Reptile

I

Introduction

Reptile, animal with tough, dry skin covered with horny scales. Some of the most widespread living reptiles are turtles, lizards, snakes, crocodiles, and alligators. Reptiles are vertebrates—animals that have a backbone. While they share characteristics common to other vertebrates—fish, amphibians, birds, and mammals—reptiles display a unique combination of characteristics that distinguishes them from other vertebrates. Like amphibians, modern reptiles are cold-blooded, or ectothermic. This means that they are unable to produce their own body heat, so they rely on the sun for warmth, and much of their behavior is directed toward regulating their body temperature. Like birds, most reptiles hatch from eggs that are laid on land, covered by a protective shell. Reptiles breathe air with lungs, as do most mature amphibians and all birds and mammals. And like amphibians and mammals, most reptiles, with the exception of turtles, have teeth. Reptile skin, with its tough, horny scales, is unique in the animal world. It is neither moist and permeable, like that of amphibians, nor feathered, like that of birds, nor covered with hair, like that of mammals.

Reptiles inhabit nearly every corner of the planet, including most of the world’s oceans. They are found in a broad range of habitats, from the bottom of ponds and lakes to the tree lines of high-elevation alpine regions. However, they are especially abundant and diverse in the tropics and in deserts. The single factor that limits their geographic distribution is their inability to generate their own body heat. For this reason, no reptiles are found in icy Antarctica or the polar oceans, and few live within the Arctic Circle.

II

Types of Reptiles

There are over 7,000 species of living reptiles, all of which belong to one of four main groups: turtles and tortoises; lizards; worm lizards; snakes; crocodiles and their relatives; and finally, the lizardlike tuatara, which has survived relatively unchanged since before the rise of dinosaurs.

A

Turtles and Tortoises

All reptiles with a protective shell are turtles, and there are over 250 species of turtles and tortoises worldwide. In North America, people use the word tortoise to designate those turtles that live entirely on land, and the word terrapin for the diamond-backed terrapin of brackish coastal marshes. Turtles and tortoises range in size from the tiny American bog turtle, which reaches just 11 cm (4.3 in), to the giant leatherback sea turtle, which has a shell up to 2.4 m (8 ft) long and can weigh over 900 kg (2,000 lb).

---

---

A turtle’s shell is formed from flat bones fused with parts of the spinal column and the ribs. In most turtles, the shell bones are covered with broad scales called scutes. Turtles that live all or part of their lives on land tend to have strong, heavy shells, while those that spend nearly all of their time in water may have lighter, flatter shells. For example, the gopher tortoise of the southeastern United States has a hard, high-domed shell. In contrast, the soft-shelled turtles of freshwater lakes and rivers have flat, rubbery shells that are covered by leathery skin instead of scutes. These turtles spend much of their time hidden in mud in shallow water.

B

Lizards

With over 4,300 individual species, the lizards are the most abundant and diverse group of reptiles alive today. Typical lizards have long, slender bodies that taper to a narrow, pointed tail. Most have four legs ending in dexterous, clawed toes, although several types of lizards, including some worm lizards, have reduced limbs or no limbs at all. Typically, lizards are between 10 and 20 cm (4 and 8 in) long, but some, such as the tiny Monito gecko, grow to only about 3.5 cm (about 1.5 in) and weigh only 0.1 g (less than 0.01 oz). The largest lizard is the Komodo dragon, a giant monitor lizard of Southeast Asia, which reaches 3 m (nearly 10 ft) in length and weighs up to 165 kg (about 365 lb).

Most lizards are small, agile insectivores that live in trees or shrubs or in rocky places. Larger lizards may eat other vertebrates, and a few species are herbivores, or plant eaters. The Galápagos marine iguanas are the only lizards that regularly venture into the ocean. These lizards feed mostly on algae that they scrape from underwater rocks with their claws and teeth. Only two lizard species—the gila monster and the beaded lizard—produce venom, although the bite of the Komodo dragon can sometimes be fatal because infectious bacteria are present in its saliva.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain