Cold-blooded animals (ectotherms) are animals that do not have an internal mechanism for regulating body temperature, relying instead on solar energy captured by the environment. Reptiles, amphibians and fish are examples of cold-blooded animals.

Reptiles will often sun themselves on a rock to absorb heat which is converted to energy. The heat raises the metabolism of the animal which results in an active period. If the weather is too warm, the animal might bury itself in the sand, seek shade, find a hollow or some other cool shelter. In this way behavioral instincts regulate body temperature within acceptable parameters. As ambient temperature drops, the animal's metabolism slows to conserve energy.

Relative to their environments, amphibians and fish have similar behaviors. A frog that gets too warm on the muddy banks of a river will either bury itself in the soft earth or seek a cooler spot in the water. Fish will change depths to regulate their temperature, seeking cooler deeper water, or warmer water nearer to the surface.

Because cold-blooded animals do not use internally-generated energy to regulate body temperature, they require far less energy than warm-blooded animals. Warm-blooded animals (endotherms) like mammals and humans, have internal mechanisms that maintain a set body temperature regardless of the ambient temperature of surroundings. This self-regulation requires vast amounts of energy obtained through frequent meals to "feed the furnace." Cold-blooded animals like snakes may only require one meal every several weeks. As a result cold-blooded animals thrive in remote areas like small islands and deserts where food is too scarce to support warm-blooded animals.

The brains of cold-blooded animals tend to be less complex, requiring less energy. At one time it was assumed dinosaurs were slow moving, dim-witted cold-blooded animals. More recent research indicates many species of dinosaurs like the infamous T-Rex (tyrannosaurs rex) were fleet-of-foot and quite intelligent, leading some scientists to hypothesize that dinosaurs were warm-blooded, though this topic is hotly debated.

Cold-blooded animals can do amazing things as a result of their physiology. For example, in winter, a wood tree frog will bury itself in a few inches of dirt or leaves, and freeze virtually solid with the soil. Heart and brain functions cease and the eyes of the frog turn a dead milky white. It appears solid as an ice cube, yet amazingly when the temperature warms the frog comes back to life as it thaws. Its brain and heart kick back in to jump-start the rest of the body and eventually it hops happily away. Research shows that starches consumed just before the stasis period are converted to blood sugar or glucose, which makes it more difficult for the fluid in the cells of the frog to crystallize, acting like a kind of biological anti-freeze. Because the cells never freeze, this makes it possible for the wood frog to thaw without damage.

Other species of frogs survive months of drought by burying themselves and entering a state known as estivation (or aestivation), surfacing when the rains come. Though they lie completely dormant for months, these amazing cold-blooded animals do not lose muscle mass. Scientists believe that a greater understanding of this ability could lead to applications in the areas of healthcare and space travel.