Most reptiles are ectotherms (cold-blooded): they maintain their body temperature by absorbing heat from the environment, so their body temperature changes with environmental temperature. In contrast, most mammals and birds are endotherms (warm-blooded): they are able to maintain a constant body temperature despite changes in the environmental temperature. Snakes, lizards, and chelonians have a 3-chambered heart. Crocodilians have a 4-chambered heart. Eggs are fertilized internally (inside the body). Some reptile species lay eggs from which the offspring hatch, and others give birth to live young.
Determining the sex of most reptiles (particularly snakes) can be tricky and in most cases is best left to a veterinarian, a herpetologist, or an experienced breeder or dealer. The males and females of some species of lizards have distinguishing characteristics such as size, color, or scale pattern. However, some reptiles are hard to reliably identify as male or female by visible characteristics. Male turtles have a longer tail than females, and the cloacal opening in males is closer to the tail tip. Among semiaquatic reptiles, males are smaller and have longer claws. They might also have a spur on the hindlegs. Terrestrial turtles and tortoises have distinct differences in the shape of their underbelly (plastron): it is concave (rounded) in males and flat in females. Some male tortoises also have a larger pair of gular scales on the front end of the underbelly.
Snakes range in size from the size of a worm to many feet long. Their skin contains scales that may be smooth or ridged. The scales on the belly (called scutes) are thicker than those on the sides and back of a snake to provide protection as it moves.
Snakes have some clear differences from other reptiles. They have no limbs, no moveable eyelids, and no ear openings. The teeth of most nonvenomous snakes form 2 rows in the upper jaw and 1 row in the lower jaw. The teeth are curved backward to help keep struggling prey from escaping. Venomous snakes have grooved or hollow fangs that they use to inject venom into their prey. Snakes' skulls contain joints to allow the snake to swallow large prey.
Internally, snakes have many sets of ribs to support their length. These ribs are somewhat flexible to allow the prey to move through the snake’s body. In addition, the organs inside a snake are long and narrow to fit inside the body cavity. Snakes move in different ways, including the familiar undulating crawl (slithering), side-winding, and an accordion-like movement used to climb trees.
Snakes rely primarily on their senses of smell and touch. Their forked tongues bring small air particles into the mouth, where an organ in the roof of the mouth identifies odors. Although snakes do not have external ears, they have an ear bone that can detect vibrations of sound waves.
There are more than 4000 species of lizards, ranging in size from a few inches to the Komodo dragon, the largest lizard, which can reach up to 10 feet in length. Lizards that are commonly kept as pets include geckos, anoles, iguanas, skinks, chameleons, and agamids (including bearded dragons).
Most lizards have dry skin made up of scales. The scales of lizards vary from the smoother scales of skinks to rough scales or even spikes. In many species, the tail is fragile and can break easily. It can regenerate, although the new growth may look different.
Lizards are adapted to many different environments. Some are good swimmers, and others spend most of their time in trees. Many have clawed feet that help them climb and cling.
Like snakes, lizards use their tongues to help them smell. The tongue captures particles of air and brings them into the mouth, where a specialized organ can detect various smells. Lizards have external ears and appear to be able to hear better than snakes. Most lizards have eyelids that clean and protect their eyes when they blink. A few, however, have fixed eyecaps like snakes.
Some lizards have developed special features to help them survive. Chameleons and some other species such as anoles can change the color of their scales to blend in with their surroundings. The males of some species have a loose flap of skin called the dewlap that can be extended to either intimidate a predator or to help attract a mate. And, as mentioned above, a lizard’s tail can break off, which can help it escape from predators.
Turtles and tortoises belong to a group of reptiles known as chelonians. They are easily distinguished by their hard shells that protect their upper and lower bodies. The upper covering is known as the carapace, and the bottom portion is called the plastron. The words “turtle” and “tortoise” are often used interchangeably, and in different parts of the world they can mean different things. The word "turtle" can refer to any chelonian or specifically to chelonian species that live mostly in water (including sea turtles and those found in ponds or rivers). Tortoises live only on land and do not swim.
Chelonians range in size from small (shells 3 to 4 inches in length) to very large (shell length of up to 8 feet). Many chelonians can be quite long-lived. Some species of tortoises have been known to survive for up to 150 years, and some aquatic turtles may live for 70 years.
The shells of turtles and tortoises are made up of a large number of bones that are covered by large scales called scutes. The shell is part of a turtle's skeleton and is permanently attached to the turtle at the spine. Some turtles can tuck their head, legs, and tails inside their shells, but others cannot. The shell enlarges as the turtle grows, either by replacing old scutes with larger ones during shedding or by enlarging the diameter of existing scutes. In some cases, this can help determine the age of the animal.
Like snakes, turtles and tortoises hear by feeling vibrations in the ground or water. Many have good eyesight and a good sense of smell to help them locate food. They do not have teeth; rather, their mouths have a sharp edge that they use to bite or tear food. Many turtles and tortoises are omnivores, meaning they eat both plants and animals (such as insects or worms), although some eat only plants.