African Treefrogs: Hyperoliidae

African treefrogs are active at night, which is when they look for food and mate. During the day, some of the species that live in hot, open areas dig down into the soil where it is cooler and moister. Scientists are not sure, but they think that some of them, including the toad-like treefrog, either remain underground for the entire dry season in a state of deep sleep, known as estivation (es-tih-VAY-shun), or come out on humid mornings to search for food. Some of these burrowing species shed their skin when they are underground, and this skin dries into a sort of cocoon. The frog remains inside the cocoon, which helps to keep the frog moist. Other species of treefrogs are outdoor types and stay outside even on very hot and dry days. They are not active during the day, however, and simply sit still on leaves until the evenings arrive. The painted treefrog survives hot and dry days by oozing mucus from its skin. This mucus is waterproof, but instead of keeping moisture out, it keeps the moisture in. Even so, they often lose some water from their bodies. Some young painted treefrogs are able to survive even after their bodies have lost half their weight in water. The sharp-nosed reed frog protects itself from dry weather simply by staying down low in grasses on hot days. There, the air is more humid.

The typical African treefrog mates during the rainy season. Males head to the water, often a small pool formed by the rains, and begin calling. Sometimes, the males reach the mating area even before the rains have filled the pools. Not all species mate in new pools on the ground. The African wart frog and others mate inside a tree hole above a puddle of water. Scientists still do not know where some species, such as the toad-like treefrog, mate.

Using their often-enormous vocal sacs, the males make a variety of calls. The greater leaf-folding frog, for instance, calls with a creaking sound followed by several clicks in a row. The bubbling kassina makes a sound like tiny bubbles popping. The males of the Afrixalus brachycnemis and other similar species are unusual because their calls have two parts. In this species, the first part is a zipping sound, and the second is a trill. The zipping sound tells other males of the same species to stay away, and the trill is an invitation to females interested in mating. The males of a few species, including the African wart frog, may not be able to call at all.

For those species that mate at tree holes, only one male uses each tree hole, and the female follows his call to him. It is not so easy for those that mate in ground pools. There, many males of several different species of African treefrogs may use the same pool for mating and call at the same time. Despite the confusion of calls and the large number of frogs, a female can pick out the call of a male from her own species and follow that call to a mate. In some species, the female is stopped on her way to a calling male by another male that is not calling, and she mates with him instead. These quiet males that hang around a calling male waiting for a chance to meet a female are called satellite (SAT-eh-lite) males.

The majority of the species in this family lay their eggs on leaves that hang over pools of water on the ground. For example, the female Betsileo reed frog of Madagascar lays her sticky eggs on leaves just above the water. These eggs hatch into tadpoles, and the tadpoles use their tails to wriggle off the leaves and fall into the water. The leaf-folding frogs also lay their eggs on grass blades or other leaves above a pool of water, but then fold the leaf around the eggs. Since the gel-covered eggs are sticky, the leaf stays folded. When the eggs hatch into tadpoles, the tadpoles wriggle off the leaf and into the water below. In at least one species of leaf-folding frogs, known as the delicate spiny reed frog, the female may mate with more than one male in a night, so the young in her batch of eggs may have different fathers.

The gray-eyed frog is the only species of African treefrog in which the parents provide a foamy nest inside the folded leaf for their eggs. The female makes mucus and then beats it with her hind legs until it turns into foam. She lays her eggs in the foam, and the male folds the leaf around them. The foam helps keep the eggs moist. The eggs hatch, and the tadpoles fall off the leaf and into the water below. Some species of treefrogs, including the kassinas and the sharp-nosed reed frog, lay their eggs right in pools of water along the ground. They typically stick their eggs, sometimes one at a time, onto underwater plants.

Several species, such as the African wart frog, mate in tree holes that hold water. The female lays her eggs on the inside wall of the hole just above the water. The eggs are coated in a gel and stick to the wall. When they hatch into tadpoles, the tadpoles drop into the water. The big-eared forest treefrog and a few other species do not lay their eggs in the water. Instead, these frogs bury their eggs in moist soil that may be 33 feet (10 meters) or more away from any water. Their eggs are large and filled with yolk, which provides food for the young. Eventually, the tadpoles are strong enough to wriggle along the ground to the water, where they continue to grow. In at least one egg-burying species, known as the giant big-eyed treefrog, the eggs do not hatch into tadpoles at all but hatch right into froglets.

Depending on the species, the number of eggs that a female lays may be as few as a dozen or as many as two hundred or more. The female sharp-nosed reed frog and the Seychelles treefrog are two examples of African treefrogs that lay large numbers of eggs. A typical clutch for a sharp-nosed reed frog is about two hundred eggs, while that of the Seychelles treefrog may be as many as five hundred. In most species of African treefrogs, the adult females lay the eggs and leave them to develop on their own. The African treefrog species known as the midwife frogs are different. The females stay with the gel-covered eggs until they hatch and then help the tadpoles escape from the gel.

The tadpoles of many African treefrogs change into froglets within weeks. Some, like the African wart frog, may wait until they are about three months old before going through the change, which is known as metamorphosis (meh-tuh-MOR-foh-sis). They may need this much time to grow because they live in small pools of water inside tree holes, where food may be very scarce. The sharp-nosed reed frog, on the other hand, lays its eggs in larger pools on the ground. When the tadpoles hatch from the eggs, they find plenty of food and grow so quickly that they can turn into froglets and be ready to mate and have their own young before the end of the breeding season in which they were born.