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What Is That Bubble Under a Frog's Neck?

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Frogs are intriguing creatures whose unique features add to the fascination that many people experience when studying these amphibians. One such feature is the large, somewhat comical vocal sac under a frog's neck, which looks like a bubble or balloon. Because of this bubble, the frog is sometimes dubbed an animal that blows bubbles. As far as their scientific names and classification, frogs are part of the Animalia kingdom, the Amphibia class and the Anura order. More than 5,000 species of frogs live around the world, according to "My Pet Frog," with new species being discovered annually.

Vocal Sac Facts

The vocal sac, or bubble, is spherical in shape, like a ball. It extends out from the front of the frog's body, just below the head, in the front of the throat area. Few frogs have necks, although the area where the vocal sac expands is just below the head and above the chest area. The area where the bubble-like apparatus expands is made of the same skin that covers the rest of the body. When it's not expanded, the skin is almost flush with the rest of the upper body. The vocal sac can be one-third the size of a frog's body, three times as big as his head, when it's expanded to its maximum size and filled with air.

Gender and Species Differences

Most frog species have vocal sacs, and it's more common for males than females to have them. In a small number of species, both genders have vocal sacs. Among frogs that have a vocal sac, most species have just one. But the edible frog species has one on each side of its mouth area, and these look like two bubbles extending outward.

Sac on a Mission

The curious-looking bubble is not just for show -- it serves a valuable purpose for frogs. The vocal sacs on a male expand when they call to females as part of mating and when they are defending their territory from other male frogs. When the vocal sac expands into its bubble-like shape, it makes the frog's voice louder. Frog voices need to be loud enough so that females from further away can hear them, which in turn increases the likelihood of males finding mates. Frog calls are audible as far as a mile away from the frogs making the sounds.

Blowing Bubbles

How the vocal sac or bubble works is fascinating. The frog keeps his mouth and nostrils closed. Air travels from his lungs over the vocal chords and into the vocal sac. The vocal sac expands as it fills with air, similar to a balloon being blown up with air. At the same time, the vocal sac echoes and increases the volume of the sound being made by the vocal chords. If you're hoping to see a frog's vocal sac bubbling up, you might be in for a challenge. This is because frogs make many of the calls that expand their vocal sacs at night.

Frogs Calling

When frogs bodies are creating these bubble-like shapes, the noise produced can be quite loud, because hundreds or even thousands of male frogs join together like a chorus as they all call and compete for females at the same time. Each frog species has its own individual call, and female bodies are designed to be able to hear and to identify the calls made by males of their own species.