The World of Reptiles and Amphibians

Two Ancient Groups

Imagine a still pond at the edge of a forest. A frog sits half-hidden in the water, its eyes peeping above the surface. Nearby, a lizard basks on a warm rock, soaking up the morning sun. These two creatures may live side by side, but they belong to two very different groups of animals: the amphibians and the reptiles.

Both groups are ancient. Their ancestors crawled across the Earth long before the first mammals or birds appeared. Today they live almost everywhere, from steaming jungles to dry deserts, and they include some of the most fascinating animals on the planet.

Scientists who study reptiles and amphibians are called herpetologists. In this book, we will discover what makes each group special, how their bodies work, and why these often-overlooked animals are so important to the health of our world.

Cold-Blooded Survivors

The first thing to know about both reptiles and amphibians is that they are ectothermic. People often call this "cold-blooded," but that name can be misleading — their blood is not always cold.

Warm-blooded animals like us are endothermic: we burn food to make our own steady body heat. Reptiles and amphibians cannot do this very well. Instead, their body temperature rises and falls with their surroundings. To warm up, a lizard will lie in the sun, or bask. To cool down, it will move into the shade or burrow underground.

This way of life has a big advantage: ectothermic animals need far less food than mammals, because they do not burn energy to stay warm. A large snake can survive for weeks, sometimes months, on a single big meal. The trade-off is that in cold weather these animals become slow and sleepy, and many spend the winter in a deep rest called brumation.

Reptiles: Scales and Shelled Eggs

Reptiles were among the first animals to live their whole lives on land, and their bodies show it. They have two key features that set them apart.

First, reptiles have dry, scaly skin. The scales are made of a tough material called keratin — the same stuff in your fingernails. This skin acts like waterproof armour, locking moisture inside so the animal does not dry out, even in a hot desert. As a reptile grows, it sheds its old skin, sometimes all in one piece, like a snake leaving behind a perfect see-through copy of itself.

Second, most reptiles lay eggs with a tough, leathery or hard shell. The shell protects the developing baby and stops it from drying out, so reptiles can lay their eggs on dry land. The young hatch out as miniature versions of their parents, ready to fend for themselves. Reptiles are divided into several main groups, and we will meet them next.

Snakes and Lizards

The largest reptile group is the squamates — the snakes and lizards. There are thousands of kinds, living on every continent except Antarctica.

Lizards usually have four legs, clawed feet and a long tail. Some, like the chameleon, can change colour and move each eye in a different direction. The geckos can climb smooth glass thanks to millions of tiny hairs on their toes. The largest lizard, the Komodo dragon, can grow longer than a tall adult and hunts large prey.

Snakes are lizards' close relatives that, over millions of years, lost their legs. A snake moves by rippling its muscles and gripping the ground with the scales on its belly. Snakes cannot chew, so they swallow their food whole, their jaws stretching wide thanks to a very flexible skull. Some snakes squeeze their prey, while others inject venom through hollow fangs. A snake also "smells" with its flicking tongue, picking up scent particles from the air and pressing them to a special sensor in the roof of its mouth.

Turtles and Crocodilians

Two more reptile groups are easy to recognise.

Turtles and tortoises carry their home on their backs. Their shell is part of their skeleton — the backbone and ribs are fused into it — so a turtle can never crawl out of its shell. Sea turtles are graceful swimmers that travel thousands of kilometres across oceans, while tortoises are slow land-dwellers. Some giant tortoises live for more than a hundred years, making them among the longest-living animals on Earth.

Crocodilians include crocodiles, alligators, caimans and gharials. These powerful hunters have changed little in millions of years. They lurk almost hidden in water, with only their eyes and nostrils showing, then lunge with astonishing speed and a bone-crushing bite. Despite their fierce reputation, crocodilians are surprisingly caring parents, often gently carrying their newly hatched babies to the water in their mouths.

Amphibians: A Life in Two Worlds

Now let's cross over to the amphibians — the frogs, toads, salamanders, newts and a strange burrowing group called caecilians. The word amphibian comes from Greek words meaning "two lives," and that is the key to understanding them.

Most amphibians begin life in water and later move to land, living a kind of double life. They have two more features that set them apart from reptiles.

First, amphibians have thin, moist skin with no scales. This skin must stay damp because amphibians can actually breathe through it, taking in oxygen straight from the water or air. That is why frogs are usually found in damp places and feel slippery to the touch.

Second, amphibians lay soft, jelly-like eggs with no shell. Because these eggs would dry out on land, most amphibians lay them in water or in very wet places.

The Amazing Amphibian Life Cycle

The double life of an amphibian is clearest in its life cycle, and the frog is the perfect example.

A frog's life begins as a cluster of jelly-covered eggs floating in a pond, called frogspawn. From each egg hatches a tadpole — a tiny swimmer with a tail and feathery gills for breathing underwater, just like a fish. At this stage, the young frog is entirely a water creature.

Then an incredible change called metamorphosis begins. The tadpole slowly grows back legs, then front legs. Its tail shrinks away, its gills are replaced by lungs, and its mouth widens. At last, a small frog climbs out of the water, now able to breathe air and hop on land. This complete change from water-breathing larva to air-breathing adult is one of nature's most remarkable transformations.

Not all amphibians follow exactly this path — some salamanders keep their gills for life — but the watery start is a feature of the whole group.

Why They Matter

Reptiles and amphibians may be quiet and easily overlooked, but they play huge roles in nature.

They are vital links in food chains. Frogs and lizards eat enormous numbers of insects, helping to keep pests under control, while snakes keep rodent numbers down. In turn, these animals are food for birds, mammals and fish. Remove them, and the whole web of life wobbles.

Amphibians are especially important as indicator species. Because their thin skin soaks up water and chemicals so easily, they are among the first creatures to suffer when a habitat becomes polluted or when the climate changes. When frogs start to vanish from a pond, scientists know something is wrong with the environment. Sadly, amphibians are among the most endangered animals in the world today.

We can help by protecting wetlands, ponds and forests, keeping water clean, and being careful with chemicals that can wash into the places these animals live.

What We Learned

We have explored two of the oldest and most fascinating groups of animals on Earth.

We learned that reptiles and amphibians are both ectothermic, relying on their surroundings to control their body temperature. Reptiles conquered the land with dry, scaly skin and shelled eggs, and include snakes, lizards, turtles and the powerful crocodilians. Amphibians live a double life, with moist skin they can breathe through and a remarkable life cycle that carries them from water-living tadpole to air-breathing adult. And we discovered that both groups are essential to healthy ecosystems, with amphibians acting as early warning signs for the whole planet.

The next time you spot a frog by a pond or a lizard on a sunny wall, take a closer look. You are watching a survivor from an ancient world.

Want to keep exploring the living world? Travel back to ancient reptiles in All About Dinosaurs, or meet creatures that come alive after dark in Creatures of the Night.