Amphibians and reptiles, including lizards, are ectotherms. An ectotherm is an organism that relies on external sources of heat to raise its body temperature. The behavioral mechanisms involved in ectothermal temperature regulation are quite straightforward. Lizards, especially desert species, are particularly good at behavioral thermoregulation. Movement back and forth between sunlight and shade is the most obvious thermoregulatory mechanism they use. Early in the morning, or on a cool day, lizards bask in the sun, whereas in the middle of a hot day they retreat to shade and make only brief excursions into the sunlight. Sheltered or exposed sites may be sought out. In the morning, when a lizard is attempting to raise its body temperature, it is likely to be in a spot protected from the wind. Later in the day, when it is getting too hot, the lizard may climb into a bush or onto a rock outcrop where it is exposed to the breeze and its convective heat loss is increased.

An animal can alter the amount of solar radiation it absorbs by changing its orientation to the Sun, its body shape, and its skin color. Lizards use all of these mechanisms. An animal oriented perpendicular to the Sun's rays intercepts the maximum amount of solar radiation, and one oriented parallel to the Sun's rays intercepts minimum radiation, so lizards adjust their orientation to control heat gained by direct solar radiation. Many lizards can spread or fold their ribs to change the shape of their trunk. When the body is oriented perpendicular to the Sun's rays and the ribs are spread, the surface area exposed to the Sun is maximized and heat gain is increased. Compressing the ribs decreases the surface exposed to the Sun and can be combined with orientation to the rays to minimize heat gain. Horned lizards (Phrynosoma ) provide a good example of this type of control. If the surface area that a horned lizard exposes to the Sun directly overhead when the lizard sits flat on the ground with its ribs held in a resting position is considered to be 100 percent, the maximum surface area the lizard can expose by orientation and change in body contour is 173 percent and the minimum is 28 percent. That is, the lizard can change its radiant heat gain more than sixfold solely by changing its position and body shape.

Color change can further increase a lizard's control of radiative exchange. Objects look dark because they are absorbing energy in the visible part of the solar spectrum, and the radiant energy they absorb warms them. The lightness or darkness of a lizard affects the amount of solar radiation it absorbs, and lizards can darken or lighten by moving dark pigment in their skin. Melanophores are cells that contain the pigment melanin. They are shaped rather like mushrooms, with a broad upper portion connected by a stalk to a lower section. When melanin granules are dispersed into the upper part of the cell, close to the skin surface, the skin appears dark, but when the granules are drawn away from the surface into the lower section of the cell, the skin appears light. Lizards heat 10 to 75 percent faster when they are dark than they do when they are light.

The repertoire of thermoregulatory mechanisms seen in lizards is greater than that of many other ectothermal vertebrates. Turtles, for example, cannot change their body shape or color, and their behavioral thermoregulation is limited to movements between sunlight and shade and in and out of water. Crocodilians (an order of reptiles that includes alligators and crocodiles) are very like turtles, although young individuals make minor changes in body shape and color. Most snakes cannot change color, but rattlesnakes and boas lighten and darken as they warm and cool.