Young frogs start their lives in eggs and then hatch as exclusively aquatic larvae known as tadpoles. Unlike adult frogs, tadpoles lack limbs and have long, paddle-like tails that allow them to survive in water. As they grow, their physical characteristics change markedly along with their behavior and feeding preferences. The life cycle of frogs displays one of the most remarkable cases of transformation, or metamorphosis, in vertebrates.
The long tail and lack of limbs are among the most striking differences between tadpoles and adult frogs, but other characteristics are also different. The jaw and skull structure of the tadpole varies greatly from that of the adult frog. Tadpoles have cartilage as opposed to bony skulls; they also have small teeth that allow them to chew up pond plants and similar organic matter.
As tadpoles grow, limbs begin to appear as their paddle-like tail recedes. The rear legs form before the front limbs and their bodies change and develop over time into a young frog. The jaw and head also change, becoming a more defined jaw, and a muscular tongue takes the place of the teeth. Gills give way to lungs and the intestines shorten to handle the more carnivorous diet of adult frogs.
The change from swimming tadpole to amphibious frog takes varying amounts of time, depending on species and geographic range. For some species, the tadpole life stage lasts only a few weeks; for others, like certain bullfrogs, the transformation may take years.
Whether they are land-dwelling, semi-aquatic or aquatic, nearly all frog species breed in or near water to ensure their tadpoles survive. Males gather in large groups and start calling to attract females; each species of frog has a distinctive call. In some cases, males of numerous species gather at the same body of water.
Most frogs reproduce with external fertilization. Male frogs grasp the female in a position atop the female's back known as amplexus. During amplexus, the male releases his sperm, fertilizing the eggs as they exit the female's body. A protective jelly-like substance that quickly absorbs water surrounds the eggs. Many species' eggs attach to vegetation in the water while others float along the surface. Only a few species reproduce by internal fertilization.