INTRODUCTION

Snake (reptile), legless animal with a long, flexible body covered with overlapping scales. Snakes are reptiles, a diverse group of animals that also includes lizards, turtles, and crocodiles. Snakes are thought to have evolved from lizards and share many characteristics with this group—particularly the so-called legless lizards, which have tiny, almost imperceptible legs. But unlike most lizards, snakes have thin, forked tongues, and they lack scales.

Like all reptiles, snakes are cold-blooded external ears. And while most lizards have movable eyelids that periodically close to protect and lubricate the eyes, a snake’s eyes are always open, protected by immobile, transparent, or more correctly, ectothermic—that is, they cannot produce their own body heat. Instead, they rely on the sun to heat their bodies and then regulate their temperature with behavior. Because they do not rely on energy from food to generate body heat, snakes can survive on an extremely meager diet. Some wait for months between successive meals, and a few survive by eating a single, large meal just once or twice a year. When they do eat, snakes swallow their prey whole rather than biting off small pieces. Many snakes have specialized jaws that enable them to swallow animals that are far larger than their own heads. Although uncommon, some snakes, such as the African rock python, have been observed eating animals as large as an antelope or a small cow.

With over 2,500 species belonging to more than 10 families, snakes are a large and successful group. They owe much of this success to their versatility—snakes occupy habitats ranging from underground burrows to the tops of trees to ocean depths as great as 150 m (490 ft). They are found on every continent except Antarctica, and although they are most abundant in tropical areas, many survive in regions marked by extreme cold. The range of the European adder, for instance, extends north of the Arctic Circle. The only places without snakes are parts of the polar regions and isolated islands, such as Ireland and New Zealand.