Reptiles and Amphibians

Long before there were mammals or birds, the land belonged to animals that could not make their own heat. Amphibians were the first vertebrates to crawl out of the water, about 370 million years ago; reptiles, their descendants, were the first to leave it behind completely. Both groups are still with us, in enormous variety, and being "cold-blooded" turns out to be less a limitation than a different strategy.

Living on borrowed heat

Mammals and birds burn food to keep their bodies warm all the time. Reptiles and amphibians do not; their body temperature follows their surroundings, and they warm up by basking in the sun and cool down in the shade. The cost is that they are sluggish in the cold and cannot live where it is always cold; there are no reptiles in Antarctica and few in the far north. The benefit is enormous. A reptile needs about a tenth of the food of a mammal the same size. A crocodile can go a year without eating. Where food is scarce or unpredictable, in deserts and on small islands, cold blood wins, and lizards and snakes are the dominant land vertebrates.

Amphibians: two lives

The word amphibian means "double life", and most of them lead one. Frogs, toads, salamanders and the worm-like caecilians generally lay soft eggs in water. The young, tadpoles, hatch as swimming, gill-breathing creatures with tails, then transform, growing legs and lungs, losing their tails and gills, and emerging onto land. Their skin stays thin and moist and must be kept that way, because they breathe partly through it; that is why amphibians are tied to damp places and why they are so vulnerable to pollution, which passes straight through.

They are also full of surprises. The wood frog of North America freezes solid in winter, its heart stopped and its blood turned to ice, protected by sugar pumped into its cells, and thaws out alive in spring. Poison dart frogs of South America carry toxins in their skin strong enough to kill a dozen men. Some salamanders can regrow a lost leg, tail, jaw or even parts of the heart and brain, an ability scientists are studying for medicine. Frogs' calls, the chorus of a tropical night, come in thousands of species-specific songs.

Reptiles: sealed for land

Reptiles made the break from water with two inventions. The first was waterproof skin covered in scales, so they do not dry out. The second was the egg with a shell, inside which the embryo floats in its own private pond, so reptiles could breed far from water. Those two changes opened the continents, and for 150 million years their descendants, the dinosaurs, ruled them.

Today's reptiles fall into four groups. Lizards, about 7,000 species, range from geckos that walk up glass using microscopic hairs on their toes, to the chameleon with its colour changes and a tongue longer than its body, to the Komodo dragon, three metres long with a venomous bite. Snakes are lizards that lost their legs around 100 million years ago; some 4,000 species swallow prey whole, their jaws unhinging to take animals wider than their heads, and the pit vipers see the body heat of prey in the dark with infrared sensors on their faces. Turtles and tortoises carry a shell of bone fused to the ribs and spine, a design unchanged for 200 million years; they are among the longest-lived of all animals. Crocodiles and alligators, the closest living relatives of birds, are ambush hunters that have barely changed since the age of dinosaurs, with the strongest bite of any living animal.

The amphibian crisis

Amphibians are in more trouble than any other group of vertebrates. About 40 percent of species are threatened with extinction, and dozens have disappeared in recent decades. Habitat loss and pollution are part of it, but the main cause is a fungus, spread around the world by human trade, that attacks the skin frogs breathe through. Because amphibians eat vast numbers of insects and are themselves food for many birds and mammals, their decline unravels wetland ecosystems.

Why it matters

Reptiles and amphibians are a living record of how life moved from water to land. They control insects and rodents, and their venoms have yielded drugs for blood pressure, diabetes and pain. The frog in the pond and the lizard on the wall are also the heirs of the first creatures ever to breathe air on land, still doing it their way, 370 million years on.