Reptiles are modern-day animals related to the dinosaurs and other prehistoric animals. Because they rely on external heat sources rather than body chemistry to warm themselves, these animals do not need to eat as much food as the warm-blooded mammals and birds, so reptiles can live in regions largely uninhabitable to other groups of animals.
It has been estimated that mammals need 30-50 times more food than reptiles of similar size.
Reptiles are also characterised by their dry, horny skin made up of a number of thickenings called scales. Reptiles often lay shelled eggs in nests, although some give birth to live young.
Other reptile characteristics include two pairs of five-toed limbs (absent in a few lizards and most snakes), young that resemble adults when hatched, respiration using lungs (not gills), and an incomplete, four-chambered heart.
Reptile evolution
It is known that reptiles evolved from amphibians some 315 million years ago. One of the earliest reptiles was called Hylonomus - a 12 inch long lizard-like creature with spiky teeth that may have eaten insects. Although the dinosaurs lived between around 250 and 65 million years ago these were not the only types of reptile at that time. Indeed, some 16 different orders of reptiles have been classified since they first evolved, of which only two (the Saurischia and Ornithischia) were dinosaurs. Of the rest, four are still in existence today (crocodiles, lizards and snakes, turtles and tortoises and the tuatara).
At the time of the dinosaurs, other contemporary groups of reptiles included the sea-dwelling ichthyosaurs, plesiosaurs, turtles and mosasaurs, the flying pterosaurs and the land living snakes, lizards and crocodiles. It is not known why the dinosaurs died out, although a combination of circumstances including climate changes and the evolution of other animal forms like the mammals which competed more successfully for available food sources were probably involved.
Reptile relatives
The animal world is made up of lower animals (usually classed as animals without backbones) and higher animals. This second group of animals ranges in complexity from primitive types of fish up to primates like gorillas and humankind.
The most primitive higher animals live in water, and the transition from water to land is seen in the amphibians, a group of moist-skinned animals which still depend on water, especially for nuturing their eggs. Closely related to the amphibians are the reptiles. These animals have however successfully made a complete transition from water to land and are found in some of the driest places on earth. Some reptiles do choose to live in water, but these groups have probably evolved from land-dwelling ancestors.
Life Cycle
Sex differences
Like many animals, most reptiles show a difference in size and shape between males and females. Male crocodiles and lizards are often bigger than the females while female snakes and turtles are usually bigger than males. Male agamid lizards and iguanas are brightly colored, while the females of the same species appear dull by comparison.
Reproduction
All reptiles fertilize internally, with the sperm deposited directly into the female's cloaca, a tube used for both excretion and reproduction found in both sexes. Unlike mammals which have a single penis, male reptiles usually have either a penis or a pair of hemipenes. One of the features which distinguishes reptiles from amphibians and other lower vertebrates is the presence of a copulatory organ for direct transfer of sperm into the reproductive tract of the female. The exception is the Tuatara - this evolutionary oddity does not have an external copulatory organ.
Eggs
Many reptiles lay eggs. Unlike the moist, gelatinous eggs of amphibians, reptile eggs are resistant to drying out: this was a major factor in freeing reptiles from the dependence on moisture that keeps most amphibians close to water. Some reptile eggs are soft and pliable. Others are hard, like birds eggs. Baby reptiles may have a special 'egg tooth' or horny growth that is used to break out of the shell.
Some reptiles give birth to live young, with an internal rather than external egg stage. These reptiles are termed 'ovoviviparous'. This group of reptiles includes sea snakes and some lizards and snakes, like the European adder. It is thought that giving birth to live young is advantageous where climate conditions are severe and external incubation would be impossible to achieve.
Looking after the young
Reptile nests may be made in hollows, or among vegetation. American alligators and king cobras make elaborate nests of mounds of leaves. Turtles lay eggs in pits excavated on sandy beaches: the eggs are then covered to protect them from predators. Most female reptiles leave the nest after laying the eggs, although some like crocodiles, skinks, cobras and pythons may stay to protect the eggs.
Although most snakes do not care for their young, females of some python species coil themselves around their eggs and remain with the eggs until they hatch: this may provide both protection and, in cooler climates, incubation for the eggs. The Indian python for example produces body heat by muscle contractions to help the incubation process.
Body Plan
Skin
The outer skin of reptiles is usually dry and scaly and is called the epidermis: it is made of a material called keratin which is thickened in places. This keratin layer is periodically shed in a process called moulting, which may occur several times a year in some snakes. The lower layer of the skin is called the dermis, and this layer contains most of the skin's pigment cells - changes in color are determined by these cells are are important in chameleons and some lizards.
The scale patterns of reptiles differs markedly from group to group. Snakes for example have large overlapping scales called scutes, whereas many lizard species have rounded scales, or scales modified into spines. Worm-lizards have concentric bands of scales that help the animal move through underground tunnels, and turtles have huge scales or scutes which cover and strengthen the bony shell.
Temperature control
Temperature control (thermoregulation) in birds and mammals is carried out by physiological mechanisms. The body chemistry in these animals (metabolism) generates much heat. Activity in muscle cells like shivering controls temperature by generating further heat in cold weather, whilst sweating and panting cools the body in summer. A system of nervous control switches these mechanisms on and off.
Reptiles lack these control systems and their internal chemistry runs at a lower metabolic rate. Instead, they rely on behavioral thermoregulation: activities like basking in the sun warm the body, and burrowing or seeking out shade cools the body down. (See: 'adapting to temperature extremes' in the Ecology chapter of this ZooGuide). Because of this reliance on external heat, reptiles are rarely seen in polar or cold temperate regions, although temperate retiles may use hibernation as a weapon to combat the winter cold.
Circulation and blood
All vertebrate animals have a circulatory system that consists of a heart pumping blood around the body via a system of arteries and veins. Vertebrates show various modifications of the circulatory system that moves from the primitive single atrium, single ventricle heart of fish up to the more efficient two atria, two ventricle heart of mammals.
The blood of reptiles collects oxygen by passing through two lungs, except for snakes in which one lung is suppressed because of the body shape. The oxygenated blood enters the heart through the left atrium. Most reptiles have two atria and a single ventricle. Crocodiles show an almost complete separation into two ventricles, but even in reptiles with a single ventricle there is little mixing of oxygenated and deoxygenated blood.
Nervous system
Reptiles have similar nervous systems to other vertebrate animals, with a central nervous system consisting of networks of neurons connected to a spinal cord within the backbone. The anterior expansion of the spinal cord is called the brain, an organ responsible for coordinating movement, for handling sensory information and for storing memory information.
The main difference between the nervous systems of different vertebrate classes lies in the complexity of the brain, and study of reptile brain anatomy shows reptiles to be more complex than amphibians and fish, but less advanced than birds or mammals, fitting with what we might expect from the behavior patterns of these animals.
Sensory system
Reptiles have developed a number of sensory specializations to help them detect prey and predators. Jacobson's organs in lizards (see animation on right) is used to detect chemical stimuli in the reptile's surroundings.
Pit vipers have sensory 'pits' on the sides of their heads which are sensitive to infrared heat rays. Boas and pythons have similar pits on their lips: these pits are so sensitive that they can detect temperature changes of as little as 0.002_F, quite sufficient to enable the snake to detect and attack unsuspecting prey in total darkness.
Movement
Reptiles were the first four legged animals to be able to lift their bodies off the ground. Unlike amphibians which wriggle along or leap on land, the legged reptiles can walk and run. This was made possible by the evolution of limbs with two right-angled bends, producing the so-called 'sprawling' gait of reptiles. Other land animals like birds, mammals and the extinct dinosaurs have the elbow joint straightened to allow faster movement.
Snakes of course have to move in a different way. There are several different kinds of snake movement, from the 'sidewinding' of rattlesnakes and vipers to the concertina-like locomotion used by snakes moving slowly forwards.
Venom
Although reptiles often look venomous, the only reptiles to produce venom are the snakes and the two species of Heloderma, including the Gila monster of the southwestern USA. Venom is used for the capture of small prey and as a defense mechanism against large animals or humans. Snake poisoning causes an estimated 15,000 deaths each year in India alone, and in the USA several hundred cases of snakebite are reported annually, although only a few deaths result.
There are two types of snake venom. The neurotoxic venom of cobras and related snakes affects the nervous system and usually causes death by asphyxiation. Rattlesnakes and vipers produce hemotoxic venom that affects the circulatory system, destroying blood cells and breaking down the delicate walls of capillaries. Hemotoxic venom usually travels slowly in the blood system, whilst neurotoxic venom can kill very quickly. One tablespoon of cobra venom could kill 165 people!
Snake bites are treated by injections of antivenin or serum, a blood extract produced from animals injected by minute doses of venom. The injected animals produce antibodies which are the active ingredient of the antivenin.
Ecology
Where do reptiles live?
Although the typical picture of a reptile is that of a lizard basking in the sun, reptiles also live in warm, constant environments like tropical rainforests. The key to reptile survival is being able to seek out warm surroundings when the reptile needs to be active - to search for food, for example.
Reptiles are found in a wide variety of habitats. Large snakes live in the tropics, and turtles and sea snakes inhabit the tropical oceans. Some lizards and snakes climb trees and rocks, and others burrow into soil or sand. Although most turtles are aquatic, box turtles live on the forest floor, and tortoises live in arid regions.
Reptile deceptions
Many kinds of reptile have developed ways to defend themselves, often by imitating other reptiles. For instance milk snakes look very similar to the poisonous coral snakes - mammals looking for food are alerted to the brighly colored bands of the coral snake and are likely to also be put off by the milk snake's similar coloring.
The hognosed snake imitates a rattlesnake when disturbed, rubbing the side of its tail against its body to make a rattling sound. This snake can also roll over possum-fashion and fake death!
Most predators do not like to eat the bodies of dead animals - the flesh may become distasteful as it decays.
The horned toad or lizard also confuses its enemies by squirting blood from its eyes - although it is not certain that this is a defense mechanism, it must certainly confuse any potential enemy.
Tha animation on this page shows how some lizards also escape from their enemies.
Searching for food
Most reptiles feed on animals, although land tortoises and turtles are plant eaters. A number of smaller reptiles prey on insects while larger reptiles eat small mammals and other reptiles. Snakes have evolved two predatory mechanisms - venom and constriction in which the prey is killed by slow asphyxiation. Other reptiles and some snakes capture prey in their jaws and kill it by crushing and swallowing.
Camouflage is an important tool for both predator and prey. Some reptiles like chameleons have the ability to change the amounts of pigment in their skin to protect them. Predators like the tree python and emerald boa show coloration designed to make them invisible to passing prey in rainforest environments.
Relations
Reptile groups
Living reptiles comprise a single CLASS of animals, and are divided into four ORDERS of reptiles. These are the lizards and snakes (order Squamata), the turtles and tortoises (order Chelonia), the crocodiles and alligators (order Crocodilia) and the single species called the Tuatara (order Rhynchocephalia).
During the Mesozoic era, some 16 orders of reptiles were known, including two orders of dinosaurs.
Suborders and families
Within the four living orders of reptiles are a number of SUBORDERS and FAMILIES. The turtles (order Chelonia) include three SUPERFAMILIES, one comprising the snapping turtles, land tortoises and common turtles, while the other two are the sea turtles and softshelled turtles. The order Squamata includes one suborder of lizards, another for the snakes and a third for worm lizards, a small group of burrowing lizards with reduced or absent limbs.
The order Crocodilia is simpler, with only three families containing the alligators, crocodiles and gharials. The fourth order (Rhynchocephalia) comprises only a single species, the Tuatara restricted to a few rocky islands off the coast of New Zealand.
Groups
Turtles
Turtles and tortoises
These reptiles are characterised by a body enclosed in an oval shell made up of a layer of plate-like bones sutured to one another. The upper or dorsal shell is called the carapace, and the lower, flatter part is called the plastron. Large scales or scutes cover the carapace - this layer is derived from skin and it strengthens the carapace. The soft-shelled turtles have a leathery outer covering which is not divided into scales.
The thoracic (chest) vertebrae and ribs of turtles are usually attached to the bony carapace. The plastron and carapace are joined by bony connectors at each side, and gaps at the front, rear and either side allow the head, tail and legs to protrude. Some turtles have hinged shells modified to allow tighter closing, either as a defense mechanism or to conserve moisture.
Turtles do not possess teeth. Instead they have horny growths that allow them to cut, tear and crush their food. In marine turtles the limbs are paddle-like for swimming, whilst land-dwelling forms have stumpy feet.
There are 13 familes in this suborder, comprising 244 species. Eight families are discussed in this section, totalling over 200 species .The families not discussed here are the Pig-nosed softshell turtle (1 species - Australasia), the Musk turtles (2 families, 23 species), the Central American River turtle (1 species - Central America) and the Big-headed Turtle (1 species).
Sea turtles
The major family of sea turtles is called the Cheloniidae, with six species found in tropical and subtropical waters, although the Leatherback sea turtle (the only species in the family Dermochelyididae) has a similar distribution. The two familes are sometimes grouped into a single superfamily called the Chelonioidea. The range of these turtles can extend into temperate seas, and the Leatherback turtle can even stand subarctic conditions.
These turtles range from 30 inches to 5 feet in length, and the Leatherback is the largest living species of turtle. Sea turtles eat jellyfish, crustaceans and mollusks, and lay up to 200 eggs on tropical beaches. One of the major differences between the Leatherback and the other sea turtles is its shell: the Leatherback has a scute-less shell covered in oily skin, while Cheloniid turtles have a hard scute-covered shell.
Snapping turtles
Snapping turtles are often aggressive and short-tempered turtles that inhabit freshwater lakes and rivers of the mideast and eastern parts of North America down to northwest South America. There are two species, the Alligator snapping turtle and the Snapping turtle. Both eat insects, fish, other turtles, mollusks and plant material, and are recognised by their strong, hooked upper jaw or beak. From 18 inches to 2 feet in length, snapping turtles have long tails and a black, gray or brown carapace. They lay between 20-50 eggs on river banks.
Pond and river turtles
This is the largest group of turtles (family Emydidae), with 85 known species. They live mainly in tropical and subtropical Asia, Africa and the New World, with some species found in Southern and Central Europe and the USA. The smallest example is the 4 inch long Bog turtle and the largest is the 2-3 ft Malaysian giant turtle.
This group also includes the River terrapin, Redbelly turtle and Stripe-necked turtle. Pond and river turtles lay relatively small numbers of eggs: usually between 1 and 30, with the smaller species producing fewer eggs.
Some species are almost wholly aquatic while others live in marshes or bogs.
Sliders comprise a variable species found in the southeastern states of the USA and Mexico. Millions of these turtles have been raised on turtle farms and sold in pet stores, and they include the yellow-bellied and red- eared forms. Sliders live in shallow streams, rivers, swamps and ponds: younger animals eat invertebrate animals - older animals are vegetarian.
Tortoises
There are 41 known species of tortoise spread throughout the Old and New World tropical and subtropical regions excepting Australia. They range from the 4 inch long Speckled Cape tortoise to the five foot Aldabra giant tortoise that may weigh 500 pounds. Other species include the Galapagos giant tortoise, the Pancake tortoise and the Egyptian tortoise.
Tortoises have a high-domed shell and heavy, elephant-like legs. They are usually plant-eaters although a few eat meat as well.
Softshell turtle
The 22 species of softshell turtle (family Trionychidae) are distributed across North America, Africa, southeast and east Asia and the islands of Australasia. They are bewteen 1-4 feet long and are mainly freshwater species although a few enter estuaries. The major US species are Trionyx species including Trionyx nigricus, or Dark softshell. Softshell turtles have reduced shells covered by leathery skin. This group also includes the flapshell turtles, named because of a pair of flaps on the rear of the plastron which cover the hind limbs when they are withdrawn into the shell. An example is the 12 inch long Indian flapshell Lissemys punctata.
Side-necked turtles
The two families of side-necked turtles are the 24 species of Afro-American side-necked or hidden-neck turtles (Pelomedusidae) and the 37 species of Austro-American or snake-necked turtles (Chelidae). These turtles are all characterised by having a neck which folds sideways into the shell. Most species are found in the Southern Hemisphere.
Representative species range in size from the 5 inch long African dwarf mud turtle to the 3 foot long South American river turtle.
Some species are plant-eaters while a few are carnivorous. This group includes the South American matamata Chelus fimbriatus, a strange-looking turtle with a jagged shell that may camouflage the animal - the shell is often covered with algae, helping to disguise the animal still further. This turtle lurks on the bottom of murky streams waiting for fish to approach. When a fish gets too close the matamata strikes by expanding its huge jaws.
Lizards
The variety of lizards
Lizards are the best known of the reptiles, with their dinosaur-like appearance. In fact, there are substantial anatomical differences between lizards and dinosaurs, but the scaly appearance and cruel-looking eyes certainly evokes pictures of the huge flesh-eaters of the past.
Lizards are typified by a slender, usually four legged body covered in scales. They are often agile and may be capable of fast movement. Lizards usually eat insects and small invertebrates and are with two exceptions non-venomous. Teeth are used as a classification feature in lizards, with some groups having pleurodont teeth (weakly attached by their roots to the inside jaw margins) and others with acrodont teeth ( firmly embedded in the jaws).
Most lizards are relatively small - up to 12 inches in length. A few like the giant Komodo dragon may reach 5 feet long, but this is very much the exception. There are about 3750 known lizard species divided into sixteen different families. In this chapter we will consider only the eleven best known families comprising some 3700 of these species. The remainder are the Blind lizards (4 species - Australasia), Xenosaurs (4 species - Central America), Snake lizards (31 species - Australasia) and the Bornean Earless Lizard (1 species - Borneo).
Agamid lizards
This family is also called the Old World or chisel-teethed lizards. It comprises about 325 species. Found throughout Africa, southern Asia and Australasia, agamid lizards range from under two inches to twelve inches in length. Some species show head ornaments, throat fans or dorsal spines and the group includes the flying dragons, moloch or horny devil and the Australian frilled lizard. These lizards have acrodont teeth.
Agamid lizards include the horned lizards (also called horned toads), a group of strange southern US and Mexican lizards belonging to the genus Phrynosoma. These lizards remain motionless when threatened, and the regal horned lizard (Phrynosoma solare) may squirt blood from the corner of its eye if disturbed - it is not known exactly why the animal does this, but it may fool the enemy into thinking the lizard is wounded. Some scientists believe the blood squirt is caused by parasites and is not a defense mechanism at all. Whatever the reasson the blood squirting phenomenon is highly impressive as the blood may travel a distance of up to three feet.
Anguid lizards
The anguid lizards are characterised by small or absent limbs and long fragile tails which regenerate easily when shed. Anguids (family Anguidae) comprise about 70 species living in Europe, southern Asia and regions of the New World. They include the Arizona and California alligator lizards and the slowworm or blindworm found in Europe, western Asia and the northern tip of Africa.
Beaded lizards
This family (Helodermatidae) consists of only two species restricted to the southwestern USA and central America. They are the only poisonous lizards and although their bite is painful there have been very few recorded deaths due to these reptiles. The Gila monster Heloderma suspectum has a stout trunk and tail with the beadlike dorsal scales which give these animals their family name. The blunt, fanglike teeth of beaded lizards are grooved, with the poison glands opening on the outer 'gum' of the lower jaw.
Chameleons
There are between 85 and 100 species of chameleon (family Chameleonidae), living mainly in Africa, Madagascar, Sri Lanka and India. Chameleons are characterized by a prehensile tail, large eyes capable of independent movement, a coiled, ejectable tongue and the ability to change color readily. The smallest chameleons may be under an inch in length, while the largest reach two feet in length.
Predominantly brown, green or yellow in color, chameleons are master color changes, and are even capable of changing to near black or white. Chameleons are good climbers making use of their prehensile tails and specialized zygodactyl feet (meaning that the toes of the same foot face opposite ways) to help them grasp branches. Chameleons also have laterally compressed bodies which may give additional stability on narrow twigs or branches.
Geckos
Like chameleons, geckos (family Gekkonidae, 800 species) are adapted for climbing, but these small lizards often have rounded adhesive pads on the gecko's feet which contain thousands of hairlike projections each around 1,000/th of an inch in length. The hairs work together to produce frictional forces helping the animal grip even to vertical surfaces.
Geckos are mainly active at night and live throughout the world in tropical and subtropical regions, and their distribution extends into the southern temperate zone. Geckos range in size from a half inch to 11 inches long, and include the banded geckos of the US and Mexico (Coleonyx sp.), the peacock gecko, day gecko and common house gecko. Geckos may get their name from the chirping and clicking noises that they make.
Girdle-tailed lizards
The 50 species of girdle-tailed lizards (family Cordylidae) are restricted to Africa south of the Sahara and Madagascar, and get their name from a whorl of sharp spines on their tails, often used as a defense mechanism. These mainly insect-eating lizards are active in the daytime and live in dry conditions. They include spiny lizards like the sungazer (Cordylus giganteus), and the armadillo girdle-tailed lizard (Cordylus cataphractus).
Iguanas
Iguanas (625-650 species - family Iguanidae) are a diverse group of day-active lizards restricted to the New World south of Canada down to the tip of South America. A few species also live in Madagascar, Fiji and Tonga. Iguanas include small lizards like the anoles (Anolis sp.) and basilisks (Basiliscus sp.) as well as the larger desert iguana (Dipsosaurus dorsalis), marine iguana (Amblyrhynchus cristatus) and green iguana (Iguana iguana).
Iguanas often have throat fans and crests on their backs. Some like the algae-eating marine iguana are good swimmers, and many bask in the sun. Iguanas are often territorial with males extending their throat fans and crests and making hissing noises.
Also included in this family are the various horned lizards (also mistakenly called horned toads) of the southwestern USA and Mexico. These small lizards (Phrynosoma sp.) include the short-horned lizard which gives birth to up to 31 live young. The chuckwalla (Sauromalus obesus) is a large potbellied lizard also found in the southwestern USA and Mexico.
Monitor lizards
Monitor lizards (family Varanidae - 31 species) include the Komodo dragon (Varanus komodoensis), the largest living lizard which may reach 6 feet in length. Komodo dragons are meat-eaters which can even attack and kill large mammals. Monitors are Old World lizards found throughout Africa, southern and southeastern Asia and Australasia. They are active mainly during daytime. Monitors have long necks and tend to swallow their prey whole like snakes. They are especially common in Australia: 24 of the known species live in Australasia.
Monitors include the common asiatic monitor, Varanus salvator, and the Cape monitor Varanus exanthematicus.
Night Lizards
This group of small lizards (family Xantusiidae -16-17 species) rarely exceed 5 inches in length. They are limited to the southwestern USA and Central America. Night lizards are active at night as their name suggests and they hide during the daylight hours in crevices or under rocks. Nights lizards do not have eyelids. The desert night lizard Xantusia vigilis of the southwestern USA may be found amongst debris under yucca trees and feeds on insects. The granite night lizard Xantusia henshawi is extremely secretive and hides under rocky flakes during the day.
Skinks
The 1000-1275 species of skink (family Scincidae) are found throughout the world's temperate and tropical regions. Skinks often have reduced or even no limbs, and have a characteristic triangular shaped head. Skinks live in a variety of different habitats, and may be tree-dwellers, burrowers or even swimmers. The Marine skink lives on the seashore and feeds on crabs and other small invertebrates. It can remain underwater for several minutes at a time.
The Giant Solomon Island skink (Cornucia zebrata) lives in trees and although it is a plant eater it is very aggressive.
The blue-tongued skink Tiliqua scincoides has a large head and heavily built body. It lives in many parts of Australia, in a variety of habitats from coastal woodlands to montane rainforest. Blue-tongued skinks feed mainly on snails, slugs and insects, although they may eat some plant material. If cornered, these skinks may open their mouths wide, making a hissing sound and exposing their bright blue tongues.
Wall and sand lizards
These lizards (family Lacertidae - 200 species) are sometimes called the Old World lizards. They live in Africa, Asia and Europe as far north as the Arctic Circle. These small lizards are active during the daytime and often only during the warmest part of the day. In cooler climates they are active only during the summer months.Wall and sand lizards eat insects and other invertebrate animals. Species include the rock lizard Lacerta saxicola and the ocellated green lizard Lacerta lepida.
Whiptails
The 200-227 species of the family Teiidae are native to temperate and tropical regions of the Americas, and are sometimes called the New World lizards. These slender lizards live in deserts and tropical rainforests and sometimes have reduced limbs. They include the Western Whiptail Cnemidophorus tigris of the southwestern USA and the larger South American tegu (Tupinambis teguixin) which grows to 3 feet long.
Snakes
Living without limbs
Snakes are a successful group of animals which may seem strange given that these reptiles have no external limbs. However, the absence of limbs confers some advantages. Large snakes can wriggle through spaces too narrow for similar-sized animals with legs, and snakes can approach their prey quietly and stealthily. The evolution of specialized sense organs has also equipped some snakes with very sensitive ways to detect their prey, from the heat-sensitive pits of vipers and rattlesnakes to the reception of chemical signals by the snake's tongue.
Snakes do not have eyelids. Instead, the eyes of most snakes are covered by a transparent layer called the brille, protecting the eye from damage. The vision of snakes varies from light-dark in the 'blind' snakes to highly acute movement detection in predators like the tree pythons.
There are about 2400-2500 species of snake, divided into three INFRAORDERS, further subdivided into eleven families. The first infraorder comprises the pythons, pipesnakes, boas and protocolubroids and are called the primitive snakes. The vipers, harmless snakes, and front-fanged snakes are in the infraorder of advanced snakes and the third infraorder is made up of four families of blind snakes.
Blind snakes
The four families of blind snakes are: the dawn blind snakes of Central and South America (Anomaleptidae - 20 species), the threadsnakes or slender blind snakes (Leptotyphlopidae - 78 species, mainly in Africa and Central and South America), the typical blind snakes (Typhlopidae) of Central and South America, southern Africa, southern Asia and Australasia, and the shieldtail snakes (Uropeltidae - 44 species, southern India and Sri Lanka).
These snakes all have vestigial eyes and reduced teeth within a tiny mouth - the pattern of teeth is used as a classification feature, as some have upper and lower teeth (shieldtail snakes) and other lack lower teeth (thread snakes, typical blind snakes). They are adapted for a burrowing lifestyle and resemble burrowing lizards as much as snakes. Blind snakes lack a left lung and have a head the same diameter as the body, giving them a worm-like appearance.
Boas
Sometimes grouped in the same family as pythons, the boas (Boidae - 39 species) are found across South America, northern and central Africa, middle and southern Asia and regions of Australasia. These so-called primitive snakes have vestigial hind limbs and two lungs, indicating that they are closer to the legged reptiles than the advanced snakes which have a single lung and no vestigial limbs.
Boas include the Boa constrictor (Boa constrictor), Emerald tree boa () and the anacondas (Eunectes sp.) These snakes are meat-eaters, killing their prey by constriction: they detect food sources using heat-sensitive pits on their lips. Boas are all live-bearing, and some larger snakes can produce up to 80 offspring. Boas are usually 6-12 feet in length but the green anaconda may reach 36 feet.
Cobras and mambas
These snakes are also called 'front fanged' snakes (family Elapidae - 186-200 species). They get their name from the pair of short erect grooved fangs in the front of the upper jaw. Front fanged snakes are so-called 'advanced snakes' found world-wide in tropical and subtropical regions, and they kill their prey using powerful venom.
Examples of front fanged snakes are the King cobra Ophiophagus hannah, Common cobra Naja naja, Black mamba Dendrosaspis polylepis and Egyptian cobra or asp Naja haje. This group also includes the coral snakes (Micrurus sp.), including the Arizona coral snake Micruroides euryxanthus. Front fanged snakes are usually between 1-5 feet in length.
Cobras are well-known for the 'hood' made of thin neck ribs which are extended when the snake rises up. The role of the hood is not certain although it certainly makes the snake look intimidating to its prey. The spitting cobras (Naja nigricollis and relatives) can spit venom up to 10 feet into the eyes of their prey or an aggressor.
Harmless snakes
Also called common snakes, snakes of the family Colubridae (5000 species found worldwide) are not strictly 'harmless' but their venom is not usually toxic enough to cause serious harm to humans. Both jaws of these 'advanced' snakes have teeth, but the snakes do not usually have elaborate fangs. This group is often divided into a number of subfamilies and includes the racers and whipsnakes, gopher snakes, milksnakes and kingsnakes, water snakes and garter snakes.
Most of these snakes prey on small mammals and birds, although the
egg-eating snakes (Dasypeltis sp.) eat only birds eggs, which in true snake tradition they swallow whole. The milk snake (Lampropeltis triangulum) is a colorful but harmless red, yellow and black banded snake that resembles the dangerous coral snakes. These two snakes differ in the color banding pattern, shown by the rhyme 'red to yellow kill a fellow, red to black, venom lack'. Milk snakes eat birds, which they kill by constriction, and they get their name because it was once thought they entered cowsheds to suck milk from cows.
The common kingsnake Lampropeltis getulus is from 3-6 feet long and lives in the southern USA and northern Mexico. This snake eats other snakes including rattlesnakes, copperheads and coral snakes, as well as mice, birds and eggs. Several subspecies of the kingsnake are found within its range, including the attractive black and cream striped form.
Pipesnakes
Pipesnakes (family Aniliidae, 11 species) are located in two main areas: the Amazon Basin and southeast Asia. These small burrowing snakes eat other snakes or eels, and include the red-and black striped coral pipesnake Anilius scytale of South America. This rainforest snake looks like the venomous coral snakes that live in the same region, although pipesnakes are not poisonous. Pipesnakes are usually 3 feet or less in length.
Pipesnakes do not have the transparent brille over the eyes found in most other snakes. Instead they have a more robust scute over their eyes - this is probably an adaptation to protect the eyes of the snake while it is burrowing.
Protocolubroids
These snakes are less well known than some of the other groups, and are restricted to Central America and northwestern South America. Several species are also found on islands in the Indian Ocean. Protocolubroids (family Tropidophiidae - 22 species) are small snakes, all under 3 feet in length. They resemble small boas, although they have some of the features of the advanced snakes: they only have one lung, and have similar throat bones.
Protocolubroid snakes include American woodsnakes and two species that live only on the island of Mauritius: the Round Island and Round Island keel-scaled snakes. These snakes are active at nighttime and eat lizards and small mammals.
Pythons
Sometimes grouped in the same family as boas, the pythons (Pythonidae - 27 species) are less widely distributed, and are found mainly in the rainforests and scrubland of the Old World tropics and subtropics from Africa across to Australasia. A single species, the Mexican burrowing python (Loxocemus bicolor), is found in the New World, although this species is sometimes placed in a family of its own. These so-called primitive snakes have vestigial hind limbs and two lungs, indicating that they are closer to the legged reptiles than the advanced snakes which have a single lung and no vestigial limbs. Pythons reach 3-30 feet in length.
Pythons include the Reticulated python (Python reticulatus), Indian or Burmese python (Python molurus), Green tree python (Chondropython viridis) and the Calabar python (Calabaria reinhardtii) of West Africa. Pythons are constricting snakes and usually feed on small mammals and birds, although larger specimens can consume large mammals and their are a few reports of humans being eaten by pythons. The womas are a small group of Australian pythons that eat other snakes. Unlike the boas, pythons all lay eggs.
Sea snakes
Like the mambas and cobras, sea snakes (family Hydrophiidae - 50 species) are also called 'front fanged' snakes and are sometimes included in the family Elapidae. Sea snakes are 'advanced' snakes with no vestigial limbs and a single lung. They have a pair of short erect grooved fangs in the front of the upper jaw.
Sea snakes have compressed tails which they use to help them swim. They are unusual because they spend their entire lives at sea and even lay their eggs in water. Sea snakes eat fish, eels and fish eggs and have very strong venom, necessary to immobilize their prey before it can swim away. Sea snakes show some specific behaviors associated with their aquatic lifestyle - they have the ability to 'tie themselves in knots', a ritual used to dislodge parasites and algae from the animal's skin.
These snakes are found in all the tropical oceans except the Atlantic. They include the Annulated sea snake Emydocephalus annulatus and give birth to live young.
Vipers and rattlesnakes
Members of this group of snakes (family Viperidae - 187-200 species) are found in all tropical, subtropical and temperate regions of the world. Like the boas, cobras, sea snakes and harmless snakes they are 'advanced' snakes with a single lung and no vestigial limbs. Vipers and rattlesnakes are poisonous, but have less toxic venom than boas, cobras and sea snakes, and their poison works on the blood and tissues of the victim rather than the nervous system. Many vipers and rattlesnakes do however produce venom that can kill humans if the victim is not treated promptly.
Vipers and rattlesnakes have a pair of fangs in the front of the upper jaw. These fangs are folded backwards when not in use, and the jawbones are rotated forwards when the snake strikes. American species include the bushmaster, Lachesis muta; the copperhead, cottonmouth and water moccasin (Agkistrodon sp.) and the rattlesnakes (Crotalus sp.), characterised by their tail rattle used to warn possible aggressors.
The Gaboon viper of equatorial Africa is the largest of this group of snakes, reaching 6 feet in length. Other Old World species include the European adder (Vipera berus), Asian tree vipers (Trimeresurus sp.) and Russell's viper (Vipera russelli). Russell's viper has a reputation of killing more humans than any other snake, but this is probably because of the wide distribution of this snake in southeastern Asia rather than the potency of its venom.
Worm-Lizards
Burrowing reptiles
Worm-lizards are not snakes but belong to a separate suborder, the Amphisbaena. They are the only group of true burrowing reptiles, making their own tunnels and living underground for most of their lives. There are 140 species in four families spread throughout much of tropical and temperate Africa, Central and South America, southern Spain and Arabia. Many species of worm-lizard have lost all traces of limbs, although one of the families is restricted to Central America and has true forelimbs.
The heads of worm-lizards are modified into tools used for digging, with concealed eyes and ears. Two separate head types are known, the shovel-snouted and keel-snouted species. The Florida worm-lizard Rhineura floridana has a shovel shaped snout, but its skull is sufficiently different from the other worm-lizards for it to be placed in its own family.
The skin of these reptiles is arranged in rings which are only attached loosely to the animal's body - the rings can fold over each other like a squeezebox and are used to grip the sides of the tunnel as the worm-lizard moves along. Worm-lizards eat worms and other invertebrate animals.
Tuatara
An evolutionary oddity
The lonely reptile Sphenodon punctatus or Tuatara is the only member of the family Sphenodontidae and is sufficiently different from other reptiles to be placed in an order of its own. Tuataras are found only on small islands off the coast of New Zealand. They appear superficially similar to lizards, but have a different skull structure more similar to that of a crocodile. They also lack a penis or hemipenes and have the vestiges of a third 'eye' on the top of their heads. Tuataras grow to about 2-2.5 feet in length and lay 6-15 eggs in holes in the ground.
Tuataras are active at night and spend the daylight hours in burrows, although they sometimes bask in the sun during the daytime. They have a low metabolic rate and can remain active at temperatures lower than most other reptiles. Tuataras eat insects, invertebrate animals and small lizards as well as eggs and chicks of seabirds. These reptiles may live up to 120 years of age. It is thought that the Tuatara has only survived because of the rocky, inpenetrable nature of its habitat: on more accessible islands this reptile is already extinct.
Crocodilians
The oldest reptiles
The order Crocodilia has only three families comprising the crocodiles, alligators and the gharial. These animals are the remains of a group of larger, even more ferocious animals that lived at the same time as the dinosaurs, and indeed have changed little since the last dinosaur died out around 65 million years ago.
Crocodilians are characterised by their long, teeth-filled snout and narrow, compressed tail. Their internal organs also show differences from those of other reptiles, including hearts and brains more 'advanced' than those of other reptiles. Crocodilians for example have a cerebral cortex, and are capable of learned behavior. The hearts of crocodilians are divided into both left and right ventricles, completely preventing mixture of arterial and veinous blood: this provides a more efficient circulatory system than that of the other reptiles.
Crocodilians also move more efficiently than other legged reptiles. They lift their bodies higher from the ground when they walk, and some species can move quickly when necessary.
Crocodilians are found in most tropical and subtropical regions of the world with some geographical separation of the three families. Gharials are restricted to India, Nepal, Bangladesh and Pakistan, whereas Alligators are found in the New World and China. Crocodiles are the most widespread of the crocodilian groups, with representatives in the New World, Africa, India and southeast Asia and Australasia.
Crocodiles
There are 14 species of crocodile spread throughout most of the tropical regions of the world. Crocodiles have longer, more pointed snouts than alligators, and are also characterised by the large fourth mandibular tooth visible when the jaw is closed. In alligators this tooth fits into the upper jaw and cannot be seen when the jaw is closed. The American crocodile Crocodylus acutus is tolerant to salt water and can occasionally be seen at sea. This species can be found from Florida across to the Pacific coast of Central America. The saltwater crocodile Crocodylus porosus is found from India across to Northern Australia, while inland waters of Australasia are inhabited by the freshwater crocodile Crocodylus johnstoni.
Alligators
The blunt-snouted alligators are grouped together with the caiman species into the family Alligatoridae (7 species in the southeastern USA, China, Central and South America). Alligators can be distinguished from crocodiles by the large fourth mandibular tooth which fits into the upper jaw and cannot be seen when the jaw is closed - in crocodiles this tooth is visible at all times. Alligators include the American alligator Alligator mississipiensis and the South American black caiman Melanosuchus niger. The Chinese alligator Alligator sinensis lives in the Yangtse River and can withstand cooler winters than most alligator species: this alligator is critically endangered and only a few hundred individuals still remain in the wild.
Gharial
Gharials (family Gavialidae; one species - Gavialis gangeticus) have a greatly elongated snout modified for fish-eating. They get their name from a mound on the male's snout which resembles a pot (in Hindu ghara = pot). The slender snout gives little resistance to water so it can be moved sideways quickly to catch prey. Gharials also have teeth specially adapted for grasping slippery fish, unlike other crocodilians that have more powerful teeth to catch and crush land animals.
Restricted to the northern inland waterways of the Indian subcontinent, gharials are an endangered species that was reduced to as few as 50-60 adults in 1974. Conservation measures over the past few years have increased this number to several thousand animals today.
