Reptiles are ectothermic animals, meaning they use their surroundings to regulate their body temperature. By basking in sunny spots to warm up and retreating to shady places to cool down, they conserve energy.
The class Reptilia includes turtles (order Testudines), crocodiles, lizards, snakes and the tuatara of New Zealand. They share a common ancestor with mammals and birds, which belong to the class Aves.
Snakes are one of the many kinds of reptiles, which also include lizards, tortoises and turtles, alligators, crocodiles and birds. All of these animals are cold-blooded, usually scaly, and live in dry, hot or wet climates. Most of them lay eggs.
Snakes, in particular, are elongated carnivorous vertebrates of the suborder Serpentes. They are limbless, ectothermic (cold-blooded), amniote reptiles covered in overlapping scales. Like all squamates, their skeletons have a front limb-like segment attached to the skull; however, front limbs are absent in most snakes because of a change in the Hox genes that control limb morphogenesis. Most snakes have a reduced axial skeleton consisting of a neck, thoracic, lumbar, sacral and caudal vertebrae; a few snakes—most notably pythons—retain the pelvic girdle and vestigial rear limbs of their ancestors.
Snakes are found on every continent except Antarctica and most smaller land masses. They have an incredible range of habitats, including temperate forests and jungles, deserts, grasslands, swamps and oceans. Most of them are primarily carnivorous, eating small mammals, birds, eggs and fish, but some snakes are herbivorous, especially the pythons and anacondas. Snakes use a variety of hunting strategies to catch their prey, such as hiding and ambushing. Some have special jaws that allow them to swallow creatures much larger than themselves. Most of the 3,000 or so snake species are nonvenomous, but some have teeth and fangs that can deliver dangerous toxins.
Lizards are well-known for the ability of some species to regenerate their tails, a trait that has led to their being valued as pets. In addition to this, their ability to regulate their body temperatures and their varied modes of reproduction make them important subjects for ecologists and behavioral ethologists. They are also valued as natural insect-control agents, particularly in tropic and desert environments. However, their role as pests in some cases—for example, in preying on ranch and farm animals and stealing chicken eggs—can also have negative effects on humans and the environment.
Like snakes, lizards are considered to be tetrapods (four-legged vertebrates that evolved from amphibians). Their thick, dry skin and hard bony skeletons offer them advantages in many habitats and conditions that are difficult for other mammals or birds to survive in.
Most lizards are low-slung creatures that writhe slowly or scamper along the ground, but the number of variations is incredible. Some lizards can climb trees, others dig burrows, and still others swim or live in water. Some lizards can even change their skin color to blend in with their surroundings.
Some lizards, such as the Tarzan chameleon of Madagascar, are green or yellow and shift to a striped pattern when threatened. Some lizards are venomous, with bites that can cause a sudden drop in blood pressure, leading to heart failure, loss of consciousness and death.
One of the most distinctive reptiles is the chameleon, which can change its skin to match its surroundings. But it’s their eyes that really set them apart. They can rotate independently and focus in two different directions at the same time, giving the lizard a full 360-degree view of its environment. They can also enlarge visual images like a telephoto lens, which helps them spot prey and avoid predators.
Most chameleons are leaf eaters, but they’ll also munch on tender plant shoots and berries to supplement their diet. They’re near the bottom of their food chain, so they need to be able to blend in with their environment to avoid being eaten by animals that prey on them, including snakes and birds.
Chameleons can display different colors to attract potential mates or warn enemies. For example, a dominant male might put on a vibrant display to attract females, while a weaker male might change color to appear more submissive. A frightened or wounded chameleon might also display dark colors.
Many species of chameleon are found in Africa, Asia, and southern Europe, but nearly half of all chameleons are located in the incredibly biodiverse country of Madagascar. There, 85 chameleon species—including the world’s smallest, the Brookesia micra—can be found, though several are endangered. Large-scale logging, agricultural fires, and human development have threatened their habitats.
Crocodiles are semiaquatic reptiles that live in the tropics of Africa, Asia and Australia. They’re the largest carnivores in their habitats, feeding on a variety of live and dead mammals, birds, fish, frogs and crustaceans. Crocodiles have acute senses, which help them to ambush their prey. They are also very strong swimmers and can hold their breath underwater for up to an hour. The skin on their sides and bellies is smooth for easier swimming, while the tops of their bodies and tails are covered in osteoderms – tough scales rich in calcium. These act as armour, providing them with good gripping power – a saltwater crocodile has been recorded applying a bite force of over 3,700 lbf (16,000 N) in a laboratory.
Like other apex predators, crocodiles help to regulate trophic dynamics within their ecosystems. By preying on a wide range of species, they ensure that certain prey animals don’t become too dominant and cause ecological changes further down the food chain.
Because crocodiles are highly sensitive to changes in water quality, they’re also good indicators of the health of their habitats. They’re also important scavengers, removing dead animals from the landscape and thereby helping to maintain a healthy balance of the food chain. Some crocodiles live solitary lives, while others form social groups during basking and eating times. This is particularly the case with saltwater crocodiles, which form tight hierarchies and are only willing to tolerate other males of similar age and size during group feeding on carcasses and terrestrial kills.