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Zoo ResearchClouded Leopard Audiology Study

Rapid response times to low frequency sounds in clouded leopards: Are we looking at an inner ear adaptation?

In order to assess the influence of environmental noise on the behavior of clouded leopards housed at the Nashville Zoo at Grassmere, the hearing of eight healthy, young adult cats was tested using an auditory evoked potential known as the auditory brainstem response (ABR). The ABR is recorded non-invasively and is commonly used to screen newborn human hearing and assess auditory function in non-human animals. As a prelude to the planned acoustic ecology study, hearing sensitivity was evaluated using the ABR to determine if clouded leopards exhibit unusual sensitivity to any portion of their audiometric frequency range. Although there was no evidence of hearing specialization, “clouds” did appear to be highly sensitive to relatively low frequency sounds, a finding that may have daily management implications for this species.

Although the primary purpose of this investigation is to understand the nature of the clouded leopard’s auditory experience at Grassmere, this research extended our understanding of felid biology, and clouded leopard biology in particular. We previously described an unusual response timing trait in the tiger (Panthera tigris) in which response times to relatively low-frequency sounds were shown to be paradoxically fast, and curves relating response timing to stimulus frequency were generally unlike those observed in other mammals studied thus far. Specifically, curves representing tigers were non-monotonic (shaped like an inverted ‘U”) and curves representing all other mammal species thus far studied are monotonically negative; i.e., responses to low frequencies occur later than those to higher frequency sounds.

This finding strongly suggested that some feature of the inner ear of the tiger had undergone adaptation. The possibility that this adaptation may appear in other, if not all members of the Pantherinae subfamily, was supported when similar preliminary findings were observed in the jaguar (Panthera onca). The prospect was further reinforced following this recent in-depth study of the relationship in the clouded leopard (Neofelis nebulosa). The main finding from this ongoing investigation is that remarkably rapid responses to relatively low frequency sounds are as evident in clouded leopards as in tigers, and the similarity of response latency vs. stimulus frequency curves spanning a frequency range between 1-2 kHz and 32 kHz is striking.

Comparative preliminary findings from 12 species representing six of the eight recognized cat lineages (Walsh et al., J. Acoust. Soc. Am. 129: 2447, 2011) suggest a fairly provocative notion. None of the smaller cats appear to exhibit the non-monotonic latency-frequency relationship observed in clouded leopards, tigers and jaguars. Curves representing these cats look like those of all other mammals thus far studied; response latencies to low frequency sounds are prolonged relative to those produced by higher frequency stimulation.

These findings are consistent with the idea that the inner ears of clouded leopards and tigers underwent adaptation early in the evolutionary history of felids. If similar findings are made in lions, leopards and snow leopards, and preliminary findings are confirmed in the case of the jaguar, a reasonably strong case may be made that this inner ear trait is characteristic of species comprising the Pantherinae subfamily. It is then possible to imagine an evolutionary scenario in which the inner ear trait was passed to the first true cats to emerge onto the evolutionary scene, the large roaring cats and their nearest relatives, only to be extinguished as other lineages continued to evolve. This account may then serve as a framework to better understand the evolution of hearing in Felidae.

Cats in general are responsive to sounds below the 1-2 kHz low frequency cut-off designated in this study. The finding reported here is focused on the 1 to 32 kHz band of responsive frequencies, which constitute the great majority of the felid responsive frequency range. Response times increase as frequencies below 1-2 kHz decrease in tigers and clouded leopards, a pattern that fits the more commonly observed response time vs. stimulus frequency relationship.