Amphibia are found everywhere in the world, except Antarctica, and there are about 5,565 different species: 88% of them are in the Anura. In number of species, they are more successful than mammals, though they occupy a smaller range of habitats. However, it is said that amphibian populations have been declining all over the world. Conservation is therefore an important concern.
Amphibians like to live near freshwater in warm weather. There have also been species which live in forests, deserts and arctic conditions. Adult amphibians use lungs, and they also get oxygen through their skin, so long as it is moist.
Amphibians may be camouflaged in brown and green, and if so they are prey for birds and reptiles. Their colour gives them camouflage, which is their main defence.
Alternatively, many other amphibia have toxic skin, which is harmful to predators. These are poisonous to eat. This is an important defence against predation. Connected to this is the use of warning colouration. They may be in vivid colours of red, black, and yellow. Research into the rough-skinned newt and the garter snake shows this is a typical case of co-evolution. Where they live in the same area, the newts get more poisonous, and the snakes develop more resistance to the poison.
Amphibians have colour vision and depth of focus for clear sight. They also have eyelids, glands and ducts which keep the eyes moist. These are adaptations to life on land: amphibia were the first vertebrates to have these features.
Some amphibians, such as the common coquí, lay eggs out of water (in this case, on palm leaves). The eggs develop directly into adult frogs, by-passing the tadpole stage. Others, like mudpuppies and olms, have a different development. In a process called neoteny, they become sexually developed as tadpoles, and continue to live in the water with gills.
The order Anura includes the frogs and toads. There is no fundamental difference between frogs and toads. Frogs have a short body, webbed digits (fingers or toes), protruding eyes, forked tongue and no tail. They are exceptional jumpers: many of their features, particularly their long, powerful legs, are adaptations to improve jumping performance. They often live in semi-aquatic or inhabit humid areas.
A popular distinction is often made between frogs and toads on the basis of their appearance. Toads' warty skin is an adaptation for making their toxic slime. Apart from these glands, their skin is dry, and that is an adaptation to drier habitats. These features have evolved a number of times independently: convergent evolution. The distinction has no taxonomic basis. The only family exclusively given the common name "toad" is Bufonidae (the "true toads"), but many species from other families are commonly called "toads".
The order Caudata is the salamanders.
Newts are salamanders which spend their life in the water even though they are adults. They are classified in the subfamily Pleurodelinae of the family Salamandridae.
Respiration differs between species of salamanders. Species that lack lungs respire through gills. In most cases, these are external gills, visible as tufts on either side of the head. Some salamanders that are terrestrial have lungs that are used in respiration, although these are simple and sac-like, unlike the more complex organs found in mammals. Many species, such as the Olm, have both lungs and gills as adults.
Some terrestrial species lack both lungs and gills and perform gas exchange through their skin. Even some species with lungs also respire through the skin in this manner.
The skin of salamanders secretes mucus. This helps to keep the animal moist when on dry land, keeps their salt balance while in water, and lubricates during swimming. Salamanders also secrete poison from glands in their skin, and some additionally have skin glands for secreting courtship pheromones.
Axolotls, from the genus "Ambystoma" (or mole salamanders), are neotenic amphibians. This means they get to sexual maturity and reproduce while still in a larval form.
Most salamanders and newts have some defence against predators, usually a poison which makes them uneatable. Their bright colours are warning colouration. If, instead, they are camouflaged, this means they are probably not protected by a toxin.
The second line of defence is to shed their tail, which can grow again. The tail wriggles a bit, attracts the predator while the business part of the salamander moves off.
There are over 350 lungless salamanders. Most of them are terrestrial and are active in daytime. Lungless salamanders may communicate with their nose. Slender salamanders are found in the Pacific Coast. They are sometimes called "worm salamanders". This is because they have slimmer (skinny) bodies than most salamanders. If touched, slender salamanders will bounce on the ground and then run away.
The order Gymnophiona includes the caecilians. These are long, cylindrical, limbless animals that look like snakes or worms. Their skin has circular folds, increasing their similarity to the segments of earthworms. Some are aquatic, but most live underground in burrows they hollow out. Many caecilians give birth to live young. In species which lay eggs, the eggs may undergo metamorphosis before they hatch. Caecilians are found in tropical Africa, Asia and Central and South America. There are 171 different species.
Their skins make a mucus that makes them slippery enough to slip through the earth. They can make a toxic material in their skins, as many frogs and toads do.
Amphibians are the only vertebrates to go through metamorphosis. This means that their young look different from their adult. Amphibians usually reproduce in early spring to late summer, though some reproduce in winter and fall. Most frogs and toads, such as the common frog ("Rana temporalis"), gather in large groups to ponds, rivers, swamps and lakes to breed. Male frogs and toads may croak to attract a female. When a female frog has chosen a mate, the male frog hops on top of her. They swim together as she lay eggs in the water. Sometimes, males fight to mate with a female. Frogs can lay up to 100 to 60,000 eggs in one clutch. This is called "frogspawn".
It is a fundamental feature of amphibia that their reproduction is, one way or another, tied to water. This is because their eggs, although covered by jelly, cannot survive long in dry conditions.
Most female amphibians lays her eggs in water. Males release sperm to fertilize them. The eggs are laid one by one or in batches. Batches of eggs can look like a long chain or a ball of foam. They may wrap their eggs around plants in the water. They do this so their eggs will not drift away.
Tree frogs usually lay their eggs on a leaf in a rainwater pool. Bullfrogs, such as the male American bullfrog and the male African bullfrog, stay with their tadpoles and protect them from predators. They also move their tadpoles by using their nose to dig a channel to another place where there is more water. They do this so their tadpoles do not dry up. Most amphibians leave their eggs to look after themselves. Fish and other animals eat most of their eggs. Male midwife toads carry their eggs on their backs. When they are ready to hatch, the toad goes back to the water and release them.
Tadpoles do not have lungs when they hatch and instead have gills. Because gills have a large surface area, tadpoles can get more oxygen by using them. Young tadpoles have their gills exposed. When they get older, their gills are covered over by skin. When they hatch, tadpoles eat constantly. The tadpoles eat what is left of their eggs, this is usually their first food.