These are the typical pond frogs, once almost unbelievably abundant but now much less common because of widespread pollution and the destruction of wetland habitats. The 21 species are rather similar in appearance, with a greenish or brownish color and irregularly shaded dark spots or splotches on their bodies. One important identifying characteristic is the presence or absence of dorsolateral ridges, a pair of folds that run down the back of certain species. The Green Frog has dorsolateral ridges; the Bullfrog, the largest frog in North America, looks quite similar but lacks these folds. Originally found only in the eastern part of the continent, the Bullfrog has been released in suitable locations throughout North America and has also escaped from "farms" where it has been raised for food or other purposes. The deep, sonorous jug-o-rum mating calls of the males are familiar spring sounds, so loud and booming that they drown out the weaker calls of smaller species. Male Bullfrogs sing solos rather than joining in choruses like most frogs. Their larger eardrums, or tympanums (the round area behind each eye), distinguish them from the females. Among the True Frogs found in South Georgia are the Green Frog, the Bullfrog, the River Frog, the Southern Carpenter Frog,  and the Southern Leopard Frog.