Reptiles were the first true land-dwelling vertebrates, appearing about 310 million years ago. They dominated Earth from about 225 to 65 million years ago, but today only four reptilian orders remain: Testudinata (or Chelonia), the turtles; Squamata, the lizards and snakes; Crocodilia, the crocodilians; and Rhyncho-cephalia, the tuatara.
The most noticeable characteristic of reptiles is their outer covering of scales. They also lay tough, leathery eggs and are poikilother-mal—that is, they lack an internal means of controlling body temperature. Because of this last feature, most of the 6,000 or so reptilian species live in the tropics or subtropics. However, some live in temperate regions, including the tuatara and the viviparous common lizard, which is found as far north as the Arctic Circle.
The reptilian body
A reptile’s body is covered by a dry, relatively thick waterproof skin, which helps to protect the internal structures and prevents dehydration. The scales on the reptilian body are really folds in the skin, composed of a hard, transparent dead material called keratin. In cheloni-ans and crocodilians, the outer keratin layer is continually worn away but a new layer always remains underneath. Snakes and lizards, however, shed their scales at regular intervals. Lizards often shed their scales in strips. But some, like snakes, shed the whole skin in one piece, a process known as sloughing. Lizards and some crocodilians have pieces of bone called osteoderms within their scales, which give the skin additional toughness.
The skeleton of most vertebrates, including reptiles, consists of bone and cartilage. The amount of cartilage in the body is highest in young reptiles, but as the reptile becomes an adult, the cartilage is usually present only over the joints, the shoulder girdle, and parts of the skull. Unlike mammalian bones, those of a reptile continue to grow throughout the animal’s life.
The ribs of reptiles extend the length of the body, from the third vertebra of the spine to the beginning of the tail, whereas in mammals, they are confined to the chest region. The skull consists of many separate bones. Similarly, the lower jaw is formed from several distinct but joined bones. The teeth are peglike or pointed and are all alike. Reptile teeth are not present in a variety of forms as are the teeth of mammals.
The reptilian brain is small in relation to the body—in many, it makes up less than 0.05 per cent of the total body weight—and reptiles are relatively unintelligent, having very little ability to learn or adapt to new situations. But they compensate for this deficiency by possessing elaborate patterns of instinctive behavior that enable them to survive and reproduce successfully.
Reptiles breathe with the aid of two lungs, one or both of which may be well developed. Some species can also absorb oxygen in the water through the membranes that line the mouth and the genital and waste-eliminating chamber called the cloaca. The cloaca is at the end of the large intestine, and opens under the body in front of the tail. It receives eggs or sperm from the sex glands as well as feces from the intestine and urine from the kidneys. The water needed for elimination of waste products is reabsorbed into the blood through the walls of the large intestine and cloacal chamber. As a result, the anal waste is solid or semi-solid, and there is little or no liquid urine, in this way, water is conserved in the body, a feature that is important for those reptiles that live in dry environments.
Senses and sense organs
Most reptiles have good eyesight. The fields of vision of each eye overlap to some extent, so that some of these animals have binocular vision, although sometimes to a limited degree. Vision is particularly highly developed in chameleons, which must be able to judge distances accurately in order to catch their prey.
Relatively little is known about hearing in reptiles, although this faculty does not seem to be as important as vision. Like birds, mammals, and amphibians, reptiles have a small bone (the stapes), which transmits sound vibrations from the eardrum to the inner ear. Most reptiles, except for snakes and a few lizards, have external ears with eardrums on the surface or sunk down on a tube at the back of the head; and, except for crocodiles, they lack earflaps. Snakes also lack the middle ear cavity—they “hear” a sound mainly by picking up vibrations from the ground, which are then transferred from the lower jaw bone to the skull.
Smell is an important sense in most reptiles. Snakes and lizards have two tiny cavities known as the Jacobson’s organ, which work in conjunction with the tongue as well as with the sense of smell. These organs are found above, and open into, the roof of the mouth and are partly lined with sensitive membranes similar to those that line the nose. As the tongue moves in and out, it picks up minute particles from the air or off the ground, which are carried to the Jacobson’s organs.
An additional sense organ—the pineal eye— occurs in the tuatara and many lizards. It was also present in many reptiles and amphibians now extinct. It is found at the top of the head and opens, by means of a small hole, into the skull. It is covered by skin, which usually has a lighter color than the surrounding skin. The function of this “third eye” is unknown, but it is light-sensitive and probably helps the animal to regulate its exposure to the sun and thus to control its internal body temperature.
The term “cold-blooded” is misleading when applied to reptiles because under certain circumstances, a reptile’s body temperature may be very high, even higher than 98.6° F. (37° C), the normal body temperature of humans. Reptiles are, in fact, poikilothermal— they do not have an internal mechanism to regulate body temperature—and therefore depend on the external temperature. Body temperature is important because it is related to activity. Under extreme conditions of heat or cold, the body becomes sluggish, because vital activities such as heartbeat and breathing slow down. But within a narrow temperature range— which varies from species to species—a reptile reaches its highest level of activity. For most, the optimum temperature is between 77° F. (25° C) and 99° F. (31 °C).
Most reptiles that live in temperate and subtropical climates breed in spring. As in most other animals, the urge to mate is triggered by an increase in the hormone levels in the sex glands and other internal organs. These changes are usually a response to changes in the environment, such as lengthening of the day, an increasing abundance of food, and a rise in air temperature. Tropical reptiles may breed several times a year, but even in these species mating usually follows a seasonal cycle.
With the exception of the tuatara, which mates by pressing its cloaca against that of the female, all male reptiles possess a penis through which sperm is introduced into the female’s cloaca. Chelonians and crocodiles have a single penis, whereas lizards and snakes have two, known as hemipenes, only one of which is used during copulation.
Most reptiles are oviparous—that is, the young hatch after the eggs have been laid. But some species of snakes and lizards are ovoviviparous (the young hatch while the eggs are still inside the mother’s body). In such reptiles, the young may still be surrounded by embryonic membranes when they are born. Exceptions to both these methods of reproduction are found in certain skinks and snakes, in which the relationship between mother and embryo is more intimate, resembling that of mammals. In these reptiles, a type of placenta develops that lies close to the lining of the mother’s oviduct.
Tortoises, turtles, and terrapins
Members of the order Chelonia—tortoises, turtles, and terrapins—differ from one another in the following way: tortoises live only on land, turtles live in the sea, and terrapins live in fresh water. In the United States, however, most chelonians are commonly referred to as turtles.
The most striking feature of this order is the presence of a shell. The shell is composed of an outer layer of horny structures called scutes, which are formed from skin tissue, and a thick inner layer of bony plates. The outer, horny layer increases in size with age, but its growth pauses during hibernation. Each dormant phase produces a ring around the previous year’s growth, similar to a growth ring in a tree. Soft-shelled turtles and the leatherback turtle have an outer layer of tough skin rather than scutes. The shell of the hawksbill turtle is the source of the commercial material known as tortoiseshell.
Chelonians do not have teeth. Instead they have a horny beak on the upper and lower jaws. Tortoises are mainly herbivorous, turtles eat a mixture of plant and animal food, and terrapins tend to be carnivorous, feeding on invertebrates and fish.
There are two suborders of chelonians, distinguished by the way the head is withdrawn into the shell. The Pleurodira, the side-necked turtles of South America, Africa, and Australia, are a small group of two families. An example is the South American matamata terrapin, which has a long snout with nostrils at the end and a long flap of skin behind each eye. It feeds on fish and other small aquatic animals.
The Cryptodira, or hidden-necks—a group that comprises seven families—can bend the neck vertically up and down. The family Testu-dinidae in this group includes the land tortoises, which are some of the most familiar chelonians, with about 40 species. The family Emydidae, with about 80 species, includes terrapins and pond tortoises. The pond tortoise and the red-eared terrapin are commonly kept as pets.
The family Chelydridae is found in North America, Central America, and northern South America, and is comprised of the musk turtles, the mud turtles, and the snappers. The common snapper is a ferocious freshwater turtle that bites sharply when disturbed. Unusually, it cannot withdraw its head into its shell, and its tail, legs, and neck are almost completely exposed on the underside. The musk turtles are exceptional among reptiles in having scent glands on their body. They are often called “stinkpots” because of the musky odor they emit when handled.
The sea turtles (families Cheloniidae and Dermochelyidae) are the most highly adapted to a watery existence. Their legs are flattened and paddle-shaped and they swim with agility. On land, however, they move clumsily and slowly. Usually only the females come ashore, specifically to lay eggs in a hole in the sand.
Lizards and snakes
These reptiles belong to the order Squamata, which is divided into two suborders: the Sauria, or lizards, and the Serpentes (also called Ophidia), or snakes.
Lizards and snakes form the largest group of reptiles and are most abundant in tropical regions. Most lizards are small and four-legged with long tails, movable eyelids, and external ear openings. Many can shed their tails to escape from predators. The tail is then regenerated but is never an exact copy of the original, being shorter and with an irregular scale pattern. Members of the family Anguidae have either one or two pairs of short, reduced limbs and some, such as the slowworm, are legless.
An interesting feature of some skinks (family Scincidae) is that the eyelids contain a transparent opening, which allows them to see while they close their eyes to protect them against flying debris.
The family Chamaeleonidae contains some 100 species of chameleons. These lizards are exceptionally well adapted to an insectivorous and arboreal life. Two of the five toes on each limb are opposed to the other three, forming a gripping “hand,” and the tail is prehensile. Each eye can move independently or both can focus on the same object. The chameleon can therefore judge distances accurately and aim precisely at the insects on which it feeds, catching them with its long, sticky tongue.
A lizard called the anole is closely related to the chameleon. There are more than 225 species of anoles. They are often called American chameleons.
Only two species of lizards are venomous: the Gila monster and the beaded lizard, both of which live in desert areas of North America. The venom is secreted along the grooves in the teeth at the base of the animal’s lower jaw.
The snakes, however, contain most of the poisonous reptilian species, although the majority of species are harmless. Venomous species are classed in two main groups— vipers and elapids—according to the position and construction of the fangs. Vipers include rattlesnakes, the adder, and the asp. When their mouth is opened, the fangs swing down and forward. Elapids include the cobras, mambas, and kraits, in which the front fangs are fixed. Most venomous snakes are front-fanged, but some are rear-fanged. The snakes in the family Boidae, such as the boa constrictor and the pythons, kill their prey by crushing it.
Unlike lizards, snakes are limbless, although some, such as the boas, have rudimentary hind legs. The tail cannot be regenerated when lost, and the transparent eyelids are fused together to form a protective covering. In addition, most snakes are able to dislocate their jaws, enabling them to swallow prey considerably larger than themselves.
All the members of the order Crocodilia— crocodiles, alligators, caymans, and gavials— are large, amphibious reptiles. Crocodilians use their long, vertically flattened tail for swimming. They usually keep their webbed feet tucked in at the sides of their body. The ears, eyes, and nostrils are located high on the head, so that the animal can remain hidden under the water and at the same time can hear, see, and breathe. When a crocodilian is totally submerged, the eyes are covered by a membrane, and the ears and nostrils are closed by special valves. In addition, a fold of skin shuts off the windpipe so that the animal can open its mouth underwater without drowning.
Crocodilians are carnivorous, and adults prey on large animals—occasionally as large as a horse—which they catch either underwater or on land. Crocodilians eat their prey whole or first tear it apart and the eat it. They often store large prey underwater until it is sufficiently decayed that they can tear it apart by the powerful teeth. Since crocodilian jaws do not move from side to side, they often must twist their whole bodies in an effort to detach a piece of flesh from a carcass.
A single species, the false gavial, makes up the family Gavialidae. Found in India, Bangladesh, Burma, and Pakistan, it is distinguished by its long, thin snout and rather bulbous head. The family Crocodylidae comprises all the other members of the order. These include the Nile crocodile, found widely in Africa; the mugger, which lives in India and Pakistan; and the American crocodile, which occurs in the southeastern United States and south to Ecuador. As in most other crocodilians, the bony plates occur only on the back and tail.
The American alligator and the Chinese alligator are the only two species of alligators. They were once much hunted for their skins and are quite scarce, although in some areas of the United States, they are now protected and making a comeback. The caymans of Central and South America are closely related to alligators but are quicker. They are mainly distinguished by the scutes, which cover both the upper and lower body.
The tuatara is the only surviving member of the order Rhynchocephalia, a group of reptiles that flourished more than 200 million years ago. This lizardlike animal is found only on a few islands off the coast of New Zealand. It preys on insects and other invertebrates and inhabits the burrows of sea birds such as shearwaters and petrels. It becomes active at night and seems able to function at lower temperatures than most reptiles.