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 How a Frog Hears
Andrea Megela Simmons and Darlene R. Ketten
Anuran amphibians (frog and toads: see are key hearing model organisms. Most anurans are highly soniferous, their croaks and calls often heralding the onset of spring in temperate zones and the rainy season in tropical zones (see During their breeding seasons, males produce prolonged, loud (to humans) advertisement calls to attract females and to announce their location, size, and status to rival males. Acoustic features of advertisement calls are critical for maintaining reproductive isolation between species. Other important calls in the frog’s vocal repertoire include aggressive calls, distress calls, and female courtship calls.
In addition to the significant role of hearing for species identification and reproduction, anurans are important bioacoustic models because of what they tell us about the evolution and development of hearing. The middle ear varies considerably across species, related to habitat (aquatic vs. terrestrial) and to evolutionary “age.” The frog’s inner ear evolved three separate organs that are
sensitive to sounds, making this ear unique among terrestrial vertebrates. The frog’s ear is also a model for understanding how genetic and environmental factors may affect hearing (Horowitz et al., 2005). About three- fourths of the approximately 6,000 anuran species undergo metamorphosis, a developmental phenomenon in which the wholly aquatic larval tadpole transforms, in a matter of weeks or months, into a terrestrial frog. For these species, metamorphosis features a switch in sound processing as the ear transforms from adapted to hearing in water to adapted to hearing in air. Understanding metamorphosis is important for understanding the mechanisms regulating growth and development as well as how organisms respond to changing environments (Simmons, 2019).
In this review, we discuss the structure and function of the anuran ear in both its mature form and across larval development.
Frogs Lack External Ears
Anurans do not have pinnae, which are the outer ear “flaps” we commonly think of as ears. Instead, the external
   Figure 1. Three-dimensional (3D) reconstructions of microCT images (Zeiss XRadia Versa 520, RadiAnt version 2020.1.1) of an adult female bullfrog. A: dorsal view of external surface of the head, showing the two tympana (t). B: ventral view of right ear showing extracolumella (ec) extending from the medial surface of the tympanum (t) to the columella (c). C: posterior view of right ear middle and inner ear structures showing outer edge of tympanum (t), columella (c), and saccular otoconia (o). Videos of the original scans through the head and a 3D rotating reconstruction in which tissues are “digitally dissected” to gradually reveal tissues from the outer ear through the middle ear can be viewed at acousticstoday/simmonsmedia). Images and multimedia used with permission, copyright © 2020 D. R. Ketten, all rights reserved.
  ©2020 Acoustical Society of America. All rights reserved.
Volume 16, issue 4 | Winter 2020 • Acoustics Today 67

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