The vocal sac
is the flexible membrane
of skin possessed by most male frogs
. The purpose of the vocal sac is usually as an amplification of their mating or advertisement call. The presence or development of the vocal sac is one way of externally determining the sex of a frog in many species.
The structure and location of the vocal sac is dependent on the species. There are three main types of vocal sac. The most common, is one large vocal sac located under the chin, called median subgular
. The other two types of vocal sac consist of two separate vocal sacs. The more common of these is two vocal sacs on the edge of the chin, called paired lateral
. The last, and least common, is two separate vocal sacs under the chin, called paired subgular
. All three types have species which inflate internall and externally. If they are inflated externally, the vocal sac can usually be clearly seen. If inflated internally, the body will look inflated. Usually, species which inflate their vocal sacs internally call from within the water or ground where lower frequencies travel better. Also, aquatic frogs would have problems with buoyancy
if the vocal sacs inflated externally.
The vocal sac is open to the mouth cavity of the frog, with two slits on either side of the tongue. To call, the frog inflates its lungs. The air is then expelled from the lungs, through the larynx, and into the vocal sac. The vibrations of the larynx emits a sound, which resonates within the vocal sac. The resonance causes the sound to be amplified, and allows the call to carry further. Muscles within the body wall force the air back and forth between the lungs and vocal sac. The frogs mouth and nose are kept shut for the duration of the call.
The development of the vocal sac is different in most species, however they mostly follow the same line. The development of the unilobular vocal sac begins with two small growths on the floor of the mouth. They begin to grow, until they form two small pouches. The pouches expand until they meet in the centre of the mouth, and form one large cavity, which grows until it is fully developed.
The primary purpose of the vocal sac is to amplify the advertisement call of the male, and attract females from as large an area as possible. Species of frog without vocal sacs may only be heard within a radius of a few metres, whereas some species with vocal sacs can be heard over away. Modern frog species (Neobatrachians
and some Mesobatrachians
) which lack vocal sacs tend to inhabit areas close to flowing water. The sound of the flowing water overpowers the advertisement call, so they must advertise by other means.
An alternative use of the vocal sac, is that employed by the frogs of the Rhinodermatidae family. The males of the two species of this family will scoop recently hatched tadpoles into their mouth, where they will move into the vocal sac. The tadpoles of Darwin's Frog (Rhinoderma darwinii) will remain in the vocal sac until metamorphisis, whereas the Chile Darwin's Frog (Rhinoderma rufum) will transport the tadpoles to a water source.