Sound is produced by frogs similarly to the way we produce vocalizations. That is, air is passed over vocal chords that vibrate, producing sounds. Frogs have some anatomical modifications that help to increase the volume of their call, thus making them discernable to potential mates for long distances. The primary structure that increases the volume of frog calls is the vocal sac (or sacs), which acts as a resonance chamber to amplify the sound they produce. When calling, the vocal sac is inflated with air and expands greatly. Many frogs have a single vocal sac below their chin but some species, such as Northern Leopard Frogs (Lithobates pipiens) and Edible Frogs (Rana esculenta) have paired vocal sacs on either side of their head.
Frogs can call from various locations including the ground, on vegetation, or while in the water. Calling, however, can attract not only mates, but also potential predators. Thus, frogs frequently stop calling if disturbed and it may take a while for them to resume calling. Some species decrease their chances of being heard by predators by calling underwater, including Gopher Frogs (Lithobates capito), Spotted Frogs (Lithobates pretiosa), and African Clawed Frogs (Xenopus laevis). Typically, such frogs have low frequency calls that transmit best underwater.
Calling by male anurans results in other costs in addition to the risk of predation because calling is energetically expensive. When you adjust for their size, many species of frogs produce some of the loudest calls of any animal. Sound production appears to be the most energetically expensive activity in which many species may engage. Researchers have shown that the calling frogs expend ten times the energy of resting frogs.
This information is used with permission from Johns Hopkins University Press, and is adapted from the following citation:
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