Page 68 - Winter 2020

FROG EARS
Figure 2. Histological cross (vertical) section through the head of an adult bullfrog (A) and an adult tropical clawed frog Xenopus tropicalis (B). In A, shades of red indicate muscle and shades of blue indicate collagen/cartilage. In B, shades of red indicate collagen/ cartilage and muscle fibers. Scale bars: 1,000 μm (A); 500 μm (B). Et, Eustachian tube; ec, extracolumella; ie, inner ear; mec, middle ear cavity. Images used with permission, copyright © 2020 A. M. Simmons, all rights reserved.
mark of the ear, if present, is the tympanum (eardrum). A top-down view of the head of a female bullfrog, Rana catesbeiana, shows the two external tympana (Figure 1A and 2A). Each tympanum is a thin membrane surrounded by a cartilage ring, the tympanic annulus, that is visible on the skin just behind the eye. In bullfrogs, the male’s tympanum is about 50% larger than the female’s, even adjusting for body size. The male’s tympanum vibrates best at frequencies around 200 Hz, within the range of the low-frequency energy in the bullfrog’s advertisement call but also radiates the high-frequency energy in the
call. How this might work is discussed by Mason (2007).
Some anuran species, like the fully aquatic adult Xenopus frogs, lack a conventional external tympanum (Figure 2B), buttheydohearandtheydovocalize.Wediscusstheways by which frogs with and without tympana can detect sounds.
Frogs Have Several Middle Ear Transmission Pathways
Tympanic Pathway
In the anuran’s tympanic pathway, similar to the mammal’s (Manley et al., 2018; Puria, 2020), vibrations of the external
tympanum are transmitted through an air-filled middle ear via a set of linked middle ear structures (see Multimedia 1 and 2 at acousticstoday.org/simmonsmedia). This set of structures in anurans is called the extracolumella and the columella (Figure 1, B and C). The extracolumella is a rod-shaped piece of cartilage connected to the internal surface of the tympanum (Figure 1B) and then joined medially to the bony columella at an angle (Figures 1C
and 2A). The other end of the columella (the footplate) inserts into the bony otic capsule at a membrane called the oval window. The extracolumella and columella function as a lever to increase the acoustic power at the inner ear, analogous to the operation of the three middle ear bones in the mammalian ear (see Acoustics Today article by Puria, 2020). Because the extracolumella is cartilaginous rather than bony, it is not very effective in transmitting high-frequency sounds. In most anurans, the high- frequency limit of hearing lies below 8,000 Hz, but there are exceptions.
The middle ear cavity opens into the mouth cavity via the wide Eustachian tubes (Figure 2A), which are permanently open in most frogs. This provides a wide path of communication between the two ears, the mouth, and the lungs. The consequence is that incoming sounds can strike both the inside and the outside of the tympanum but with unequal pressures. This is an example of a pressure-gradient receiver (Mason, 2007). Pressure-gradient receivers are inherently directional because of the differences in pressure between the inside and outside of the tympanum. For frogs with their small heads and poor sensitivity to high frequencies, this kind of receiver can improve their ability to localize sounds.
Some unique frogs, such as the concave-eared torrent frog Odorrana tormota, produce advertisement calls that include energy extending into the ultrasonic range (Feng et al., 2006). Torrent frogs live in rapidly flowing streams in mountainous areas of southeast Asia in habitats containing considerable
68 Acoustics Today • Winter 2020