Chapter 2

What is an Amphibian?

Soft-skinned shape-shifters that live a double life in water and on land.

The word amphibian comes from Greek and means "double life" — because most amphibians start life in the water and finish it on land. They are cold-blooded vertebrates with soft, moist skin and include frogs and toads, salamanders and newts, and the worm-like caecilians. There are around 8,000 species worldwide, and they were the very first backboned animals to crawl out of the water onto dry land, over 370 million years ago.

Sections

Lifecycle

Almost every amphibian begins life in water. Adults lay soft, jelly-covered eggs in ponds, streams, or damp places. Tiny larvae hatch out — tadpoles for frogs and toads, or gilled larvae for newts and salamanders — and they swim, breathe through gills, and feed until they are big enough to change into their adult form.

Skin

Amphibian skin is thin, smooth, and moist. It has no scales, so it must stay damp to work properly. Many species can even breathe through their skin, taking in oxygen straight from the air or water. Special glands produce mucus to keep the skin slippery, and some frogs make bitter or toxic chemicals to keep predators away — the poison dart frogs of South America are famous for this.

Breathing

Amphibians breathe in more ways than any other vertebrate. Tadpoles use feathery gills, adults usually have lungs, and almost all of them also absorb oxygen through their damp skin. A few salamanders have no lungs at all and rely entirely on skin-breathing, which is why they must live in cool, wet places.

Metamorphosis

Metamorphosis is the amazing change from larva to adult. A tadpole slowly grows back legs, then front legs, its tail shrinks and is reabsorbed, its gills close up, and lungs take over. The mouth changes shape so a plant-eating tadpole becomes an insect-hunting frog. This whole transformation can take a few weeks or several years, depending on the species.

Habitats

Amphibians live on every continent except Antarctica, but they always need moisture. You will find them in ponds, streams, swamps, rainforests, and damp leaf litter. Some tree frogs live high in the rainforest canopy, some toads survive in deserts by burrowing underground, and a few salamanders live only in cold mountain caves.

Conservation

Amphibians are the most threatened group of vertebrates on Earth — around 40% of species are at risk. Their thin skin absorbs pollution easily, so they are quickly harmed by dirty water, pesticides, and climate change. A deadly fungus called chytrid has also wiped out many populations. Protecting wetlands and forests is the best way to help them recover.

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