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dolphin emitted signals. Bill’s work was groundbreaking echolocation research (Evans, 1967). Using Bill’s idea but in reverse, Moore developed the concept of “jawphones” to test dolphin interaural hearing by measuring interaural intensity and time differences. The first pair of jawphones used a Brüel & Kjær (B&K) 8103 miniature hydrophone positioned hori- zontally along the lower jaw of the animal for maximum ef- ficiency (Figure 6). This position was used because the path- way of sound to the ear in dolphins is through the lower jaw (Brill and Harder, 1991).
Heptuna had no issues with the jawphones because he was trained to wear them as eye cups to occlude his vision for past echolocation experiments. The jawphones were attached to Heptuna’s lower jaws, and subsequent thresholds for the pure-tone stimuli were determined. To assess Heptuna’s inte- raural intensity difference threshold, the level of the stimuli was set at a 30-40 dB sensation level (SL). The study used wideband clicks that were similar to dolphin echolocation clicks but that were more suited to the animal’s hearing and better represented signals that the animal would naturally encounter. Stimuli were set to a repetition rate corresponding to a target echolocated at 20 meters (40 ms). Using a modi- fied method of constants and a two-alternative forced-choice response paradigm, data were collected for both interaural intensity and time difference thresholds. The results clearly indicated that the dolphin was a highly sophisticated listener and capable of using both time and intensity differences to localize direct and reflected sounds (Moore et al., 1995).
Heptuna Moves to San Diego
In 1992, the Hawaii laboratory was closed and the personnel and animals moved to what is now the Space and Naval War- fare Systems (SPAWAR) Center in San Diego, CA.
Randy Brill, who was then working at SPAWAR, wanted to try to see if there were specific areas of acoustic sensitivity along the lower jaw of the dolphin and other areas around the head. The first thing Randy wanted was to collect thresholds from Heptuna and a second younger animal named Cascade. Using the matched jawphones, it was possible to collect in- dependent thresholds for both the right and left ears of both animals in the background noise of San Diego Bay.
The resulting audiograms for Cascade revealed well-matched hearing in both ears (Brill et al., 2001). However, the results for Heptuna were startling because they showed that Hep- tuna, now about 33 years old, had hearing loss in both ears, with a more substantial loss in his right ear. Furthermore, Heptuna now had a significant hearing loss above 55 kHz.
Figure 6. Heptuna wearing “jawphones” during one of Patrick Moore’s studies in the early 1990s.
In contrast, when Heptuna was tested at age 26 with the jaw- phones, his hearing was considered unremarkable because independent thresholds for his ears were closely matched for test frequencies of 4-10 kHz (Moore and Brill, 2001). These data for Heptuna are consistent with the findings of Ridgway and Carder (1993, 1997) showing that dolphins experience age-related hearing loss. Heptuna was another example of a male dolphin losing high-frequency hearing with age, a con- dition that is similar to presbycusis in humans and that is now known to be common in older dolphins (see the article in Acoustics Today by Anderson et al., 2018 about age-related hearing loss in humans).
The results of the free-field thresholds for Cascade at 30, 60, and 90 kHz provided additional support for the use of jawphones as a means to place the sound source in closer proximity to the animal and concentrate the source in a small, localized area. Jawphones have become a tool in the exploration of hearing in dolphins and are used in many ex- periments conducted at the US Navy Marine Mammal Pro- gram and other facilities and in the assessment of hearing in stranded and rehabilitating odontocetes.
Heptuna and Beam Control
Heptuna’s hearing loss notwithstanding, the investigators forged ahead to explore the idea that dolphins may control their emitted echolocation beam, allowing them to better detect targets. This involved animals free swimming in the open ocean as they echolocated. Using a new research device that could be carried by the dolphin, the Biosonar Measure- ment Tool (Houser et al., 2005), it was found that a dolphin could detect echoes from a target before the target entered to animal’s main echolocation beam.
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