Amero-Australian Treefrogs: Hylidae

Almost all of the treefrogs are active at night and hide during the daytime. Their hiding places may be underneath loose bark on the side of a tree, between two leaves or between a leaf and a stem, in a crack in a rock, or tucked into any number of other tiny openings. A few species, including the northern cricket frog of North America, are active during the day instead. Those that are active at night spend most of the time sitting still on leaves or branches, or on the ground, waiting for a meal to wander past. People usually see them most often during or after a rain when the frogs move around more.

The treefrogs that live in deserts and grassy fields take special steps to keep from drying out. The water-holding frog of Australia lives in very dry parts of Australia. To survive, it spends most of its life underground. Like other burrowing treefrogs, it has shovel-like bumps, called tubercles (TOO-ber-kulz), on its feet to help it dig backward into the ground. Once it is completely buried, it sheds its skin, which hardens into a waterproof coat. The frog remains inside the coat until the rainy season arrives, and then comes out of its burrow to mate and to eat until the dry weather returns. This resting period is called estivation (es-tih-VAY-shun). Some treefrogs in hot places in South America can survive above the ground. They ooze a goop from their skin and smear it on the rest of the body. The goop is waxy and prevents the frog from drying up.

Frogs that live in cold climates, such as the northern United States and Canada, spend the winter in a state of deep sleep. This is called hibernation (high-bur-NAY-shun). Cope's gray treefrog is an example. It crawls into a hiding place under leaves or underground and stays there until spring. During hibernation, much of the frog's body freezes solid, but the frog thaws out safely when the weather starts to warm up.

The typical treefrog avoids many of its predators by staying out of sight during the daytime and by remaining still for most of the night and blending into its surroundings. If a predator spots a treefrog and draws near, most treefrogs are good jumpers and will simply leap away. A few, like the black-eyed leaf frog, can soar many feet (several meters) after leaping from a high branch. Others, such as the northern cricket frog, can hop to the water and continue scooting across the surface for a short distance. Some species of Amero-Australian treefrogs do not try to leap away and instead play dead by tucking their legs against the body and freezing in that position. Often a predator will lose interest and wander away, leaving the frog alive. The Amazonian milk frog and several other treefrogs have another way to avoid predators. These frogs ooze a milky substance from the skin that hurts the eyes and mouth of a predator.

Breeding season for the frogs that live in moist forests, such as cloud forests in the mountains or rainforests, may continue off and on throughout year after almost any heavy rain. These frogs usually mate in streams or ponds that are filled with water all year long. The frogs that live in cooler climates typically breed in the spring and use temporary pools of water that will dry up later in the year. Treefrogs that live in dry areas also use temporary pools, but they breed only when the rainy season arrives, which sometimes does not happen for a year, two years, or more. While many other species of treefrogs will call and mate in large groups, the males of those species that breed on land usually call alone. For example, a male spiny-headed treefrog calls alone from a tree hole or from a bromeliad (broh-MEE-lee-ad), which is a plant that grows on trees and holds little puddles of water between its leaves. When the male pairs with a female, the two also mate away from the other frogs.

The males' calls announce the breeding season. The males usually have a single vocal sac, a bit of flesh on the throat area that blows up and deflates when they call. Most call from the ground or on plants or trees. A few species, like the Manaus slender-legged treefrog, have two vocal sacs that inflate upward around the head, and they call while floating in the water. If they had the typical single vocal sac that inflates out and down from the throat, they would bob around in the water. In most species, the male finds a good spot for breeding and makes his calls not only to attract a female but also to announce his territory to other males of his species. If another male tries to move into his territory despite his calls, the two may wrestle with one another, sometimes even resorting to biting. Perhaps the most vicious fighters are the male gladiator frogs, which have sharp spines on the inner toe of each front foot. Two battling males will swipe their spines at each other, often causing bad cuts and sometimes death. Spiny-headed treefrogs may also fight by gashing one another with their head spines.

Once a female approaches a male for mating, he scrambles up and lays flat against her back while hanging on with his front legs wrapped around the top of her back. The females of some treefrogs, including many of the species in North America, lay their eggs in the water, and the eggs hatch into tadpoles. Some treefrogs instead dig a shallow dip or a deeper hole in the ground or find an already-made dip and lay their eggs there. When rain floods the dip or the burrow, the tadpoles hatch and float off to a nearby pool of water. The females of other treefrog species lay their eggs on leaves that hang over the water or make foamy nests for them on plants above the water. A number of treefrog females lay their eggs on the sides of tree holes or on bromeliad leaves above a little puddle of water. These eggs hatch into tadpoles and slide off the leaves, drop out of the nests, or wriggle from the sides of bromeliad leaves and tree holes to fall into the water below. In some species of treefrogs, the eggs spend time inside a pouch on the female's back or simply stuck to her back. Among several of these species, the eggs hatch into tadpoles inside the pouch or on the female's back, and she drops them off at a pond or in a puddle inside a tree hole or bromeliad. The eggs in a few species never become tadpoles at all and hatch right into froglets.