Frogs (and other amphibians) are unique to other animals as they have the ability to live both on land and in water, performing metamorphoses during their life cycle; in the case of frogs, developing from eggs/larvae, to tadpoles and finally to adult frogs.
The frogs moist skin (along with their lungs) aids in conducting gas exchanges; the skin is also used to secrete poisons, aiding as a form of defense.
There are approximately 4,000 frog species worldwide. The Commonwealth of Australia's website entitled "Australian Frogs", notes that Australia has 208 frog species, many of which are found only within Australia.
Frog Reproduction
With the exception of a few species, mating of frogs occurs in the water, with males climbing onto a female's back, and grasping her around the body with his legs (known as 'amplexis'), either in front of her hind legs, just behind the her front legs, or around her head (however, this is rare).
R G Zweifel, in the 1993 edition of The Encyclopedia of Animals, entitled "Frogs and Toads", explains that frogs maintain the amplexis position for up to several days, with females laying masses of eggs at regular intervals. Once layed, the eggs will immediately be fertilised by the male as he sheds his sperm externally over them in the water. Following this, they will become surrounded by a transparent jelly-like substance which serves to protect them.
Tadpole Development
Dorota in "Weird Frog Facts" details that after fertilisation, the eggs will begin to develop into tadpoles, escaping their protective jelly between six and twenty-one days, where they will attach themselves to pieces of pond grass or weeds with the use of a sucker found between the mouth and belly area. At this stage, the tadpoles are extremely undeveloped, containing external gills, a mouth and a tail (AllAboutFrogs.org)
After approximately one week, the tadpoles will begin to swim around and feed on algae; by four weeks, the external gills will disappear behind a structure known as the operculum, which aids in protecting them, and will develop tiny teeth which they use to feed on vegetation.
Within six to nine weeks, the tadpoles will begin to grow hind limbs, followed by front limbs. Their head will become more distinct, their bodies elongated and lungs form. By this stage, the tadpoles will begin feeding on insect larvae and crustaceans, becoming predominantly carnivores.
Frog Development
Zweifel in The Encyclopedia of Animals, further explains that from 12 to 16 weeks (depending on the temperature and quality of food and water), the tail is completely reabsorbed into the body, whereby the tadpole becomes a small frog. The frog will leave the water and begin to live close to the water's edge amongst the grass and undergrowth, where it will develop into an adult frog, where the cycle will begin again.
