monitor lizard

Reptiles are cold-blooded, hairless, egg-laying vertebrates. They are they divided into four orders: 1) snakes and lizards, 2) turtles, 3) crocodiles and alligators, and 4) the tuatara (a creature found in New Zealand that looks like a lizard). Most 7,200 of so reptile species are either lizards or snakes.

The ancestors of modern reptile are though to have emerged around 240 million years ago. The oldest fossils date to between 170 million and 200 million years ago. The most famous reptiles from the past are dinosaurs who many scientists believe are more closely related to modern birds than they are to modern reptiles.

Reptiles live mostly on land. They are found everywhere in the world except in polar regions. They are most plentiful in tropical regions.

Websites and Resources on Animals: ARKive Animal Info ; Animal Picture Archives (do a Search for the Animal Species You Want) animalpicturesarchive ; BBC Animals Finder ; Animal Diversity Web ; International Field Guides ; ; Encyclopedia of Life ; World Wildlife Fund (WWF) ; National Geographic National Geographic ; Animal Planet ; Wikipedia article on Animals Wikipedia ; ; Endangered Animals ; Endangered Species Resource List ; Biodiversity Heritage Library

Websites and Resources on Reptiles: Reptile Database ; Reptileweb ; Reptile Channel ; Wikipedia article Wikipedia ; Reptile Phylogeny

Reptile Characteristics

lizard heart

Reptiles have a distinctive hearts with three chambers---two rear chambers (auricles) and one front chamber (ventricle)---with a partition that divides the heart almost completely in two. By contrast mammals have a four-chambered heart and amphibians have a three-chambered heart without a partition.

Reptiles swallow rather than chew their food. Many have jaw bones that bend and pivot and even come unhinged to allow the reptile to manipulate large prey. These developments freed the tongue from manipulating prey and allowed it become a sense organ . The forked tongue that many snakes and reptiles possess dates back to around 65 million years ago. It picks up chemical clues in “stereo” which allows reptiles to locate things.

Reptiles shed their skin. Most have scales. Many are well camouflaged, which means they are given some protection from predators but are forced to stay to certain habitats. Those who blend in with leaves for example stand out on rocks. Their limited range results in many becoming fiercely territorial.

Reptiles such as iguanas show physiological changes associated with pleasure---a rise in heart rate and body temperature---like mammals but amphibians don't.

Cold Blooded Reptiles

Reptiles are ectothermic (cold blooded), which means they can not regulate their body temperature internally and can not create heat with their bodies like mammals can and are at the mercy of the sun and their surroundings for heat. This explains why reptiles are less common in cold and temperate areas. The few that live in these places often hibernate when the weather is cold.

lizard skin

Because they are cold blooded, reptiles can move vigorously when the weather is hot but are sluggish and move slowly when it is cool. At night a reptile’s body heat falls with dropping temperature of its surroundings. Each morning it must regain heat. Reptiles often bask in the sun to build up heat. Reptiles can also overheat. When it gets too hot for them they seek shelter in the shade.

Being cold blooded has its advantages. Because reptiles receive much of their energy from outside their bodies and do not have to create it, they are more energy efficient and need less food. This and the fact they have salt-excreting glands also means that they are more tolerant to salt, and thus are capable of surviving long periods at sea, which explains how they have traveled across seas and inhabited islands that no mammal, until the arrival of man, could ever make it to.

During the summer, the weather is often too hot for reptiles. At this time of the year reptiles become more active at night. In the middle of the day they seek shelter in the shade. As the weather cools they become more active. If they absorb enough heat in the afternoon they can remain active through the evening and night before seeking refuge in a burrow, in leaf litter or under a log or rock.

Reptile Reproduction

Reptiles were the first creatures to develop eggs. They passed on the ability to birds. Today, most reptiles lay eggs but not all of them. Boa constrictors, rattlesnakes and chameleons give birth to live young while pythons, cobras and iguanas lay eggs.

Reptiles that don't lay eggs produces eggs inside their bodies that either have very thing shells or no shell at all. In the case of the former the young hatch from eggs while still in their mother's body and emerge live.

Reptile eggs contain considerable amounts of yoke. Some reptiles species provide their young with additional nourishment by producing eggs that attach to the uterus. The uterus and the embryo generate interlocking blood vessels that allow nutrients to be passed from the mother to her young.

Most reptiles bury their eggs or conceal them somehow and then abandon them. Some snakes actively guard them and fight off predators.

Many reptiles have no sex chromosomes. Instead gender is determined by temperature. In crocodiles for example males are hot: eggs incubated in sand above a certain “pivotal temperature” almost always hatch males. This could spell trouble if global warming takes hold and female crocodiles---and reptiles---become scarce.


Lizards and snakes are regarded as members of the same order (Squamata, meaning “scaly skins”) within the reptile class of animals because of anatomical similarities. Some true lizards such the glass snake are actually legless lizards and some snakes have rudimentary legs with their bodies. What distinguishes a lizard from a snake is that snakes have a flexible skull to swallow prey larger than they are. Lizards don’t have this.

Lizards evolved about 200 million years ago during the Jurassic Period. Their scaly skins helped protect their bodies from desiccation. The adaption allowed the move away from water into a drier environment. The lizard and snake branches are thought to have diverged about a 100 million years ago when a group of reptiles that evolved into snakes began adopting a burrowing lifestyle.

There are about 4,500 species of lizard, compared to about 4,000 mammals. Most lizards are predators with smaller species feeding on spiders, worms, larvae pill-bugs and various ground-dwelling insects. Large ones eat small mammals, eggs. Some even eat other lizards and their own kinds.

Lizard Characteristics

Lizards have long tails and wide mouths. They differ from similar animals such salamanders in that they have scales, claws, movable eyelids and ear openings.

Most lizards run by alternately moving the front right and rear left legs together and then the front left and rear right legs. This motion gives lizards a distinctive “wriggling” motion. Lizards can’t breath and run at the same time. The lack a diaphragm muscle to push their lungs. Their rib muscles which expand the chest during each breath also brace the forelimbs during locomotion, especially running. This means that lizards can run for only a short time and need to pant afterwards to restore depleted oxygen.

Many species are able to discard their tails to distract predators. They do his by squeezing special muscles, causing the still-wriggling tail to pops off while muscles pinch close the blood vessels, minimizing blood loss. Lizards can regenerate a new tail but it takes a lot of energy to do this.

Some lizards have blue tails designed to attract predators and break off, allowing the lizard to escape. Most legless lizards are either long-tailed surface dwellers that “swim” through grass or short-tailed burrowers. The surface dwellers have long tails that can be bitten off by predators without causing serious harm to the lizard. Burrowers that live underground don’t need this defense.

Scientist that catch lizards work in teams. One man approaches the lizards and drives it towards a catcher hidden behind the bushes. As the lizard approaches the catchers snags with fishing lines fashioned into a noose.

Lizard Senses

Lizards have movable eyelids (snakes don't have these) and most species have excellent eyesight. Some species of lizard even have a "third eye." on top of their head. This eye does not form an image but may help a lizard distinguish between light and dark. Lizards also have external eardrums and can hear very well.

Lizards and snakes are both very good at sensing and analyzing smells and message-carrying chemicals Many have a vomeronasal organ embedded in the roof of their mouth that detects heavy non-airborne molecules taken in through the mouth. It supplements olfaction which is the ability to smell airborne molecules that enter the nostrils and is distinct from taste, which analyzes chemicals that come into contact with taste buds on the tongue. These senses help reptiles locate prey and help warn them or potential prey that might be toxic. It also frees up the eyes to locate prey and find mates.

The vomeronasal organ is sometimes called the Jacobsen's organs. Lizards and snakes with forked tongues have these on either side of the roof of their mouth. Chemicals are picked up from the environment with their forked tongues then transfer to these organs.

Lizards and snakes with forked tongues constantly flick their tongues in and out of their mouths, bringing in new samples of chemicals on either side of the tongue through the chemical equivalent of stereoscopic vision. Not only can they determine the presence of chemicals they can also determine the direction which they are coming from and detect edges and dimensions of the sources.

Lizards and snakes use their forked tongues and sense organs in their mouth to locate food, enemies and mates. And this they can do without even opening their mouths. Predators rely on smells and message-carrying chemicals to locate their prey and use their eyes to determine the location of the prey for the final lunge.

Lizard Mating

The males of some lizard species mate by clamping their jaws onto the rear leg of a female and then slide their bodies underneath to copulate. The eggs are fertilized in the bodies. The females of some species will mate several time over a period of several months. The eggs hatch in a month to a month and half, with the lizards emerging fully-formed and ready to fend for themselves.

Most lizards lay eggs with tough, leathery shells warmed by the sun. Some bring forth live young from eggs hatched in their bodies. The young are generally not taken care of by adults after they hatch and have to fend for themselves.

At least 27 species in seven different families exist mostly or entirely as females and produce eggs that develop without sperm and produce clones of the single parent. None of these females can reproduce by themselves. They need the stimulation of with the help of pseudo-male female


Skinks are kinds of lizards with small legs and bodies that can squirm like a snake. They are quick and their tails break off and will grow back. They often have stripes. Some are bluish in color. Skinks and rattlesnakes give birth to live young.

Sometimes when the skinks tail comes off it keeps moving. Skins perform this dramatic form of self-amputation by suddenly contracting it muscles, which it turn causes a fragile vertebrae to break. The new tail often looks different from the original. Instead of bonding to vertebrae it has a tube of cartilage.

Turtles and Tortoises

Turtles and tortoises are reptiles with shells. Turtles live mostly in the water and tortoises live mostly on land. According to the official Red List by the World Conservation Union almost half the turtles and tortoises are threatened.

Turtles are common sites in ponds, lakes, rivers and streams. They are commonly seen sunning themselves on rocks and logs. During the winter in temperate areas turtles hibernate by spending months in the mud without taking a breath.

The males and females of many turtles species bob their heads up and own, sometimes for hours, before mating. The males of some species bite the females head and suck on her feet to get her in the mood. During copulation the male often bites into the shell of the female to steady himself. Some males have to prop themselves up in an almost vertical position on the female to achieve penetration.

Origin of Turtles

Oliver Rieppel, curator of fossil reptiles at the Field Museum in Chicago, told the Chicago Tribune, “The origin of the turtle shell has been a big debate in paleontology for a long time. The turtle shell is such a specialized, unique feature. No other vertebrate animal group has this kind of body plan that has always been a big mystery.”

It appears that turtles first had shells only on their bellies. In an article in Nature in November 2008, Rieppel described a fossil---found in what used to be a shallow sea in Guizhou Province of southwestern China---of a turtle that lived 220 million years ago, a time when dinosaurs first appeared. The toothy aquatic creature---named Odontochelys semitestacea Latin for ---half-shelled turtle with teeth--- was about 40 centimeters long and had a shell on its belly (a plastron) but lacked one on its back (the carapace). Its ribs and backbones were beginning to expand and grow together in such a way that millions of year larger would yield a carapace.

Some scientist dispute the conclusion. Robert Reisz, an evolutionary biologist at the University of Toronto, said he thinks Odontochelys once had a carapace and lost as an adaption to its environment the same ways some modern sea turtles have.

The oldest previously known turtle fossil has a full shell, beaked mouth and no teeth like modern turtles. It was would found in Germany and dated to about 206 million years ago.

Turtle Characteristics

Turtles don't have teeth. They chew and grasp things with horny jaws that contain sharp, almost knifelike cutting edges. Some turtles are completely vegetarian. Others are completely carnivorous. Many are omnivorous, eating whatever they can find. Turtles can not breath like human by expanding and contracting the rid cage. Instead they rely on muscles around their lungs to do the job.

Turtle shells consist of two parts: 1) the carapace, the upper, arched part; and 2) the plastron, the flat lower part. The carapace is attached to the backbone and ribs. The palstron is fused to the breastbone. In hard shelled turtles the bone is covered by a shield made of a horn-like material. Soft shelled turtles have a covering of tough skin over the bony shell.

The carapace consists of two layers: an inner core of bony plates that are fused together, and an outer layers of shields made of hornlike keratin, called scutes. The shape and patterns of the scutes is a useful clue in identifying different species.

A turtle's shell grows along with the animal and, unlike the skin of lizards and snakes, is not periodically shed. Most turtles when flipped on their back can turn themselves back over.


Soft-Shell and Two-Headed Turtles

Soft shelled turtles like snapping turtles lack the keratin shields of other turtles. Their carapaces instead are covered with a layer of leathery skin. They are generally more aggressive than hard-shell turtles and can deliver a nasty bites with the sharp edges of their jaws.

Most soft-shelled turtles are carnivorous while hard-shelled varieties are most vegetarians. Soft-shelled turtles spend much of their time lying motionless on the floor of a pond, lake or swamp, waiting for passing fish, crayfish, and aquatic insects which they snap up with surprising speed.

Many soft-shelled turtles have greatly elongated nasal cartilages that serve as snorkels when the turtle is under water. With these devises the turtles can stay submerged for long periods of time, occasionally projecting their nose above the water to breath.

Occasionally two-headed turtles or tortoises are born. The owner of a two-headed Mediterranean. Spur-thighed tortoise told National Geographic that his pet had a slightly misshapen shell and back legs stuck out he side of the shell more than with one-head turtles. He said the right head was dominant and it sometimes moved the body while the other head was eating.

Image Sources:

Text Sources: Mostly National Geographic articles. Also “Life on Earth” by David Attenborough (Princeton University Press), New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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