Aude F. Pacini

Cognitive Adaptation of Sonar Gain Control in the Bottlenose Dolphin

Echolocating animals adjust the transmit intensity and receive sensitivity of their sonar in order to regulate the sensation level of their echoes; this process is often termed automatic gain control. Gain control is considered not to be under the animals cognitive control, but previous investigations studied animals ensonifying targets or hydrophone arrays at predictable distances. To test whether animals maintain gain control at a fixed level in uncertain conditions, we measured changes in signal intensity for a bottlenose dolphin (Tursiops truncatus) detecting a target at three target distances (2.5, 4 and 7 m) in two types of sessions: predictable and unpredictable. Predictable sessions presented the target at a constant distance; unpredictable sessions moved the target randomly between the three target positions. In the predictable sessions the dolphin demonstrated intensity distance compensation, increasing the emitted click intensity as the target distance increased. Additionally, as trials within sessions progressed, the animal adjusted its click intensity even from the first click in a click train, which is consistent with the animal expecting a target at a certain range. In the unpredictable sessions there was no significant difference of intensity with target distance until after the 7th click in a click train. Together, these results demonstrate that the bottlenose dolphin uses learning and expectation for sonar gain control.

Expectancy and conditioned hearing levels in the bottlenose dolphin (Tursiops truncatus)

ABSTRACT The hearing sensitivity of a bottlenose dolphin for a warning sound, when the exact time of the arrival of a loud sound could or could not be predicted, was measured. Sensitivity was measured when the time of onset of the loud sound was randomly varied (random-variation sessions) and when the time of onset of the loud sound and the pattern of stimulus levels was constant (fixed-stimulus sessions). The loud sound was kept the same in both of the series. The mean duration and mean range of the levels of the test/warning signal were also kept equal across experimental sessions. Hearing sensitivity was measured using the auditory evoked potential method with rhythmic trains of short pips as test stimuli. With randomly varied warning sounds, thresholds before the loud sound were on average 10.6 dB higher than the baseline thresholds. With fixed warning signals, thresholds were on average 4.4 dB higher than the baseline thresholds. Considering that the loud sounds were identical, the difference between the random-variation and the fixed-stimulus sessions cannot be explained by a direct (unconditioned) influence of sound exposure. Therefore, the data provide reliable evidence for the conditioning nature of the hearing-dampening effect and also demonstrate that hearing sensitivity change also depends on when the animal can expect the loud sound to occur. Summary: Dolphin hearing sensation levels, measured just before a loud sound, dampen more when the animal does not know when the loud sound will arrive than when the arrival time is fixed and predictable.

Evidence of hearing loss due to dynamite fishing in two species of odontocetes

Blast fishing is an illegal and unsustainable practice that is often reported in Southeast Asia and Africa. Its impact on fish and reef-building corals is well documented, yet there is limited information on the effects on other larger species and near-shore predators. In recent years, several marine mammal strandings in the Philippines have coincided with underwater explosions associated with blast fishing. The goal of this study was to measure the hearing of stranded dolphins, including two spinner dolphins (Stenella longirostris) and two rough-toothed dolphins (Steno bredanensis), that were rehabilitated in Subic Bay at Ocean Adventure in cooperation with the Philippines Marine Mammal Stranding Network and Wildlife in Need. Hearing measurements were conducted using noninvasive auditory brain stem responses (ABRs). Test stimuli consisted of tone pips ranging from 8 to 128 kHz. The results indicated elevated thresholds and limited hearing range, including three individuals with no hearing response beyond...

Conditioned hearing sensitivity change in the harbor porpoise (Phocoena phocoena).

Hearing sensitivity, during trials in which a warning sound preceding a loud sound, was investigated in two harbor porpoises (Phocoena phocoena). Sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. When a hearing test/warning stimulus, with a frequency of either 45 or 32 kHz, preceded a loud 32 kHz tone with a sound pressure level of 152 dB re 1 μPa root mean square, lasting 2 s yielding an sound exposure level (SEL) of 155 dB re 1 μPa(2)s, pooled hearing thresholds measured just before the loud sound increased relative to baseline thresholds. During two experimental sessions the threshold increased up to 17 dB for the test frequency of 45 kHz and up to 11 dB for the test frequency of 32 kHz. An extinction test revealed very rapid threshold recovery within the first two experimental sessions. The SEL producing the hearing dampening effect was low compared to previous other odontocete hearing change efforts with each individual trial equal to 155 dB re 1 μPa(2) but the cumulative SEL for each subsession may have been as high as 168 dB re 1 μPa(2). Interpretations of conditioned hearing sensation change and possible change due to temporary threshold shifts are considered for the harbor porpoise and discussed in the light of potential mechanisms and echolocation.

Four Odontocete Species Change Hearing Levels When Warned of Impending Loud Sound

Hearing sensitivity change was investigated when a warning sound preceded a loud sound in the false killer whale (Pseudorca crassidens), the bottlenose dolphin (Tursiops truncatus), the beluga whale (Delphinaperus leucas) and the harbor porpoise (Phocoena phocoena). Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. When the test/warning stimuli preceded a loud sound, hearing thresholds before the loud sound increased relative to the baseline by 13 to 17 dB. Experiments with multiple frequencies of exposure and shift provided evidence of different amounts of hearing change depending on frequency, indicating that the hearing sensation level changes were not likely due to a simple stapedial reflex.

Auditory sensitivity areas of head to local underwater stimulation in a bottlenose dolphin (Tursiops truncatus)

Sensitivity to the local underwater acoustic stimulation of the ventro-lateral head surface was investigated in a bottlenose dolphin (Tursiops truncatus). The stimuli were tone pip trains of carrier frequencies ranging from 32 to 128 kHz with a pip rate of 1 kHz. Auditory evoked potentials (the rate following responses) were recorded. For all the tested frequencies, a low-threshold region was revealed at the lateral side of the middle portion of the lower jaw. This result differed from that obtained in a beluga whale, Delphinapterus leucas (Popov et al., JASA 2016, 140: 2018) revealed a maximal sensitivity region next to the medial side of the middle portion of the lower jaw. The comparative analysis of these data and their extrapolation to all odontocetes in generally is discussed.Sensitivity to the local underwater acoustic stimulation of the ventro-lateral head surface was investigated in a bottlenose dolphin (Tursiops truncatus). The stimuli were tone pip trains of carrier frequencies ranging from 32 to 128 kHz with a pip rate of 1 kHz. Auditory evoked potentials (the rate following responses) were recorded. For all the tested frequencies, a low-threshold region was revealed at the lateral side of the middle portion of the lower jaw. This result differed from that obtained in a beluga whale, Delphinapterus leucas (Popov et al., JASA 2016, 140: 2018) revealed a maximal sensitivity region next to the medial side of the middle portion of the lower jaw. The comparative analysis of these data and their extrapolation to all odontocetes in generally is discussed.

Auditory sensitivity of various areas of the head to underwater acoustical stimulation in odontocetes

Sensitivity to the local underwater acoustic stimulation of the ventro-lateral head surface was investigated in a bottlenose dolphin (Tursiops truncatus). The stimuli were tone pip trains of carrier frequencies ranging from 32 to 128 kHz with a pip rate of 1 kHz. Auditory evoked potentials (the rate following responses) were recorded. For all the tested frequencies, a low-threshold region was revealed at the lateral side of the middle portion of the lower jaw. This result differed from that obtained in a beluga whale, Delphinapterus leucas (Popov et al., JASA 2016, 140: 2018) revealed a maximal sensitivity region next to the medial side of the middle portion of the lower jaw. The comparative analysis of these data and their extrapolation to all odontocetes in generally is discussed.

Transmission beam characteristics of a spinner dolphin (Stenella longirostris)

Transmission beam characteristics have been described in a small number of odontocete species, providing insight into the biological and ecological factors that have influenced the design of the outgoing echolocation beam. The current study measured the on-axis spectral characteristics and transmission beam pattern of echolocation clicks from a small oceanic delphinid, the spinner dolphin (Stenella longirostis). A formerly stranded individual was rehabilitated in captivity and trained to station underwater in front of a 16 element hydrophone array. Preliminary analysis of a subset of recorded clicks showed on-axis spectral characteristics with a mean center frequency of 68 kHz and a mean peak frequency of 52 kHz. The dolphin exhibited both a circular beam shape and also varying degrees of a dorso-ventrally compressed transmission beam. The mean angular beamwidth for all clicks was 16.6 and 14.3 degrees in the horizontal and vertical planes, respectively, though some clicks exhibited horizontal beamwidths ...

Echolocation detection of fishing hooks and implications for the Hawaiian longline fishery

Interactions between marine mammals and fisheries have a biological and economic impact that is often detrimental to both fishermen and species of concern. False killer whale bycatch in the Hawaii longline fishery has exceeded the potential biological removal (PBR) triggering the designation of a take reduction team under the Marine Mammal Protection Act (MMPA). As an attempt to understand the importance of acoustic cues in depredation events, this study presents preliminary data looking at the echolocation ability of a false killer whale (Pseudorca crassidens) to detect a longline fishing hook at various distances. Using a go/no-go paradigm, the whale was trained to report the presence of the hook at distances varying in 50 cm increments. A total of 28 sessions of 25 trials each were collected and echolocation signals were recorded using a nine element acoustic array. Number of clicks, acoustic parameters, decision time and performance were recorded. The subject successfully reported the presence of the ...

Hearing in Whales and Dolphins: Relevance and Limitations.

Understanding the hearing of marine mammals has been a priority to quantify and mitigate the impact of anthropogenic sound on these apex predators. Yet our knowledge of cetacean hearing is still limited to a few dozen species, therefore compromising any attempt to design adaptive management strategies. The use of auditory evoked potentials allows scientists to rapidly and noninvasively obtain the hearing data of species rarely available in captivity. Unfortunately, many practical and ethical reasons still limit the availability of large whales, thus restricting the possibility to effectively ensure that anthropogenic sounds have minimum effects on these species. The example of a recent Blainvilles beaked whale (Mesoplodon densirostris) audiogram collected after a stranding indicated, for instance, very specialized hearing between 40 and 50 kHz, which corresponded to the frequency-modulated upsweep signals used by this species during echolocation. The methods used during a stranding event are presented along with the major difficulties that have slowed down the scientific community in measuring the audition of large whales and the potential value in obtaining such results when successful.