Exploratorium: Frogs: The Amazing, Adaptable Frog / page 5 of 6

Page 5 of 6


	Clusters of frog eggs in the moist environment at water's edge . [ Click for a larger image.] Photo by Amy Snyder.

Frogs eggs hatch anywhere from three to 25 days after they are laid. Most hatch not into frogs, but into fish-like tadpoles, complete with gills and a tail. Round, neckless creatures, tadpoles eat voraciously to fuel their metamorphosis into frogs. Hardly anything is beyond the appetite of a tadpole; they'll eat plants, decaying animals, and sometimes even frog eggs and other tadpoles.

Metamorphosis usually takes several weeks. Species that breed in temporary ponds may develop more quickly than this, for if the pond dries up too soon, the underdeveloped tadpoles will perish. By contrast, bullfrog tadpoles may take several years to grow into frogs.

The changes begin when the hind legs sprout. Soon after, lungs develop and the front legs appear. Meanwhile, the tail gradually shrinks. Just before becoming a frog, the tadpole's gills disappear. The tiny froglet emerges from the water with just a stump of a tail, which soon disappears.

In the wild, most frogs live for just a few years. In captivity, well-cared-for frogs can live for 10 or even 20 years. One toad reportedly lived for 36 years under the front steps of a house in England. This toad is said to have been so tame that often came out to be fed and have its back scratched.

Most frogs lay thousands of eggs and then abandon them, leaving their fates up to chance. But some frogs have developed more careful approaches to bringing the next generation into the world. The marsupial frog keeps her eggs in a pouch like a kangaroo. When the eggs hatch into tadpoles, she opens the pouch with her toes and spills them into the water. A Surinam toad carries her young embedded in the skin of her back, where they develop until hatching fully formed.


Pipa pipa, the Surinam toad of South America, carries her young embedded in the skin of her back, where they develop until hatching fully formed. [ Click for a larger image.] Photo by Amy Snyder.

The gastric brooding frog of Australia swallows her fertilized eggs. The tadpoles remain in her stomach for up to 8 weeks, until finally hopping out of her mouth as little frogs. During the brooding period, gastric secretions cease--otherwise she would digest her own offspring. Among Darwin frogs, it is the male who swallows and stores the developing tadpoles--in his vocal sac.

Midwife Toads attach the fertilized eggs to their hind legs. The male carries the eggs around this way for about a month, sometimes dipping them in water to keep them moist. When the eggs are ready to hatch, he brings them to a pond and releases them.

Barking frogs, which live on rocky cliffs in Texas, are equally devoted fathers. (This toad-like frog is so named because it has a voice like the yapping of a small dog.) After the female lays her eggs in a protected crevice, the male fertilizes and then guards them. For four or five weeks he waits by the eggs, moistening them with his urine when they become too dry, until finally the froglets emerge.