Reptiles first appeared on the earth about 315 million years ago, having apparently arisen from an amphibian ancestor. They share unique anatomical traits, including special modifications of their skin and eyes, the skull and backbone, and an external nasal gland. Over 6500 species are distributed world-wide on all continents except Antarctica. Due, in part, to their minimal food requirements, reptiles occupy nearly every habitat imaginable, including the world's oceans, desert and mountainous regions as high as 16,500 feet above the sea-level. Most reptiles are ectotherms, maintaining appropriate body temperatures by basking in optimal conditions, although some hormonal and muscular systems are involved in heat production in some species. During inclement seasons reptiles can suspend many functions and remain dormant.
There is considerable size variations among reptiles: tiny geckos and chameleons measure less than one inch in length; Komodo monitors reach 10 feet.; Anacondas and areticulated Pythons may be 25 feet and more; Leatherback turtles and Indopacific Crocodiles have weights approaching a ton.
Typical reptiles possess dry skin covered with scales. Snakes also have a brille, or protective eye covering. Like other animals, reptiles shed their skin, but they do so at one time rather than continually. They may be limbless or nearly so, and some lizards and some worm lizards, possess only one pair of limbs. Limbless terrestrial reptiles move via some form of undulation. Those with limbs crawl, while some lizard species rear up, using only the hind limbs, when running fast.
The limbic system developed with two things in mind. First, emotions had to be connected to memory, because emotionally charged events needed to be remembered. You don't want a child touching a hot stove more than once. Attaching an emotional handle to events not only makes it easier to remember, but it also makes us want to remember it. The second task for the developing limbic system was to express emotions with changes in heart rate, breathing, blushing sweating, bowel control and all the other features of the fight or flight response. To do this, the limbic system was attached to the hypothalamus, the part of the brain that sends out signals to express our emotions physically through the sympathetic and parasympathetic nervous system.
Fertilization is internal, and in most reptile species there are sex differences in size and color. Certain lizard populations consist entirely of females, each genetic duplicates of the other. One island population of snakes is entirely hermaphroditic, with all individuals possessing both male and female characteristics. The majority of reptiles produce leathery-shelled eggs which, in contrast to amphibians, are resistant to drying.
Crocodilians and most turtles construct nests of vegetation or excavate burrows for their eggs. All other reptiles deposit their eggs in humus, or the forest floor, or adhered to bark or something similar. Many kinds of snakes and lizard display the same type of viviparity, producing live young. Parental care is largely absent in reptiles, with offspring quite capable of subsisting on their own from birth. However, some lizards and snakes brood their eggs, and egg-brooding and parental care in crocodilians is widely documented. In some species of turtles and crocodilians, the sex of the offspring is determined by the temperature during incubation.
Reptiles are preyed upon by a variety of organisms including humans, other reptiles, amphibians, birds, mammals, fishes, and many kinds of invertebrates. Some of their defense mechanisms involve camouflage (direct resemblance to something in the environment), cryptic or disruptive coloration (rendering the animal difficult to see), mimicry (resembling something noxious), a wide variety of their displays (including puffing, flattening, gaping, and hissing), death feigning, fleeing the scene (even via gliding), biting, scratching, and lashing or breaking the tail.
This diverse group of animals has influenced humans throughout recorded history. Objects of both revulsion and veneration since ancient times, reptiles figure strongly in myth and folklore. They have gained great popularity recently as the burgeoning field of herpetology reveals their interesting lifestyles. Ranging from the bizarre to starkly beautiful, reptiles present a panorama of shapes and colors.
Although most of negative image of reptiles is unfounded, some species are capable of causing suffering or death in humans. Several species of crocodilians have preyed on humans, sometimes in considerable numbers, especially in Africa and Indonesia. There are many dangerously venomous species of snakes throughout the world, and even the venomous Gila Monster (Heloderma) and the large Komodo Monitor (Varanus) have caused human fatalities. Thus, it is not surprising that reptiles have been the object of considerable toxicological and behavioral research.
Pet alligators and snakes do not respond to cuddling and petting because they don't feel emotions beyond anger, fear, and pleasure. Mammals, in contrast, nurture and care for their young. They have a sense of family and community. They play and bond with one another. Many mammals live and hunt in packs, and they guard and protect members of the pack. For this, you need more than a reptilian brain had to offer. What eventually developed was a small collection of gray matter, called the limbic system, located immediately above and incircling the brainstem.