High-resolution measurement of a bottlenose dolphin's (<i>Tursiops truncatus</i>) biosonar transmission beam pattern in the horizontal plane

Dolphins fishing alone in open waters may whistle without interrupting their sonar clicks as they find and eat or reject fish. Our study is the first to match sound and video from the dolphin with sound and video from near the fish. During search and capture of fish, free-swimming dolphins carried cameras to record video and sound. A hydrophone in the far field near the fish also recorded sound. From these two perspectives, we studied the time course of dolphin sound production during fish capture. Our observations identify the instant of fish capture. There are three consistent acoustic phases: sonar clicks locate the fish; about 0.4 s before capture, the dolphin clicks become more rapid to form a second phase, the terminal buzz; at or just before capture, the buzz turns to an emotional squeal (the victory squeal), which may last 0.2 to 20 s after capture. The squeals are pulse bursts that vary in duration, peak frequency and amplitude. The victory squeal may be a reflection of emotion triggered by brain dopamine release. It may also affect prey to ease capture and/or it may be a way to communicate the presence of food to other dolphins. Dolphins also use whistles as communication or social sounds. Whistling during sonar clicking suggests that dolphins may be adept at doing two things at once. We know that dolphin brain hemispheres may sleep independently. Our results suggest that the two dolphin brain hemispheres may also act independently in communication.

Section: Discussionmentioning

confidence: 97%

Section: Study Subjectsmentioning

confidence: 99%

On doing two things at once: dolphin brain and nose coordinate sonar clicks, buzzes, and emotional squeals with social sounds during fish capture

Ridgway

Dibble

Alstyne

et al. 2015

“…Beam directivity has recently been shown in captivity to increase with target range in both delphinids (Finneran et al, 2014) and phocoenids (Wisniewska et al, 2015). In the wild, marine delphinids seem to follow the same overall pattern .…”

Section: Biosonar Beamwidth Broadens During Prey Capturementioning

confidence: 77%

Amazon river dolphins (Inia geoffrensis) modify biosonar output level and directivity during prey interception in the wild

Ladegaard

Jensen

Beedholm

et al. 2017

Toothed whales have evolved to live in extremely different habitats and yet they all rely strongly on echolocation for finding and catching prey. Such biosonar-based foraging involves distinct phases of searching for, approaching and capturing prey, where echolocating animals gradually adjust sonar output to actively shape the flow of sensory information. Measuring those outputs in absolute levels requires hydrophone arrays centred on the biosonar beam axis, but this has never been done for wild toothed whales approaching and capturing prey. Rather, field studies make the assumption that toothed whales will adjust their biosonar in the same manner to arrays as they will when approaching prey. To test this assumption, we recorded wild botos (Inia geoffrensis) as they approached and captured dead fish tethered to a hydrophone in front of a star-shaped seven-hydrophone array. We demonstrate that botos gradually decrease interclick intervals and output levels during prey approaches, using stronger adjustment magnitudes than predicted from previous boto array data. Prey interceptions are characterised by high click rates, but although botos buzz during prey capture, they do so at lower click rates than marine toothed whales, resulting in a much more gradual transition from approach phase to buzzing. We also demonstrate for the first time that wild toothed whales broaden biosonar beamwidth when closing in on prey, as is also seen in captive toothed whales and bats, thus resulting in a larger ensonified volume around the prey, probably aiding prey tracking by decreasing the risk of prey evading ensonification.

“…Further lab and field experiments should be performed to verify these results and to tease apart the nature of the relationship between beam width and range under different environmental conditions and sensory challenges. However, the increased field of view at short range is comparable to the increasing field of view of trained harbour porpoises (Wisniewska et al, 2012) and bottlenose dolphins (Finneran et al, 2014). This indicates that both phocoenids (family Phocoenidae, using narrow-band high-frequency signals) and delphinids (family Delphinidae, using broadband biosonar signals) employ a dynamic biosonar beam that allows them to expand their field of view when approaching objects or prey animals, and that these sensory adaptations seem to be important for animals in the wild.…”

Section: Angle Of Incidence (Deg Vertical)mentioning

confidence: 84%

“…Echolocating delphinids studied so far also demonstrate some control over their biosonar beam. Trained delphinids are capable of changing the source level (Moore and Patterson, 1983), frequency content (Moore and Pawloski, 1990) and directionality (Au et al, 1995) of their biosonar signals, and they control their field of view further by steering the beam direction and by controlling the width of the biosonar beam (Finneran et al, 2014;Moore et al, 2008). Most of these adjustable properties may be linked to changes in biosonar frequency, and it is possible that, like in bats, control over the biosonar field of view is primarily a by-product of frequency control.…”

Section: Introductionmentioning

confidence: 99%

Single-click beam patterns suggest dynamic changes to the field of view of echolocating Atlantic spotted dolphins (Stenella frontalis) in the wild

Jensen

Wahlberg

Beedholm

et al. 2015

Echolocating animals exercise an extensive control over the spectral and temporal properties of their biosonar signals to facilitate perception of their actively generated auditory scene when homing in on prey. The intensity and directionality of the biosonar beam defines the field of view of echolocating animals by affecting the acoustic detection range and angular coverage. However, the spatial relationship between an echolocating predator and its prey changes rapidly, resulting in different biosonar requirements throughout prey pursuit and capture. Here, we measured single-click beam patterns using a parametric fit procedure to test whether free-ranging Atlantic spotted dolphins (Stenella frontalis) modify their biosonar beam width. We recorded echolocation clicks using a linear array of receivers and estimated the beam width of individual clicks using a parametric spectral fit, cross-validated with well-established composite beam pattern estimates. The dolphins apparently increased the biosonar beam width, to a large degree without changing the signal frequency, when they approached the recording array. This is comparable to bats that also expand their field of view during prey capture, but achieve this by decreasing biosonar frequency. This behaviour may serve to decrease the risk that rapid escape movements of prey take them outside the biosonar beam of the predator. It is likely that shared sensory requirements have resulted in bats and toothed whales expanding their acoustic field of view at close range to increase the likelihood of successfully acquiring prey using echolocation, representing a case of convergent evolution of echolocation behaviour between these two taxa.