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Checklist of Amphibian Species and Identification Guide

Identification Terms - Tympanic Membrane


picture of bullfrog
The tympanic membrane, or typanum, can be called the frog's eardrum. It is composed of nonglandular skin and unlike our own eardrums it is situated externally on the frog. It is located in most species directly behind the eye and can be quite large and visible as in this Bullfrog (Rana catesbeiana). They function in a similar fashion to our own eardrum. There is a bone attached to the tympanic membrane which also attaches to the oval window of the inner ear. When sound waves strike the membrane vibrations are transferred via this bone to the fluid of the inner ear. This in turn causes receptors within the inner ear to be stimulated, sending a transmission to the brain for processing much the same that our own ear does.
pair of bullfrogs
Determining the sex of frogs by their ears.

In some closely related species of frogs there is a sexual dimorphism in the shape of the tympanic membranes. This simply means that females and males have differently shaped ears (tympanic membranes)! In males the tympanum is clearly larger than the eye, while in females it is about the same size, as in this pair of Bullfrogs to the left. The males in this species also have a yellow throat, in females it is white like the belly. In all, there are six species of North American Frogs that show this sexual dimorphism or difference (dimorphism = 2 forms). These are sometimes referred to as frogs of the "Catesbeiana Complex":


American Bullfrog - Rana catesbeiana
Green Frog - Rana clamitans
Pig Frog - Rana grylio
River Frog - Rana heckscheri
Mink Frog - Rana septentrionalis
Carpenter Frog - Rana virgatipes

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