IV.

Frog Behavior

Many aspects of frog behavior help the animals avoid conditions that are too hot, too cold, or too dry. Frogs are ectotherms (commonly referred to as cold-blooded), which means that their body temperature depends on the temperature of the surrounding environment. Few species can tolerate temperatures below 4°C (40°F) or above 40°C (104°F), and many species can survive only within a narrower range of temperatures. In addition, frogs’ thin, moist skin offers little protection against water loss, and when on land the animals must guard against drying out. Many frogs are active at night because temperatures are cooler and humidity is higher than during the day. In seasonal environments, frogs may remain dormant for months at a time when conditions are inappropriate. For example, many frogs that live where the winters are cold spend the winter months buried in the mud at the bottom of a pond, and desert-dwelling species may burrow under the sand during the dry season or to escape extreme heat. Frogs do not drink, but they can replace lost water by absorbing it through the skin. A frog may sit in water or completely submerge to replenish moisture in its body. When a frog sits on moist ground, it absorbs water through the skin on its belly and the underside of its hind legs.

On land, frogs move from place to place by jumping, and some species, especially toads, move by crawling. Jumping enables frogs to move rapidly over land without leaving a scent trail, helping them escape from predators such as bats, herons, raccoons, snakes, turtles, fish, and even tarantulas. Although frogs escape from predators by jumping, they typically catch their own prey by simply sitting in one place and waiting. Most frogs feed on insects and other small invertebrates, such as worms, spiders, and centipedes. Aquatic frogs sometimes eat other frogs, tadpoles, and small fish. Larger frogs eat animals as large as mice or small snakes. When a frog sees that an insect or other prey animal is within range, it rapidly flips out its tongue, which is attached at the front of the mouth and catapults over the lower jaw. The tongue is coated with a sticky substance that holds on to the prey. Frogs generally swallow their prey whole. When a frog swallows, its eyes sink down through holes in the skull and help push food down the throat.

Frogs advertise their presence and communicate with other frogs using a variety of complex calls, including ribbets, croaks, and other sounds. They produce these sounds in much the same way as humans speak, by forcing air from their lungs over their vocal cords, located in the throat. Frog communication is particularly important during the mating season, when male frogs call to attract females (see Animal Courtship and Mating). The males of many species possess vocal sacs, expandable pouches of skin in the throat or on each side of the mouth. When the frog calls, the vocal sacs inflate and act as resonating chambers, amplifying the volume of the call. Each frog species has a distinctive call, enabling females to find an appropriate mate even when several different species are calling and breeding in the same area at once. If a female frog is clasped by a male that she does not want to mate with, she may give a call that tells the male to release her.

The breeding behavior of frogs is extremely variable. Some species congregate in large numbers around a pond, where they breed and lay their eggs in the water. Typically, after the female enters the water, the male frog positions himself behind her and grasps her by the waist. Once a suitable egg-laying site has been found, he releases sperm as she releases eggs, and fertilization—or union of the sperm and egg—takes place outside the body. Other species breed on land, and here again the male clasps the female from behind, releasing sperm as she lays her eggs on trees or other vegetation. Some male frogs are territorial during the breeding season, engaging in biting or wrestling combat with other males to compete for females.

Most frogs do not care for their young—the adults simply mate and abandon the eggs. A few frogs provide varying degrees of parental care, of which there are some spectacular examples. Either the male or the female may provide this care, depending on the species. Among some frogs one of the parents stays near the clutch of eggs and guards it from harm. The female marsupial frog takes her eggs with her, incubating them in a pouch on her back. In one Australian frog species, the female swallows the eggs and the young develop in her stomach and emerge from her mouth as tadpoles or froglets. In Darwin’s frog, the male cares for the young, carrying the eggs and tadpoles in his vocal sac until they have matured into adults.