History of Dolphin Biosonar Research

History of Dolphin Biosonar Research Whitlow W. L. Au Research into the most sophisticated short range sonar. Postal: Discovery of Biosonar in Animals Marine Mammal Research Program Hawai’i Institute of Marine Biology Research in biosonar can be traced back to the work of the Italian scientist Lazzaro University of Hawai’i Spallanzani who, in the 1770s, observed that bats could fly freely in a dark room. P.O. Box 1346 However, it was not until Pierce and Griffin (1938) detected, using an ultrasonic Kaneohe, Hawai’i, 96744 detector and a superheterodyne receiver, biosonar signals of the little brown bat (Myotis lucifugus) and big brown bat (Eptesicus fuscus), confirming the notion that USA Figure 1. First behavioral audiogram of an Atlantic bottlenose dol- Figure 2. Example of representative biosonar signals used by bottle- ultrasonic signals were involved in echolocation. phin (Tursiops truncates; the “Johnson curve”) along with a human nose dolphins in concrete tanks and in open waters (Kaneohe Bay). Email: audiogram (solid line). Symbols are different types of sound projec- The waveforms are shown in the top panels and the frequency spectra [email protected] In 1947, Arthur McBride, the first curator of Marine Studios (later Marineland, Florida) noted that bottlenose dolphins (Tursiops truncatus)1 were able to avoid tors used in the study. Adapted from Johnson (1966). and peak-to-peak sound pressure levels (in dB re 1 μPa) are shown in the bottom panel. From Au and Hastings (2008), with permission nets and find openings in enclosing nets at night and in murky waters, leading from Springer-Verlag. him to suspect that bottlenose dolphins had a biosonar capability (McBride, 1956). Two Decades of Biosonar Research Other investigators such as Winston Kellogg, Forrest Wood, and the husband-and- (1973–1993) wife team of William Schevill and Barbara Lawrence conducted the preliminary In 1968, the Naval Ocean System Center (NOSC) opened a target. Again, the results from the noise-masking experi- investigations on dolphin biosonar (Schevill and Lawrence, 1953). field station in Hawai’i on the Kaneohe Marine Corps Air ments were consistent with range detection experiments. Kenneth Norris and colleagues provided the first conclusive demonstration of Station that extended into Kaneohe Bay. The mere fact that Another important experiment measured the discrimina- bottlenose dolphin echolocation by placing rubber suction cups over the eyes of a bottlenose dolphins could project their signals into a large tion acuity of an echolocating dolphin. Cylinders of the bottlenose dolphin and found that it continued to swim normally through a maze bay without tank walls to reflect their signals made a tre- same height (17.8 cm) and outer diameter (3.81 cm) but with of vertically hanging pipes and other obstacles while emitting clicks (Norris et mendous difference in how they used their biosonar. Team- varying wall thicknesses were used. The standard cylinder al., 1961). They also speculated that the sonar sounds were directional and were ing up with Robert Floyd, Ralph Penner, and Arthur “Earl” had a wall thickness of 6.35 mm. Two targets (one standard projected from the top of its forehead (melon). Norris next showed that a Pacific Murchison, we were the first to measure biosonar clicks that and one nonstandard) separated by 22° in azimuth at a range white-sided dolphin (Lagenorhynchus obliquidens) also possessed a biosonar capa- had peak frequencies as high as 120 kHz, which was an oc- of 8 m were presented to a bottlenose dolphin stationed in bility. In 1967, two French scientists trained a blindfolded harbor porpoise (Pho- tave higher than signals measured in concrete tanks (Au, a hoop. The bottlenose dolphin was required to locate the coena phocoena) to swim through a maze of vertically hanging wires (Busnel and 1993). We also measured peak-to-peak source levels as high standard target. The cylinders had the same target strength, Dziedzic, 1967). as around 227 dB re 1 µPa, which was over 40 dB (100 to 1,000 times) higher than previously in-tank measurements with the only difference being the wall thickness of the cyl- Most of the early biosonar experiments were conducted by scientists in the US (Figure 2). inders. The results of the experiment are shown inFigure 3. Navy Marine Mammal Program. C. Scott Johnson was the first to apply a rigorous At the 75% correct performance threshold, the bottlenose psychophysical procedure to study hearing in Tursiops (Johnson, 1966). He found In the 1970s and 1980s, we found that bottlenose dolphins dolphin was able to discriminate a difference of -0.23 mm that Tursiops could hear sounds from 100 Hz to 150 kHz (Figure 1), the widest use a pulse system in which they adjusted the repetition rate and 0.27 mm. We also measured the biosonar beam in both frequency range for any mammal. He also performed a masked hearing study to of outgoing clicks so that the echoes were received before the horizontal and vertical planes for several bottlenose dol- determine the critical ratio at different frequencies and demonstrated that the bot- the next click was emitted. We also conducted experiments phins and obtained broadband 3-dB beam widths between tlenose dolphin integrated acoustic energy in a manner similar to most mammals. to determine the biosonar detection range of Tursiops. The 10 and 11° in both planes. Forrest Wood and William Evans used suction-cup blindfolds to demonstrate distance a bottlenose dolphin could be to detect a target at echolocation in a Pacific pilot and killer whales (Wood and Evans, 1980). It soon a 50% correct threshold was 113 m for a 7.62-cm-diameter The receiving beam was also measured at 120, 60, and 30 become accepted that odontocetes (toothed whales) that emitted clicks were echo- water-filled stainless steel sphere and 93 m for a 2.54-cm- kHz in the vertical and horizontal planes by Patrick Moore locating. The early history of the Navy program was summarized in the book The diameter solid steel sphere. These two results with differ- and myself. The frequency of 120 kHz matched the peak fre- Dolphin Doctor by Sam H. Ridgway, the primary veterinarian in the Navy Marine ent sized spheres agreed within 1.5 dB when the difference quency of typical biosonar signals used by our bottlenose Mammal Program (Ridgeway, 1987). in target strength was considered (Au, 1993). Experiments dolphins. The receiving beam pattern at 120 kHz was slightly were also conducted with the 7.62-cm sphere at different larger than the transmitting beam pattern (Figure 4). The 1 Note about the use of animal names. The term “dolphin” refers to all members of the family fixed ranges in the presence of a broadband masking noise received beam got larger as the frequency decreased in a Delphinidae, a group within the odontocete (toothed whales). When a specific species of between a bottlenose dolphin stationed in a hoop and the manner similar to a standard planar hydrophone. dolphin is referred to, a full common name (e.g., bottlenose dolphin) is used. 10 | Acoustics Today | Fall 2015 | volume 11, issue 4 ©2015 Acoustical Society of America. All rights reserved. Fall 2015 | Acoustics Today | 11 History of Dolphin Biosonar Research by digitizing the animal’s outgoing biosonar signal, convolv- ing each click with the previously collected transfer function of different targets, and playing the simulated echoes back to the dolphin. This technique provided us with another tool by which “targets” could be manipulated and used to investigate biosonar processing by echolocating bottlenose dolphins. My student Stuart Ibsen then used the phantom echo technique and bandlimited echoes to determine what Figure 4. Transmitting and receiving beam patterns of a bottlenose dolphin in the vertical (left) and horizontal (right) planes. The darker part of target echoes were used by bottlenose dolphins in pattern represents the transmitting beam. From Au (1993). discrimination between water-filled steel and brass spheres. Using a bottlenose dolphin that emitted clicks with a center Figure 5. Circles with numbers are relative decibel values of points area of the lower jaw. frequency close to 35 kHz, he found that a discrimination of stimulation. The larger the number in each circle, the higher the threshold occurred when the phantom echoes were band- sensitivity. The point marked “38” in white is the point of maximum Ted Cranford pioneered the use of X-ray computer tomog- passed by 13 kHz (Ibsen et al., 2009), which agreed with the sensitivity. The point marked NR had no response and X is the point raphy (CT) and magnetic resonance imaging (MRI) scan- of the minimum delay. The arrows refer to the sensitivity values on Figure 3. Bottlenose dolphin wall thickness discrimination perfor- 10-dB bandwidth of the dolphin auditory filter that we mea- ners to examine the acoustic structures in a dolphin head the midline under the jaw. The suction cup with the lead zirconate mance as a function of wall thickness difference. Thinner and Thicker sured later (Lemond et al., 2012). (Cranford et al., 1996). He showed where the low-velocity titanate (pzt) crystal is shown in the lower right panel. From Mϕhl are relative to the standard wall thickness. From Au (1993), with per- et al. (1999). mission from Springer-Verlag. core of the melon was situated geometrically. He also found Another important project was the auditory pathway mea- a small pair of fatty bursae (monkey lips-dorsal bursae com- surements conducted with Bertel Møhl (University of Aar- plex) “embedded in a pair of connective tissue lips, a carti- hus, Denmark). A bottlenose dolphin was trained to beach false killer whale was able to decrease its hearing sensitivity laginous blade, a stout ligament, and an array of soft tissue on a mat and we measured the auditory brainstem response Moore and I also measured the critical bandwidth of a when a loud sound was preceded by a precursor (Nachtigall air sacs” within the nasal structures of many odontocetes (ABR) of the animal as we played clicks through a piezo- bottlenose dolphin and found that the ratio of the critical and Supin, 2013).

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