Most frogs have a life cycle with two distinct stages, typically living in water as young and on land as adults. Adult frogs lay large numbers of eggs in water or other moist places (A). The eggs hatch into small, fishlike larvae called tadpoles (B), which grow larger as they feed on algae and bits of plant material in the water (C). As it begins metamorphosis, or change into the adult body form, the tadpole grows hind limbs (D). After both pairs of limbs are complete and the animal has exchanged its gills for lungs, the young frog crawls out on land (E). The frog resorbs the rest of its tail and continues to grow until it reaches its adult body size (F).

Most frogs undergo a two-stage life cycle. Eggs hatch into fishlike young called tadpoles, which grow and eventually undergo metamorphosis, a change in body form, to become adults. Frogs have developed many variations on this common theme, and a few species provide striking exceptions to the rule. For example, frogs in the tropical genus Eleutherodactylus do not have a tadpole stage. Instead, tiny froglets hatch directly from the eggs. In the African toad, fertilization occurs internally, and the young are born alive.

Frogs often produce large numbers of eggs—up to 10,000 or more—surrounded by a jellylike coating that protects the eggs from predators and keeps them from drying out. Those that provide some degree of care for their young usually produce relatively few eggs. Species that breed in ponds typically lay their eggs in clumps or strings in the water. Those that breed in streams often attach their eggs to the undersides of rocks. Others construct foamy nests in the water or on vegetation near the water. Some frogs even attach their eggs to the undersides of leaves that hang over the water. When these tadpoles hatch, they drop into the water below. Many species lay their eggs in water trapped in tree cavities or cup-shaped parts of plants. In warmer climates, eggs may hatch within 1 or 2 days, while in colder environments they may take 30 to 40 days to hatch.

Becoming a Frog

The legless tadpoles that hatch from a floating mass of frog eggs are the animal’s fishlike larval stage. Part of a true metamorphosis, they have gills and a tail, both of which disappear as the tadpole feeds and grows. When limbs and air-breathing lungs develop, the young frog, now a miniature replica of its parents, emerges from water to land.

Tadpoles, which usually live in water, look somewhat like small fish. A tadpole has an oval body, gills for breathing, and a long, muscular tail with fins along the upper and lower edges for swimming. Tadpoles have two small eyes, one on each side of the head. A tadpole’s mouth has a horny beak and rows of tiny, comblike teeth that the animal uses to scrape algae from underwater plants and bite off bits of plant material. In a few species, tadpoles eat the eggs and tadpoles of other frogs. Tadpoles pump water through their mouths, over the gills, and out through an opening called the spiracle, which is usually located on the left side of the body. The gills extract oxygen from the water. The tadpole stage lasts anywhere from a few weeks in species that lay eggs in temporary ponds to three years in the bullfrog. Typically, frogs spend a few months as tadpoles before metamorphosing into adults.

Metamorphosis involves radical changes in both external and internal body parts. The tadpole grows legs—the hind legs appear first—and resorbs its tail. It loses its gills and grows lungs, and the structure of the heart, digestive system, and skeleton changes. The horny beak and other mouthparts adapted for eating algae disappear and are replaced by the long, sticky, projectile tongue that helps adult frogs catch insects. Frogs reach reproductive age anywhere from several months to several years after metamorphosing. Most frogs probably live only a year or two in the wild, but bullfrogs live several years, and the African clawed frog has been known to live 35 years in captivity.

Metamorphosis of European common frog

Within 3 days of hatching, the tadpole has acquired gills for obtaining oxygen and has an open mouth that it uses to feed on microscopic plants. Within 12 weeks of hatching, the tadpole has hind legs that have developed from nubs along the sides of its body, and lungs, which it uses to take in air at the water’s surface. At about the same time, a wall of tissue divides the atrium, one of the components of the tadpole’s heart, so that the heart now has three chambers rather than two. This change makes it easier for blood to move between the heart and the tadpole’s newly formed lungs. At about 16 weeks, the tadpole has four legs and bulging eyes, and its tail has begun to be reabsorbed into its body. The tadpole loses its many tiny teeth, and its mouth grows larger while its intestines grow smaller, in preparation for a diet consisting largely of insects. At this point the frog has achieved its adult form, and it climbs out of the water to make its home on land.

Courtesy of Microsoft® Encarta® Encyclopedia 2002. © 1993-2001 Microsoft Corporation.