True Frogs: Ranidae

Although most members of this family are active at night, some that live next to lakes and ponds, or in cooler areas, are out and noticeable during the daytime. They sunbathe, or bask, by sitting in a warm spot on land or, if they are in the water, by floating in the warm, top layer. Some commonly seen daytime-basking frogs in North America include the green frog and leopard frog.

Those true frogs that live in moist forests in warm climates remain active all year. Others do not. Some live in dry areas and have to find ways to survive the weather. The African bullfrog, for example, makes its home in dry regions of southern Africa. It becomes active and breeds during the rainy season, but when the ground dries out, it buries itself underground. It then sheds a few layers of skin that it wears like a watertight coat to keep its skin from losing too much water. This skin cocoon stays around its body while the frog rests during a period that is known as estivation (es-tih-VAY-shun). The frog remains in its cocoon until the next rainy season arrives.

Frogs that live in places with cold winters also enter a resting period, known as hibernation (high-bur-NAY-shun). Both hibernation and estivation are long resting periods, but one happens when the weather is cold, and the other when the weather is dry. The hibernating frogs typically bury themselves in the ground or in mud at the bottom of their pond or wetland and stay there until the temperatures warm the following spring. A few species, like the wood frog of North America, are able to freeze solid in the winter and recover the next spring to live another year.

Most of the species in this family avoid predators by restricting their activity to the dark of night or by remaining still and blending in with the background. Since many of them live near the water, they also have the option of leaping and then diving down to make a fast escape if predators come too close. A few, like the African bullfrog, will stand their ground. They will nip at predators that approach them or their young.

Most of the frogs in this family breed during one season a year. Those that live in dry areas mate during the rainy season. Others that live in climates with cold winters start their breeding seasons when warmer spring temperatures arrive. Some breed in early spring, and others in late spring. Those that live in warm, tropical areas may breed more than once a year, often following heavy rains. Often, true frogs mate together in large groups. In the wood frogs, for example, dozens of males will hop over to the water and begin calling all at once. This type of group calling is called a chorus (KOR-us). With their quacking calls, the wood frog chorus sounds something like a large group of ducks. They, like many other species of true frogs, are explosive breeders, which means that they breed over a very short period of time. All of the wood frogs in a population, for instance, may mate over just seven to fourteen days. The male true frogs may have one or two balloon-like vocal sacs in the throat area. These fill with air and deflate as the frog makes his call. Many species, like the green frog and leopard frog, have one large vocal sac. The wood frog is one of the species with two smaller vocal sacs. They typically both inflate at the same time—one on each side of its throat.

Like females of other frog species, female true frogs follow the males' calls. Different species have different calls. The green frog, for instance, has a short "gung" call, while the leopard frog's call is more of a snoring sound. In many species, including the bullfrog, males may wrestle with one another for a good calling area. Once males and females are together, one male and one female typically pair off, the male scrambles onto her back and hangs onto her while she lays her eggs. The male in most species clings to the female by gripping near her front legs. Depending on the species, the female may lay a few or many eggs. The female Penang Taylor's frog, for instance, lays five to thirteen large eggs; the female Sanderson's hook frog lays twelve to seventeen; the female African bullfrog lays three to four thousand; the female Roesel's green frog lays two to six thousand; and the bullfrog female can lay up to twenty thousand eggs. In some species, like the African bullfrog, one male may mate with more than one female.

In most species, the females lay their eggs in the water, and the eggs hatch into tadpoles. The tadpoles may turn into froglets in a few weeks or, in some cases, in a few years. Bullfrog tadpoles are an example. They can survive as tadpoles for up to four years and even hibernate just as their parents do. In a few species, like Penang Taylor's frog, the female lays her eggs in a moist place on land, and the eggs hatch right into froglets, skipping the tadpole stage. In either case, eggs typically hatch in a matter of days to weeks. In most true frogs, neither parent provides any care for the young once the eggs are laid. In others, however, the parent may remain nearby to make sure that predators do not eat the young and/or to help keep the eggs moist if they are laid on land. The female Sanderson's hook frog even returns to her eggs every night to cover them with her body. In the African bullfrog, it is the male that watches over his young. He will bite at any intruder who comes close to his eggs or tadpoles, even if the intruder is as large as a lion or a person. If the weather becomes dry very quickly and the tadpoles are trapped in a small puddle away from the main pond, he may also dig a path through the mud to the pond so the tadpoles can swim from the puddle to the deeper water of the pond, where they continue their development.