the observed dampening of hearing sensitivity (Nachtigall and Supin, 2014). The dampening effect did not depend on inter-trial intervals, i.e.…”

“…Therefore, for the conditioning sessions, these faint sounds preceding the loud one are referred to as test/warning stimuli, and the 5-30 s time of their presentation is referred to as test/warning time. The 5-30 s test/warning time was used because this rather short warning resulted in successful conditioned hearing dampening, whereas longer warning (15-75 s, mean 45 s) resulted in a less prominent effect (Nachtigall and Supin, 2014). Within the chosen limits of durations, trial-by-trial randomization of the test/warning time served to exclude the linking of a conditioning effect to a particular time after the warning signal onset.…”

“…Later this capability was also demonstrated in conditions when it might serve to protect the hearing from the action of a loud sound. It has been found that a false killer whale and a bottlenose dolphin Tursiops truncatus are capable of dampening their hearing when a loud sound is preceded by a warning faint sound (Nachtigall and Supin, 2013;Nachtigall and Supin, 2014). This in-advance damping of hearing had typical features of a conditioning effect and may therefore be an effective mechanism for hearing protection.…”

Effects of multiple impulses from a seismic air gun on bottlenose dolphin hearing and behavior

“…Given the orientation of the auditory bulla and air spaces in the head, the dolphins in the present study may have been positioning themselves in such a way as to shield their ears from the impulse and lower the effective received level. Anticipation and self-mitigation of an intense noise exposure has previously been demonstrated in a false killer whale (Nachtigall and Supin, 2013) and bottlenose dolphin (Nachtigall and Supin, 2014), although in these cases, no overt behavioral response was observed.…”

Can fear conditioning repel California sea lions from fishing activities?

“…A possible explanation for this is that the animals developed strategies to reduce hearing sensitivity that could not be observed behaviorally. In some cetacean species, for instance, the Pavlovian response to a warning signal preceding intense acoustic stimuli reduces hearing sensitivity (Nachtigall & Supin, , ), a possibility that has not been explored with pinnipeds. In contrast to previous studies, we used a relatively long CS tone that may have provided the animals with a relatively long warning period.…”