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Frogs and toads belong to the amphibian class of vertebrate animals. Amphibians have the distinguishing characteristic of starting their lives as larvae, during which time they reside under water and breathe through gills, and undergoing metamorphosis that transforms them into adult land dwellers who breathe air through their lungs. The life cycle of a frog begins as an egg that is laid in a body of water. There are thousands of known frog species that inhabit a diverse range of climates throughout the world. The incubation periods for frog eggs vary between species, from as little as 48 hours to as long as 23 days. Once the egg hatches, the larval stage begins. The organism is called a tadpole or pollywog at this stage, and it is characterized as having a mouth at one end, a long tail at the opposite end, no legs or arms and gills instead of lungs.
The Life and Times of Tadpoles
During the larval stage of the frog life cycle, tadpoles are primarily herbivores, feeding off of algae and dead plant matter within the water. They soon grow teeth to prepare them for their adult life as a carnivore whose diet will consist of worms, flies and other insects. Skin begins to form over the gills at approximately four weeks. Between 6 and 9 weeks old, on average, the tadpole will begin to develop legs and a more defined shape to his head. The rear legs begin to appear before the arms. Once the legs have begun to grow, the tadpole will be approaching metamorphosis, the most delicate and crucial phase of his development. Metamorphosis occurs over roughly a four-week span, beginning at an average age of nine weeks.
Metamorphosis: From Froglet to Frog
At this stage, the tadpole may be called a froglet. He looks more like a frog than a fish, and it is during metamorphosis that he undergoes his final transformation. Once you observe arms budding from your tadpole's body, it is time to provide a land area in addition to his aquatic zone. Failure to do so will result in the frog's death by drowning when he scrambles to escape the water to breath. During metamorphosis, the froglet will complete the development of his lungs. Once these lungs are fully functional, he will require oxygen from the air to survive. In addition to lung development, the froglet will undergo the following changes:
- The gills will cease to function once the lungs take over.
- The shape of the mouth and jaws will change and a long tongue will develop.
- The gastrointestinal tract changes to adapt from digesting an herbivorous diet to a carnivorous diet.
- The tail shrinks and disappears as it gets reabsorbed into the body.
- The skin is shed and is replaced by a permeable skin that enables respiration through the skin.
- The skeleton continues to develop and the legs and arms grow longer.
Hopping Through Adulthood
Once metamorphosis is complete, the adult frog will spend the majority of his life on land, taking to the water only for occasional splashing, sunbathing on lily pads and to reproduce and lay the eggs of the next generation. There are a few frog species, such as the African clawed frog, that remain aquatic throughout their lives, taking in air from just above the water surface into their lungs. From tadpole through metamorphosis, the transformation period varies among the different species and their climates. Those that reside in colder environments, such as the grass frog of northern Europe, take longer to develop, and they also take longer to reach full maturity, than those in warmer environments, such as the Pacific tree frog. The age at which frogs are fully mature ranges from 2 months to 3 years.