Why Do Frogs Croak? Mating Calls, Choruses and Vocal Sacs

The main reason: attracting a mate

Every frog species has its own distinctive call, and females recognize the calls of their own kind. A male’s call works like an advertisement that says, in effect, “I am here, I am the right species, and I am ready to breed.” Females move toward the calls they prefer, often choosing males whose calls are deeper, longer, louder or more frequent, which can signal a larger or fitter male.

This is why frog calls are so varied. A chorus pond in eastern North America may hold the high peeps of spring peepers, the duck-like clucks of wood frogs, the deep drone of the American bullfrog and the banjo-like twang of green frogs, all at once. Each female listens only for her own species.

Warning rivals and defending territory

Calls are also messages to other males. In territorial species such as the bullfrog, males space themselves along the shore and call from their own patches. When a rival comes too close, a resident male switches to a different, more aggressive call. If that fails, the two may wrestle.

Bullfrogs have at least three types of call: the advertisement call to attract females, a territorial call aimed at other males, and an encounter call given just before a fight. The famous jug-o-rum is the advertisement call, and it can carry for more than 1 km (0.6 mi) on a still night.

How frogs croak: lungs, vocal cords and vocal sacs

Frogs make sound in a different way from mammals. With the mouth and nostrils closed, a calling frog squeezes air from its lungs across vocal cords in its larynx. The air then flows into one or more vocal sacs, thin elastic pouches of skin, and back into the lungs, so the same air can be used again and again.

The vocal sac acts like a resonator and amplifier. It helps radiate the sound into the air and lets the frog call quickly without having to take a new breath each time. Vocal sacs come in several forms:

In the American bullfrog, the throat swells as the frog calls, but studies have shown that much of the sound is actually broadcast through its large eardrums. That may be one reason males have eardrums bigger than their eyes.

Why frogs call in choruses

Male frogs often gather and call together in a chorus. A large chorus is louder than a single male and can attract females from farther away. It also offers some safety in numbers, because a predator cannot focus on every caller at once.

Choruses are not random noise. In many species neighboring males take turns or time their calls to avoid overlapping, so that each call stands out. Some frogs call faster in warmer weather, since their muscles work faster when they are warm.

Choruses usually happen at night and after rain. Darkness hides callers from many predators, and damp air keeps their thin skin from drying out. Rain also fills the temporary pools that many species use for breeding.

Other frog calls: release, distress and rain calls

Not every croak is about courtship.

• Release calls: when a male mistakenly grabs another male, or a female that has already laid her eggs, the grabbed frog gives a short chirp or grunt, often with body vibrations, that says “let go.”
• Distress calls: some frogs, including bullfrogs and common frogs, may give a loud scream with the mouth open when seized by a predator. The sudden noise can startle an attacker into letting go.
• Rain or territorial calls: some tree frogs call away from the breeding pond in damp weather, and many species have short calls used only between males.

How the common frog and the bullfrog compare

Europe’s common frog is a good contrast to the bullfrog. Common frogs gather in ponds early in the year, often from February to April in western Europe, and males make a low, purring or grunting croak, quiet enough that it is easy to miss from more than a few meters away. Their breeding season lasts only days to a couple of weeks, so the chorus is short and intense.

The bullfrog, by contrast, breeds late, in warm water from late spring into summer, and its loud calls continue for weeks. The two species show how call style matches lifestyle: a brief, crowded breeding season does not need long-distance calls, while a long season with spaced-out territories rewards loud ones.

Unusual frog calls and silent frogs

Some frogs have found other ways to be heard. The concave-eared torrent frog of China lives beside noisy streams and uses ultrasonic calls, above the range of human hearing. Some frogs in Borneo and India that live near loud waterfalls wave their feet as a visual signal in addition to calling.

At the other extreme, tailed frogs (Ascaphus) of the Pacific Northwest have no vocal sacs and are not known to give mating calls, perhaps because their fast mountain streams are too noisy. Scientists still use calls for most frogs, though: in North America and Europe, volunteer call surveys help track frog populations by listening to choruses at night.