While most grade school children can rattle off a list of traits usually associated with reptiles -- they have scales, they lay eggs, they're cold-blooded -- few characteristics are common to every species in the group. Even devising a definition of “reptile” is challenging, although most scientists consider lizards, snakes, turtles, crocodiles, birds and tuataras to to be reptiles. Tuataras are absent from the pet trade, but specimens of all of the other major subgroups find themselves in the aquariums and cages of reptile enthusiasts.
The Linnaean classification system groups animals based on their physical characteristics. Most snakes and lizards have scaly skin and reproduce by depositing eggs, so Linnaeus placed them in the same group. However, this method of classification is based on human-selected criteria rather than biological reality. Accordingly, some anomalies occur with the scheme.
Beginning in the 1940s, biologists began grouping species in a different way. Instead of grouping by shared characteristics, they emphasized the evolutionary relationships among the species. With this scheme, all descendants of the first reptile species become reptiles. This means that your pet cockatoo, parrot or finch is also a reptile, closely related to crocodiles, and is an evolutionary descendant of theropod dinosaurs.
From a pet-keeper’s point of view, Linnaean classification system is more useful. After all, caring for a pet snake is a far different endeavor than maintaining a parakeet. However, it is important to understand that, from an evolutionary perspective, birds are just as much reptiles as snakes are.
While crocodiles, birds and turtles all have four limbs, some lizards have only two legs; others lack legs entirely. Snakes, which are technically a subgroup of the lizard clade, lack fully functioning legs, although some primitive lineages -- including many boas and pythons -- have a reduced pair of vestigial legs. Aside from those losing their tails to predators or conspecifics, all reptiles have tails.
Save for a few extremely rare individuals, all reptiles have scaly skin. However, these scaly coverings vary from one lineage to the next. For example, many geckos are covered in fine, delicate scales, which gives their skin a soft feel. By contrast, crocodilians have bony plates in some of their scales that provide protection. All reptiles shed their scales periodically, whether they replace all the scales at one time as snakes do or a few at a time as most others do.
Matters of Metabolism
Birds are endothermic or warm-blooded animals, but most non-avian reptiles are ectothermic or cold-blooded creatures who depend on external sources of heat to drive their metabolisms. This means that, when they are suitably warm, reptiles can forage, eat, mate and defend themselves but, if the temperatures drop too low, they become lethargic. Exceptions exist; a 2010 study published in “Plos One” concluded that leatherback sea turtles have some internal control over their body temperatures.
By any definition of the group, all reptiles practice internal fertilization, which distinguishes them from amphibians, most of whom engage in external fertilization. Additionally, most reptiles, including all turtles, crocodilians, birds and the tuatara, reproduce by depositing shelled eggs -- although several snake and lizard species give live birth.
Usually, the method by which reptiles reproduce -- live birth or shelled eggs -- is consistent among close relatives. For example, most natricines -- a group of snakes that includes water snakes, garter snakes and their kin -- give birth to live young. By contrast, most lampropeltines, a group that includes king, milk and rat snakes, deposit eggs. Some species within these clades reproduce in the opposite manner, generally in response to climactic variables.