Science, Tech, Math › Animals & Nature Top 5 Characteristics of Reptiles Know How to Distinguish Reptiles From Amphibians, Fish, and Mammals Share Flipboard Email Print Sami Sert/Getty Images Animals & Nature Reptiles Amphibians Birds Habitat Profiles Mammals Wildlife Conservation Insects Marine Life Forestry Dinosaurs Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated August 22, 2019 What exactly is a reptile? Although it's easy to say that snapping turtles, Galapagos land iguanas, and leaf-tailed geckos are reptiles, it's more challenging to explain precisely why they are reptiles and what differentiates them from amphibians, fish, and mammals. 01 of 05 Reptiles Are Four-Legged Vertebrate Animals All reptiles are tetrapods, which simply means that they have either four limbs (like turtles and crocodiles) or are descended from four-limbed animals (like snakes). More broadly, reptiles are vertebrate animals, meaning they have backbones housing spinal cords that run down the lengths of their bodies—a characteristic they share with birds, fish, mammals, and amphibians. In evolutionary terms, reptiles are intermediate between amphibians (which have moist skin and need to stay near bodies of water) and mammals (which have warm-blooded metabolisms and have diversified into every habitat on Earth). 02 of 05 Most Reptiles Lay Eggs Reptiles are amniote animals, which means that the eggs, laid by females, contain an elastic sac within which the embryo develops. Most reptiles are oviparous and lay hard-shelled eggs, but a few squamate lizards are viviparous, giving birth to live young that develop inside the females' bodies. You may be under the impression that only mammals are viviparous, but this isn't true; not only do some reptiles give birth to live young, but so do certain species of fish. Most reptiles differ from mammals in that they lack placentas—the tissue structure with which developing embryos are nourished within the womb. 03 of 05 The Skin of Reptiles Is Covered With Scales (or Scutes) The scales of reptiles, which develop from the epidermis (the outermost layer of skin), are small, hard plates made of the protein keratin. Scutes, such as the shells of turtles and the armor of crocodiles, are similar in appearance and function to scales but are bony structures that form in a deeper layer of the skin, the dermis. Scales and scutes provide reptiles with physical protection and prevent water loss; in many species, the shapes and colors of these structures play a role in territorial disputes and courtship displays. Bear in mind that, although all reptiles have scales, this is not a unique reptile characteristic; butterflies, birds, pangolins, and fish have scales as well. 04 of 05 Reptiles Have Cold-Blooded Metabolisms The body temperature of cold-blooded animals is determined by the temperature of their environment. This contrasts with warm-blooded animals—the body temperature of which is maintained within a small, constant range largely independent of external conditions. Because they are cold-blooded, or ectothermic, reptiles must bask in the sun to increase their internal body temperatures, which in turn allows for a higher level of activity (as a rule, warm lizards run faster than cool lizards). When they overheat, reptiles shelter in the shade to cool back down to a safer temperature. At night, many species are virtually immobile. 05 of 05 Reptiles Breathe With the Aid of Lungs One of the most important characteristics of animals is how efficiently they gather and utilize oxygen, the molecular fuel that powers metabolic processes. All reptiles, including snakes, turtles, crocodiles, and lizards, are equipped with air-breathing lungs, though different kinds of reptiles employ different means of respiration. For example, lizards breathe using the same muscles with which they run, which means they have to hold their breath while in motion, while crocodiles have more flexible diaphragms that allow for a wider freedom of movement. As a general rule, the lungs of reptiles are more advanced than those of amphibians but less sophisticated than those of birds and mammals.