the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the main leg muscles accounting for over 17% of the total mass of the frog.Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time thespecies spends in the water.The completely aquatic African dwarf frog(Hymenochirussp.) has fully webbed toes, whereas those of White's tree frog(Litoria caerulea), an arboreal species, are only a quarter or half webbed.Arboreal frogshave pads located on the ends of their toes to help grip vertical surfaces. These are not suction pads, the surface consisting instead of columnar cells with flat tops with small gaps between themlubricated by mucous glands. When the frog applies pressure, the cells adhere to irregularities on the surface and the grip is maintained through surface tension. This allows the frog to climb on smooth surfaces, but the system does not function efficiently when the pads are excessively wet.In many arboreal frogs, a small "intercalary structure" on each toe increases the surface area touching the substrate. Furthermore, since hopping through trees can be dangerous, many arboreal frogs have hip jointsto allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes. This allows the frogs to "parachute" or make a controlled glide from one position in the canopy to another.Ground-dwelling frogs generally lack the adaptations of aquatic and arboreal frogs. Most have smaller toepads, if any, and little webbing. Some burrowing frogs such as Couch's spadefoot(Scaphiopus couchii) have a flap-like toe extension on the hind feet, a keratinisedtubercleoften referred to as a spade, that helps them to burrow.Sometimes during the tadpole stage, one of the developing rear legs is eaten by a predator such as a dragonfly nymph. In some cases, the full leg still grows, but in others it does not, although the frog may still live out its normal lifespan with only three limbs. Occasionally, a parasitic flatworm(Ribeiroia ondatrae) digs into the rear of a tadpole, causing a rearrangement of the limb bud cells and the frog develops an extra leg or two.Northern leopard frog(Rana pipiens) moulting and eating its skin.SkinA frog's skin is protective, has a respiratory function, can absorb water and helps control body temperature. It has many glands, particularly on the head and back, which often exude distasteful and toxic substances. The secretion is often sticky and helps keep the skin moist, protects against the entry of moulds and bacteria, and make the animal slippery and more able to escape from predators.The skin is shed every few weeks. It usually splits down the middle of the back and across the belly, and the frog pulls its arms and legs free. The sloughed skin is then worked towards the head where it is quickly eaten.
Being cold-blooded, frogs have to adopt suitable behaviour patterns to regulate their temperature. To