Reptiles can be distinguished from other animals because of their scaly skin and, except for snakes, true claws. This means that reptiles do not hold their body temperature constant like mammals, but their internal temperature depends upon that of the surrounding environment. The exception is some large sea turtles which have some ability to regulate their body temperature metabolically. It is believed that some dinosaurs may also have been warm-blooded.
Reptiles have complex, shelled eggs similar to a bird's, which are laid on land.
In these cases the young are born directly rather than being laid as eggs. Unlike bird eggs, the shells of reptile eggs are often soft and leathery. In many species the eggs are laid in moist soil. They absorb some water and may enlarge in size after being laid. Unlike amphibians, all reptiles have internal fertilization of the eggs by the male.