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broadband, predominantly low-frequency noise. The use of ultrasound allows these frogs to exploit a relatively silent acoustic niche. Ears of male torrent frogs show some modifications of the tympanic pathway that facilitate detection of high frequencies. These modifications include a very thin and recessed tympanum, a short external ear canal, and a short, low-mass extracolumella and columella (Feng et al., 2006). The tympana of female torrent frogs are larger and thicker than that of males and vibrate best in response to lower frequencies (Shen et al., 2011). These anatomical differences suggest that females are primarily listening for the nonultrasonic frequencies in the male’s advertisement calls, whereas males may use ultrasonic frequencies in territorial interactions.
Fully aquatic Xenopus frogs detect sounds via a cartilaginous tympanic disk that lies below the fatty tissue and skin behind the eye (Figure 2B). The tympanic disk is larger in males than in females, even correcting
for body size (Mason et al., 2009). The tympanic disk including its extracolumella is coupled to the columella through an air-filled middle ear cavity. As in the bullfrog ear, this middle ear apparatus acts like a lever to increase force on the oval window. In Xenopus, vibrations of the tympanic disk produce comparatively larger vibrations of the footplate of the columella than those observed in bullfrogs (Mason et al., 2009). These larger vibrations may be an adaptation for underwater hearing.
Extratympanic Pathways
Aside from the tympanic pathway, several “extratympanic” (not involving the tympanum) routes to the inner ear have been identified. These include transmission via the operculum (Figure 3), the lungs, the mouth cavity, and through bone conduction via the skull. Extratympanic sources of input do not always act independently from each other or from the tympanic pathway, and how these multiple transmission pathways interact is still being studied.
In evolutionary time, extratympanic pathways are older than the tympanic pathway (Christensen-Dalsgaard and Manley, 2013). Extratympanic pathways are also more enduring, being present in one form or another in all living frogs. These pathways provide good sensitivity to low-frequency sounds and vibrations. On the other hand, many anurans do not have a tympanum or even a middle ear cavity. There are, for example, about 200 species of
“earless” toads (Pereyra et al., 2016). Earless and eared toads (Bufonids) have similar hearing sensitivities at low frequencies (<1,000 Hz), but eared toads hear better at high frequencies (Womack et al., 2017). This suggests that the pressure for evolution of the tympanic pathway was to extend the species’ audible range.
The operculum (Figure 3) is a structure unique to the anuran ear. It consists a piece of cartilage located at one end of the oval window; the footplate of the columella is located at the other end (Figure 3C). The operculum may have several important functions in hearing (Mason, 2007). In earless species without a columella, the operculum provides an essential route of transmission to the inner ear. In both earless and eared frogs, the operculum may improve detection of low-frequency vibrations via its attachment to the opercularis muscle and then to the shoulder girdle. A frog sitting on the ground or in a tree could pick up seismic vibrations with its forelimbs; these vibrations are transmitted to
 Figure 3. Schematics of the middle ear apparatus in an adult terrestrial frog (A) and at progressively earlier stages of development: B: at the end of metamorphosis; both hind and fore limbs are present; C: tadpole with hind limbs but fore limbs still emerging; D: before development of hind limbs. Schematic in A shows all components of the tympanic pathway from the oval window to the external tympanum. op, Operculum; cf, columella footplate; c, columella; ec, extracolumella; t, external tympanum; ow, oval window. Schematic in A redrawn from Christensen-Dalsgaard and Manley, 2013; schematics in B-D based on data from bullfrog tadpoles from Horowitz et al., 2001; Simmons, 2019. Images used with permission, copyright © 2020 A. M. Simmons, all rights reserved.
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