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led Earl to suggest that Heptuna (and by extension other dol- phins) had the ability to focus his echolocation attention on a segment of time that encompassed the returning target echo, allowing the dolphin to ignore sounds before and after the echo (Murchison, 1980).
When Earl finished his range resolution studies, Heptuna be- came available for other research. Whit Au wanted to contin- ue to explore Heptuna’s hearing abilities. Because Whit and Patrick Moore had worked together on earlier experiments involving sea lion sound source localization (Moore and Au, 1975), they teamed with Heptuna to better understand his hearing using receiving models from classic sonar acoustics. This began a multiyear research effort to characterize Heptu- na’s hearing (e.g., Moore and Au, 1983; Au and Moore, 1984; Branstetter et al., 2007).
The first task was to collect masked hearing thresholds at un- explored high frequencies and compute critical ratios and critical bands (Moore and Au, 1983). Armed with these data, it was possible to start a series of experiments to measure Hep- tuna’s horizontal- and vertical-receiving beam patterns (Au and Moore, 1984). This was the first attempt to quantify the receiving beam pattern for an echolocating marine mammal. The investigators used a special pen with a 3.5-meter arc that filled two sides of a 9-square-meter pen. Heptuna stationed at the origin of the arc on a bite plate and was required to remain steady during a trial. Having Heptuna grab the bite plate was easy as this was something he had done in earlier studies using a chin cup. Still, having Heptuna transition to the new 1.5-me- ter depth of the bite plate proved to be a challenge. Heptuna did not like the aluminum pole used to suspend the bite plate above his head to hold it in position for the horizontal mea- surements. Wild-born dolphins avoid things over their heads, which is why it is necessary to train them to swim through underwater gates with overhead supports.
Once Heptuna was satisfied that the overhead pole was not going to attack him, the experiment began. The arc in the pen allowed positioning of the signal source about Heptuna’s location in 5° increments. For the horizontal beam measure- ments, two matched noise sources were placed at 20° to the animal’s left and right and the investigators could move the signal source. During all of the testing, Heptuna’s stationing was monitored by an overhead television camera to ensure he was correctly stationed and unmoving.
After data acquisition for the horizontal beam was finished, Heptuna moved to the vertical beam for measurements. Again, Heptuna proved to be a dolphin of habit. He tried ev-
ery possible way to station on the vertical bite plate just as he had before on the horizontal bite plate except turning on his side. This was perplexing because this was a behavior that Donna McDonald had used in her study (it was found later that he twisted in the opposite direction for Donna). The is- sue was overcome by slowly rotating the bite plate from the horizontal position to the vertical position over several train- ing sessions and then Heptuna started the vertical measure- ments. From these data, it was possible to compute Heptu- na’s directivity index and model a matched pair of receiving transducers that had the same directivity as the animal. Two major observations of the beam patterns were that as the fre- quency increased, the receiving beam became more narrow (a similar result was found in the bat; Caspers and Müller, 2015) and that the receiving beam was much broader and overlapped with the transmit beam (Au and Moore, 1984).
Heptuna and Controlled Echolocation
Moore’s observations over several echolocation experiments found a wide variation in clicks, some with a high source level but with a lower frequency and vice versa. The ques- tion became, “does the dolphin have conscious control over click content?” Can he change both the level and frequency of the click as needed to solve an echolocation problem or is it a fixed system so that as the dolphin increases the level, the peak frequency also increases?
Again, Heptuna was chosen to help answer this question. This began a very difficult and long experiment involving echolocation. Heptuna took part in a series of experiments designed to tease out if the dolphin actually had cognitive control over the fine structure of the emitted click. Dr. Ron- ald Schusterman had already demonstrated tonal control over a dolphin’s echolocation click emission. Schusterman et al. (1980) trained a dolphin to echolocate on a target only when a tone stimulus was present and to remain silent when it was not. This experiment started with the attempt to place both Heptuna’s echolocation click source level and frequency content under stimulus control while he was actively detect- ing a target echo.
Heptuna had to learn to station on a bite plate and then place his tail on a tail rest bar behind him, close to his fluke. This stationing procedure was necessary to ensure that Heptuna was stable and aligned with the click-receiving hydrophone, ensuring on-axis sampling of his clicks. Heptuna found this new positioning not at all to his liking. And much like the vertical bite plate with the beam pattern measurements issue mentioned in Heptuna and Echolocation Studies, Heptuna
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