Frogs are amphibians. Most frogs are characterized by long hind legs, a short body, webbed fingers or toes, protruding eyes, and the absence of a tail. Frogs are widely known as exceptional jumpers, and many of the anatomical characteristics of frogs, particularly their long, powerful legs, are adaptations to improve jumping performance. They are the best jumper of all vertebrates. The Australian rocket frog, Litoria nasuta, can leap over 50 times its body length (5.5 cm), resulting in jumps of over 2 metres. That would be equivalent to a person who is 2 metres tall (or approximately 6 feet) jumping 100 m!
Due to their permeable skin, frogs are often semi-aquatic and inhabit humid areas. Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area. However, the permeability of a frog's skin can also result in water loss. Some tree frogs reduce water loss with a waterproof layer of skin. Camouflage is a common defensive mechanism in frogs. Most camouflaged frogs are nocturnal, which adds to their ability to hide. Nocturnal frogs usually find the ideal camouflaged position during the day to sleep. Some frogs have the ability to change colour, but this is usually restricted to shades of one or two colours. For example, White's tree frog varies in shades of green and brown.
Many frogs contain mild toxins that make them unpalatable to potential predators. For example, all toads have large poison glands. These are called the parotoid glands, which are located behind the eyes on the top of the head. Some frogs, such as the poison dart frog, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors. Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller 200 times more potent than the human drug morphine, is found in some species of poison dart frogs. In the future, other chemicals could be isolated from the skin of frogs that may one day offer resistance to other human diseases.
The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are a number of blood vessels near the surface of the skin. When a frog is underwater, oxygen is transmitted through the skin directly into the bloodstream. On land, adult frogs use their lungs to breathe. Their lungs are similar to those of humans, but the chest muscles are not involved in respiration, and there are no ribs or diaphragm to support breathing. Frogs breathe by taking air in through the nostrils (that often have valves that close when the frog is submerged), causing the throat to puff out and compress the floor of the mouth, which forces the air into the lungs.
Frogs are known for their three-chambered heart, which they share with all tetrapods except birds, crocodilians, and mammals. In the three-chambered heart, oxygenated blood from the lungs and deoxygenated blood from the respiring tissues enter by separate atria The thin-walled chamber of the heart that receives blood from veins (plural: atria). , and is directed via a spiral valve to the appropriate vessel – aorta for oxygenated blood and pulmonary artery for deoxygenated blood. This special structure is essential in keeping the mixing of the two types of blood to a minimum, which enables frogs to have higher metabolic rates and to be more active.
The frog's digestive system begins with the mouth. Frogs have teeth along their upper jaw called the maxillary teeth that are used to grind food before swallowing. These teeth are very weak and cannot be used to catch or harm agile prey. Instead, the frog uses its sticky tongue to catch food such as flies or other insects. The food then moves through the esophagus into the stomach and then into the small intestine ( duodenum and ileum) where most of the digestion occurs. Frogs secrete pancreatic juices (produced from the pancreas) and bile (produced by the liver) through the gall bladder to the small intestine where the fluids digest the food and extract the nutrients. When the food passes into the large intestine, the water is reabsorbed and wastes are routed to the cloaca. All wastes exit the body through the cloaca and the cloacal vent.
Frogs typically lay their eggs in puddles, ponds, or lakes, and their larvae, called tadpoles, have gills and develop in water. Adult frogs follow a carnivorous diet, mostly of arthropods, annelids, and gastropods. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their mating season.
The distribution of frogs ranges from tropic to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5 000 species, they are among the most diverse groups of vertebrates. However, populations of certain frog species are declining significantly due to human activities. A Canadian study in 2006 suggested that heavy traffic near the frog habitats was a large threat to the population.