Most people are familiar with the croaking calls of frogs. The range of calls is however very diverse – from the deep guttural bellow of the bullfrog to the high pitched whistle of reed frogs. Some calls are melodious and pleasing to the ear others harsh, irritating and strident in their repetitiveness.
Frog calls are critical for the survival of the species because the mating calls of frogs are unique and is the signal by which females recognise partners of their own kind. Because each kind of frog has a unique call this is the best way to learn to identify the species. Frogs also produce sounds which are used to alarm predators, to keep males spaced apart at the breeding site and to summon other males to the chorus. Some females have their own peculiar call which, when emitted, terminates mating.
Frogs breathe by lowering the floor of the mouth to increase the volume of the oral (buccal) cavity. As the volume is increased, air is drawn in through the nostrils. The nostrils are now closed by skin flaps and the floor of the mouth is raised which forces air through the open glottis into the lungs. To exhale the muscles of the body contract, forcing air from the lungs and out of the nostrils..
To produce sound, air being forced from the lungs passes through the larynx where paired vocal chords vibrate to produce the particular call of the species. If the male has a vocal sac this expands as the call is being made to act as a resonator to amplify the sound. This is clearly shown in the images. The top images show a guttural toad (Bufo guttularis). Identification of the lower image is not certain but is thought to be a golden leaf-folding frog (Afrixalus aureus).