Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane, and members of the class Sauropsida. Today they are represented by four surviving orders:
- Crocodilia (crocodiles, gharials, caimans and alligators): 23 species
- Sphenodontia (tuataras from New Zealand): 2 species
- Squamata (lizards, snakes and amphisbaenids (“worm-lizards”)): approximately 7,900 species
- Testudines (turtles and tortoises): approximately 300 species
Modern reptiles inhabit every continent except for Antarctica, although their main distribution comprises the tropics and subtropics. Though all cellular metabolism produces some heat, most modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as “cold-blooded” or ectothermic (the Leatherback Sea Turtle might be an exception). Instead, they rely on gathering and losing heat from the environment to regulate their internal temperature, e.g, by moving between sun and shade, or by preferential circulation – moving warmed blood into the body core, while pushing cool blood to the periphery. In their natural habitats, most species are adept at this, and can usually maintain core body temperatures within a fairly narrow range, comparable to that of mammals and birds, the two surviving groups of “warm-blooded” animals. While this lack of adequate internal heating imposes costs relative to temperature regulation through behaviour, it also provides a large benefit by allowing reptiles to survive on much less food than comparably sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile moves very quickly.
Except for a few members of the Testudines, all reptiles are covered by scales.
Most reptile species are oviparous (egg-laying). Many species of squamates, however, are capable of giving live birth. This is achieved, either through ovoviviparity (egg retention), or viviparity (babies born without use of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals (Pianka & Vitt, 2003 pgs: 116-118). They often provide considerable initial care for their hatchlings.