Merck Manual

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Description and Physical Characteristics of Reptiles


Roger J. Klingenberg


Last full review/revision Jul 2011 | Content last modified Nov 2016
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There are many differences between reptiles and other vertebrates (animals with backbones). Most reptiles maintain their body temperature by absorbing heat from their environment and have a body temperature that changes according to the temperature of the local atmosphere, whereas most mammals and birds are able to maintain a constant body temperature despite changes in the temperature of their environment. With the exception of the crocodilians, reptiles do not have a heart with 4 chambers; yet the heart functions like a 4-chambered heart. Reptiles have both kidneys and a liver (see descriptions in the sections below). Fertilization of their eggs occurs internally, and the embryos develop within amnionic eggs; however, some reptiles lay eggs from which their offspring hatch, while other reptiles 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 or to 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, Gila monsters, bearded lizards, and some skinks are hard to reliably identify as male or female. Male turtles have a longer tail than females, and the cloacal opening in males is more toward 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 scales on the head end of their underbelly (the gular scales).


Snakes range in size from small (the size of a worm) to many feet in length. The skin of snakes is made up of 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. Most nonvenomous snakes have teeth that include 2 rows on top and 1 row on the bottom. The teeth are curved backwards to help keep struggling prey from escaping. Venomous snakes have grooved or hollow fangs that they use to inject venom into their prey. The bones in the lower jaw of snakes are long and flexible, which helps enable the snake to swallow large prey.

Anatomy of a snake

Anatomy of a snake

Internally, snakes have many sets of ribs to support their length. These ribs are also 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 allow them to fit inside the body cavity. There are several different ways in which snakes can move, including the familiar undulating crawl (slithering), side-winding, and the accordion-like movement used to climb trees.

Snakes rely primarily on their senses of smell and touch. Their forked tongues assist in bringing small air particles into the mouth, where an organ on the roof of the mouth is used to identify smells. While snakes do not have external ears, they do have an ear bone that is used to detect vibrations of sound waves that move through the ground.


There are more than 4,000 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).

Anatomy of a lizard

Anatomy of a lizard

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; however, the new growth may look different.

Lizards are adapted to many different environments. Some are good swimmers, while 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 do 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

Turtles and tortoises belong to a group of reptiles known as chelonians. They are easily distinguished by their hard protective shells that protect their upper and lower bodies. The upper covering is known as the carapace, while the bottom portion is called the plastron. The words “turtle” and “tortoise” are often used interchangeably by people, and in different parts of the world they can mean different things. In general, however, a turtle spends most or a large portion of its time in water (including sea turtles and those found in ponds or rivers), whereas a tortoise generally lives on land.

Anatomy of a turtle

Anatomy of a turtle

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 in captivity 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 permanently attached at the spine and rib cage. 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 (during shedding) with larger ones 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.

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