How Does a Frog Croak?
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Production of Sound
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A frog creates vocal sound in much the same way a human does. The frog first fills its lungs with air, then expels the air with high pressure across its larynx, or vocal chords. The vibration of the vocal chords as the air moves across them produces sound.
The Vocal Sac
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Most people are familiar with the quintessential image of the frog inflating the skin beneath its chin. This area is called the vocal sac; when inflated, it creates a resonating chamber for the sound produced in the frog's vocal chords. In the same way that the hollow wood on a guitar amplifies the sound of the strings or a hollow chamber creates an echo, the vocal sac makes the croak audible across long distances--sometimes as far as a mile or more.
The vocal sac is not a part of the frog's mouth or cheeks, but a separate chamber which is connected to the mouth through two slits which are positioned on either side of the frog's tongue. Some frogs may have two vocal sacs and/or sacs which inflate internally (causing the frog's entire body to look inflated, rather than just the area under the chin).
A few frog species lack vocal sacs, but make up for it with an enlarged, dome-shaped buccal cavity (mouth cavity) which allows for the same resonance.
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The Purpose of Croaking
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Frogs croak for a variety of reasons. The primary purpose of croaking is for a male frog to attract a mate, and some female frogs have a reciprocal call in response to this. Some male frogs will croak in order to mark their territory or to ward off another male who may be mistakenly trying to mate with them. Certain species of tropical frogs will croak in anticipation of rain during periods of high humidity.
During most calls, a frog keeps its mouth closed. When set upon by a predator, a frog may make a higher-pitched distress call by keeping its mouth open.
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