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The uniqueness of the amphibian and basilar papillae lies in the absence of a basilar membrane and the largely nonoverlapping range of sound frequencies processed by these two organs. In both papillae, the sensory hair cells lie on a fixed tissue support and extend their hairs toward an overlying tectorial membrane (Simmons et al., 2007). The absence of a basilar membrane means that there is no shearing force generated by differential movement between two flexible membranes as in mammals (Manley et al., 2018); instead, shearing action occurs between the fixed tissue support and the tectorial membrane. This mechanical difference, along with the differences in the tympanic pathway, contributes to the overall weaker sensitivity of the anuran compared with the mammalian ear.
The amphibian papilla (Figure 4B) is larger than the basilar papilla (Figure 4C), and it varies in size and shape across different anuran species (Lewis, 1981). In the earless Pacific tailed frog (Ascaphus truei), the amphibian papilla consists of one club-shaped region of sensory hair cells (Figure 5). In eared frogs, the amphibian papilla has two contiguous regions, a club-shaped patch and a longer, curved tail. There is considerable species diversity in the shape and length of the tail (Figure 5). Even accounting for body size, species with a longer tail hear a broader range of sound frequencies than those with only the club-shaped region. Lewis (1981) proposed that the tail elongated under selective pressure to extend the range of hearing to higher frequencies.
The amphibian papilla is organized tonotopically. Low frequencies (<300 Hz) are represented in the club-shaped region, and higher frequencies (up to about 2,000 Hz in the bullfrog) are represented progressively further along the tail (Lewis et al., 1982). Thus, this organ is sensitive to a wide range of sound frequencies, those found both in communication calls and in signals from other sources (predators, abiotic sounds).
The basilar papilla (Figure 4C) is a small cup-shaped organ with a similar shape in all species. In ultrasound- detecting frogs, it is reduced in size, with a lower mass tectorial membrane and shorter, stiffer hairs on the sensory cells (Arch et al., 2012). In contrast to the amphibian papilla, the basilar papilla is not tonotopically organized. Instead, eighth nerve fibers innervating this organ all respond best to (are tuned to) the same restricted range of high frequencies. Thus, the basilar
papilla operates as a simple resonator (Simmons et al., 2007). Unlike that of the amphibian papilla, the tuning of the basilar papilla is species specific in a high-frequency range (Capranica and Moffat, 1983). For example, the male bullfrog’s advertisement call contains maximal energy in two frequency ranges, a low-frequency range around 200 Hz and a higher frequency range around 1,400 Hz. The bullfrog’s amphibian papilla is tuned to frequencies between 100 and 1,000 Hz, whereas the basilar papilla is tuned to frequencies around 1,400 Hz (Lewis et al., 1982). The advertisement call of the Puerto Rican coqui frog consists of two tones, a “co” note at 1,100 Hz and a “qui” note with energy between 1,800 and 2,400 Hz. The amphibian papilla and the basilar papilla in this frog are tuned to these two nonoverlapping frequency ranges (Narins and Capranica, 1976).
In some frogs, the frequency sensitivity of the basilar papilla varies with the sex of the animal. In the Puerto Rican coqui frog, nerve fibers innervating the female’s basilar papilla are tightly tuned to the frequencies in the male’s qui note (around 2,100 Hz), whereas the male’s basilar papilla is more broadly tuned between 2,000 and 3,500 Hz, extending
  Figure 5. Evolution of the amphibian papilla, showing the relatively stable club-shaped region (magenta) and the more variable tail (hatched). Top: Pacific tailed frog (Aschapus truei); center: oriental fire-bellied toad (Bombina orientalis); Bottom: bullfrog. The club-shaped region responds best to low frequencies below about 300 Hz and is not tonotopically organized, whereas the tail region responds to higher frequencies and supports tonotopy. Top two schematics redrawn from Lewis, 1981; bottom schematic based on an original image supplied by E. R. Lewis.
Winter 2020 • Acoustics Today 71

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