Whatever their musical qualities, vocalization are conspicuous feature of the behavior of most frogs and the toads. Although the function of frogs calls were not well understood until relatively recently most early naturalists realized that calls are given almost exclusively by males are associated with reproduction, and probably serve to attract mates.
Mechanism of sound production:
The basic mechanism of sound production in most anurans is relatively simple system. Air is forced from the lungs by the contraction of muscles in the trunk region and moves through the larynx into the buccal cavity. As the air passes through the larynx, vibrations of the vocal cords and the associated cartilage produce sound. An action of the larynx muscles shapes the sound in variety of forms.
The sound producing system involves three major functional units:
1. The trunk muscles that powers the system.
2. The larynx apparatus that produces the sound.
3. The buccal cavity and the vocal sac that transmit the sound.
Bogert (1960) classified anuran calls into six categories based on the context in which they occur:
1. Mating calls.
2. Territorial calls.
3. Male release calls.
4. Female release calls.
5. Distress calls.
6. Warning calls.
1. Advertisement calls: bogert used the term mating calls to describe the principal signals given by the males during the breeding seasons. These calls are now called as advertisement calls (wells 1977) because they often serve for more than one function or convey more than one message
2. Male courtship calls: male frogs often alter vocal behavior when females are nearby , producing calls that render the male more conspicuous in a chorus
3. Female courtship calls: some female frogs give call in response to the call of male and these are often called as reciprocation calls (littlejohn 1977) they tend to be given at very low intensity and therefore are hard to hear.
4. Aggressive calls: many male frogs defend their calling sites and have distinctive aggressive vocalization.
5. Release calls: male anurans usually give release calls when clasped by other males, either when the male being clasped is alone or in complexus with a female. The females that have completed oviposition also give similar calls. Usually these calls consist of a series of rapidly repeated broad spectrum notes.
6. Distress calls, alarm calls, and defending calls: bogert used the tem distress calls to describe vocalization given by the frogs being attacked by predators. Usually these are loud screams, often with the mouth open.
Energy cost of vocalization by frogs:
Ted taigen and kent wells experimented on hyla versicolor. The rates at which individual frogs consumed oxygen is directly proportional to their rates of vocalization
At low calling rates, about 150 calls per hour, oxygen consumed was barely above resting stage. However at the highest calling rates, about 1500 calls per hour the frogs were consuming oxygen at a rate even higher than they consumed during their highest locomotors activity.
Costs and benefits of vocalization:
The vocalization of male frog is costly in two senses. The actual energy that goes into calls production can be very high and the variation in calling pattern that accompany several interactions among male frogs in a breeding chorus can increase the cost per calls.
Another cost for the vocalization for a male frog is the risk of predation.
A critical function of the vocal calls is the permit the female frog to locate the male frog.
