Another well-known case is that of most amphibians, frogs and toads. These amphibious animals are the evolutionary step between aquatic and terrestrial life, and for that reason they must always reproduce in water. Their life cycle comprises the following stages:
- Eggs: The life of a frog begins when a female lays her eggs in a body of water, such as a river, lake or puddle of some kind, next to a male who immediately proceeds to fertilize them. The eggs then mature, as the life inside them reaches the right point, and they hatch to release tadpoles.
- Tadpoles: This is the name given to frog “larvae”, that is, to their infantile forms, which lead a 100% aquatic life. In fact, they have long, limbless bodies, similar to fish, equipped with tails, gills and a suction cup to hold on to objects. Tadpoles live for about three months, feeding on everything they can, during which time they gradually change as they accumulate the energy necessary for metamorphosis. Then, the changes begin: the hind legs begin to sprout, then the front ones, and the body increases considerably in size. The lungs develop inside the young frog, as the tail and gills are gradually reabsorbed, preparing the animal for its future terrestrial life.
- The frog: Once metamorphosis is complete, an adult frog leaves the water and begins its adult life on dry land, although returning from time to time to hydrate and spawn, thus repeating its life cycle.