Frogs mating in the water Amphibians are a small group of cold-blooded, vertebrate animals. They include two main groups: the salamanders and newts, and the frogs and toads. The word “amphibian” means “double life”. Most amphibians spend the first part of their lives underwater, breathing through gills. Their adult lives are generally spent on land, breathing through lungs. They usually return to the water to lay eggs. All amphibians, in their adult forms, are predators.
The first amphibians
Amphibians were the first vertebrates to live on land more than 360 million years ago. They evolved from fish that had moved on to the land and developed lungs. Early amphibians looked similar to salamanders. Some were huge, reaching 4 metres (13 feet) in length. They fed on fish, insects and other invertebrates. All early amphibians had to return to water to lay their eggs.
All amphibians have moist skin, through which they can “breathe in” oxygen dissolved in water. They can also take in or lose water from their bodies through their skin. Because they need to keep their skin moist, most amphibians spend much of their time in cool, damp conditions. Many live in or near water, even as adults.
All the stages in the metamorposis of a frogAmphibians undergo metamorphosis, a change in the body from young, called larvae, to adults. The most obvious change in salamanders and newts is that they lose their feathery gills and develop lungs. Frogs and toads change from plant-eating tadpoles with gills, a tail and no legs, to carnivorous, tail-less adults with lungs and long legs. Some salamander larvae become adults without undergoing full metamorphosis. Axolotls, for example, have features that are typical of salamander larvae, including external gills and a long fin extending the length of their backs.
Many amphibians can produce bad-tasting or poisonous substances from their skins to deter predators. Brightly coloured skin is a warning sign. If attacked, some salamanders and newts will lash out with their heads or tails, where their poison glands are located. Others suddenly freeze, hoping to confuse their attackers.
Frogs and toads
About 90% of all amphibian species are anurans: the frogs and toads. Frogs are short, tail-less amphibians with long hind legs and smooth skins. They live either on land or in trees, although all species return to fresh water to lay their eggs, called spawn. Their young are called tadpoles. Frogs can make huge leaps and swim quickly. People use the word "toad" when describing anurans that have drier, lumpier skins, that live mostly on land and that crawl rather than hop.
Salamanders and newts
Salamanders are amphibians with long bodies and tails. Some are fully aquatic, while others live on land and return to the water only to lay their eggs. Many species are brightly coloured to warn that they are toxic. Newts are a type of salamander with long, flattened tails.
A ringed caecilian slithers through the undergrowth
Caecilians are long, snake-like amphibians with no limbs. They have small, almost sightless eyes and live underground in wet tropical regions. Their skins contain toxic glands to deter predators. Though not blind, a caecilian's eyes are tiny and covered by skin for protection: they can sense little more than the difference between darkness and light.
Consultant: Chris Jarvis