Page 69 - Summer2020

However, it can, and should, be argued that the trait dif- ferences shown in Figure 3 can be misleading when body size differences are not taken into consideration. When scaling factors are considered, the frequency-selectivity differences between the domestic cat and the tiger are not particularly surprising. In accordance with the principle of allometric growth in which the growth of one feature relative to another is proportional, it is notable that small animals generally exhibit proportionally shorter cochlear lengths and, in some cases, higher upper frequency hear- ing limits than larger animals. The best way to think about these differences is probably within the framework of inner ear frequency-mapping constants (i.e., the length of the BM devoted to a given frequency bandwidth; Figure 3, inset). Because the BM of tigers is longer than that in domestic cats and other typical laboratory animals (Ulehlová et al., 1984; Walsh et al., 2004) and because the high-frequency hearing limit is lower than that observed in common laboratory animals, the mapping constant of tigers is, theoretically, significantly larger than that in domestic cats, assuming other cochlear variables are com- parable. The upshot of this consideration is that there is no evidence suggesting that inner ear filter outputs break the uniform mold of other less famous felids if, that is, we frame the question in terms of biological scaling.
Tiger findings considered here are also of interest when thinking about earlier claims that inner ear mechani- cal filters are unusually sharp in humans. As also seen in Figure 3, the sharpness of inner ear filters in the tiger closely approximates the cochlear sharpness measured in humans. This finding suggests that the predicted cochlear mapping constant of tigers is much like that observed in humans (von Békésy, 1960; Shera et al., 2010), a finding with considerable scientific importance when biological scaling questions arise. These results also suggest that other cochlear features contributing to tuning, such as longitu- dinal coupling via tectorial membrane traveling waves, are also most likely comparable when humans and tigers are considered (Sellon et al., 2019); again, body size matters.
On the surface, therefore, for all of their otherwise mag- nificence, tigers are not, it would seem, particularly noteworthy from an auditory performance/processing perspective. However, all of that changes when the analyti- cal lens shifts to focus on the timing or latency of neural responses following stimulation in the frequency realm. In the real world, response timing can make the difference
between life and death or between a successful hunt and an empty stomach, for example. When concentrating on response timing, the generalist impression is at least par- tially upended. Outcomes of studies examining response timing in the stimulus frequency space of tigers reveal nonmonotonic profiles (Figure 4A). Increasing from highly unanticipated short-response latencies to low- frequency stimulation, latencies reach a maximum in the midfrequency range and steadily decrease with increasing frequency such that the latency to the highest frequency studied is higher than the latency to the lowest frequency studied. This stands in stark contrast with findings from other mammals (Ruggero and Temchin, 2007) studied thus far; latencies generally decrease exponentially with frequency, as shown for the modern day workhorse labo- ratory animal, the mouse (Mus musculus; Figure 4B), as well as in a squirrel monkey (Saimiri sciureus; Figure 4C). The differences are striking, and they are confusing in light of contemporaneous models of inner ear mechanics.
Although space limits won’t permit an in-depth consider- ation of a similar discovery made recently in the clouded leopard (Neofelis nebulosa), the implications of the find- ing (Figure 4D) may have real relevance in efforts to understand the evolution of the timing trait observed in tigers (Walsh et al., 2017). Not only is the resemblance of response-timing profiles, in our view, stunning, it takes on evolutionary relevance when the taxonomic proxim- ity of the genus Neofelis to Panthera is considered. Based on the close taxonomic relationship between tigers and
Figure 4. The relationship between auditory response latencies and stimulus frequency taken from scalp recordings for tigers (A), a laboratory mouse (B), a squirrel monkey (C), and clouded leopards (D). SPL, sound pressure level. Recordings in A-C were made at the same location using the same setup.
Summer 2020 • Acoustics Today 69