An Amphibian is a group of animals that are born and live in water when they are young (larvae) and live mostly on land which is near a water source when they are adults.
The word ‘amphibian’ comes from the Greek word ‘amphibios’ meaning ‘double life’ as these animals are able to live both on land and in water. Amphibians were the first land animals and have kept close links to water where their ancestors originated. Many species of amphibians live in temperate regions, however, some species are found in more humid conditions such as tropical forests. When the weather turns very cold, amphibians hibernate by burying themselves in the mud at the bottom of ponds or under stones or logs.
Amphibians have been around for around 230 million years and are known to have evolved from fish. They evolved and adapted to surviving in moist, freshwater habitats. Amphibians are cold-blooded (ectothermic) vertebrates that draw their body heat from their environment. They bask in the sun to warm-up and enter the water to cool down. Amphibians have smooth skin, 4 limbs and are divided into 3 groups:
Caecilians Frogs and Toads Newts and Salamanders
Amphibians are carnivores and feed upon living prey rather than carrion (dead animals). Most amphibians have large, wide mouths to consume large animals. Frogs and Toads can eat large animals such as mice, birds, small reptiles and small snakes. One of these meals will satisfy their energy requirements for a long time. Typically, an adult amphibian diet will consist of insects, snails, slugs and spiders. Amphibian larvae feed upon aquatic invertebrates. Sometimes larvae can become cannibalistic and consume their own kind. Some species such as the Mexican Burrowing Toad have small mouths and feed only upon termites.
Amphibians are typically aquatic as larvae and terrestrial as adults. They depend highly on water, especially freshwater. No amphibians inhabit salt water (oceans and seas). Amphibian eggs generally lack shells and all are produced in water. As the eggs develop into larvae, they survive for long periods of time in the water before undergoing the complex transformation of metamorphosis. Many changes happen to their body form that will eventually equip them for life as adults in terrestrial habitats. Larval gills are lost and replaced by air-breathing lungs and in the case of frogs and toads, their tails (which they have as tadpoles) are reabsorbed and they develop limbs.
Parental care varies in different species of amphibians. Some lay many eggs and do not protect them at all. Some have evolved parental care and lay a small number of eggs which are protected by both parents. In caecilians, newts and salamanders, the female usually provides protection whereas in frogs and toads, the male protects the eggs from predators. Because amphibian eggs lack shells, protection is very important to ensure the eggs do not dehydrate or become infected by fungi, as well as being eaten by predators.
The skin of an amphibian is kept moist by special glands under the surface of the skin that produce a sticky substance called mucus. They do not have scales, feathers or hairs. Some amphibians draw in oxygen through their skin and adults are able to breathe through both their lungs and their skin. As blood flows through an amphibians gills at the same time water flows over the outside, oxygen passes out of the water straight into the amphibians bloodstream.
Most amphibians are brightly coloured, particularly poisonous species. These bright colours serve as a warning to potential predators that they are not good to eat and also for camouflage. Some amphibians can change the colour of their skin in response to changes in temperature or light levels.
Facts about amphibian anatomy
Amphibian skeletons have far fewer bones than those of other vertebrate species.
Amphibians have a 3 chambered heart, not 4 as in mammals.
Amphibians have 5 basic senses, taste, hearing, smell, sight and touch, just like humans. However, they can also detect the Earths magnetic field and infrared and ultraviolet light.
Toads and Frogs (order: Anura) have flattened skulls.
Toads and Frogs have short backbones and enlarged hind legs.
Salamanders and Newts (order: Caudata/Urodela) have longer backbones than toads and frogs. Their limbs are roughly the same size and they have long tails.
Newts have flattened tails which makes them excellent swimmers.
Caecilians (order: Gymnophiona) have no limbs and move along by muscle contractions that move along their body. With their segmented bodies, they are highly specialized for burrowing.
Caecilians have a tentacle on each side of the head located between the eye and the nostril. The tentacles are used as scent detectors to locate prey.
All caecilians have internal fertilization. Some caecilians are oviparous and have aquatic larvae. The larvae have gill slits but no external gills. Around 75% of the species are viviparous, meaning that they give birth to already developed offspring.
Some amphibians such as snakes and lizards shed their skin. Some toads such as the European Toad sheds its skin several times during the summer months and then eats it. This recycling habit keeps the toads skin healthy and full of goodness.