Reptiles are cold-blooded animals. This means they are unable to regulate their own body temperature like birds and mammals do. Therefore, reptiles must modify their activity and behavior to accommodate changing environmental temperatures. They must seek shelter during excessive heat (to prevent over-heating) and extreme cold (to prevent hypothermia). But being cold-blooded has its advantages too. It has enabled reptiles to enjoy success in habitats that mammals and birds find challenging. Since reptiles do not need to burn calories to fuel a constant body temperature, they can survive on much less food intake that birds and mammals. For this reason, reptiles are the dominant vertebrate in desert habitats.
There are two characteristics of reptiles that enabled them to colonize terrestrial habitats more extensively than their amphibian ancestors—scales and the ability to lay hard-shelled eggs. Reptiles' scales provide them with a tough, protective layer to their skin. They also help to minimize the loss of body moisture. The scales of a reptile consist of a protein called keratin. Reptile scales are not individual structures, like those of a fish, but are instead a continuous sheet of epidermal tissue.
Hard-shelled eggs provide a protective environment in which the embryo can develop and enables reptiles to lay their eggs in dry environments. In contrast, amphibian eggs do not have a hard shell coating and consequently must lay their eggs in or near water.
The reptilian skeleton differs from other vertebrates in various ways. For example, mammals have a single lower jawbone called the mandible but reptiles have several bones in their lower jaw that enable them greater bite mobility. Also, reptiles have only one bone in each ear (the stapes) whereas mammals have three small bones in each ear (the malleus, incus and stapes). Reptiles also have only one occipital condyle (a protrusion on the skull that forms a joint that enables movement of the head) while mammals and amphibians have two occipital condyles.