Why does a lizard need to lie in the sunshine to get energy?

The lizard is a reptile, a cold-blooded animal that is unable to internally control its own body temperature. In order to warm up or cool down, lizards and other reptiles—such as snakes, turtles, and crocodiles—move to different areas of their environment. They also use certain other behavioral traits to keep their body temperatures constant. For instance, if a lizard is starting to feel the intensity of the tropical sun, it might head into the shade or take a dip in a pool of water. The same lizard might also bask in the sun to warm up. Frilled dragons and collared lizards run on their hind legs in the heat of the day, making an artificial breeze to help cool themselves off. And another reptile, the crocodile, holds its jaws open to cool down on hot days. The blood vessels in its mouth are close to the skin surface, and help transfer heat. Lying quietly is another technique the crocodile uses to warm its body and help digest its food. Because they are cold-blooded, reptiles can survive on much less food, compared to warm-blooded small mammals and birds, which burn much of their food to keep warm.