A cold blooded animal, or ectotherm, is one that does not have an internal mechanism for regulating its body temperature. Instead, a cold blooded animal relies on solar energy captured by the environment. Reptiles, amphibians and fish are examples of cold blooded animals.
Reptiles will often sun themselves on rocks to absorb heat. The heat raises the metabolism of the reptiles, which results in an active period. If the weather is too warm, a reptile might bury itself in sand or seek shade in a hollow or some other cool shelter. In this way, the cold blooded animal's behavioral instincts keep its body temperature within the proper range. As ambient temperature drops, the animal's metabolism slows to conserve energy.
Fish and Amphibians
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 either cooler deeper water or warmer water that is closer to the surface.
A cold blooded animal does not use internally generated energy to regulate its body temperature, so it requires far less energy than warm blooded animals, or endotherms. Warm blooded animals, such as humans, other mammals and birds, have internal mechanisms that maintain their body temperature within a certain range, regardless of the ambient temperature of surroundings. This self-regulation requires vast amounts of energy that is obtained through frequent meals. A cold blooded animal doesn't need to eat as often and might eat one meal every few weeks. As a result, cold blooded animals are able to thrive in remote areas such as 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 and use less energy. At one time, it was assumed dinosaurs were slow-moving, dim-witted cold blooded animals. More recent research indicates that many species of dinosaurs, such as the tyrannosaurs rex, were fleet-of-foot and quite intelligent, leading some scientists to hypothesize that dinosaurs were warm blooded.
The Case of the Wood Tree Frog
Cold blooded animals can do some unusual things as a result of their physiology. For example, in winter, a wood tree frog will bury itself under dirt or leaves and freeze virtually solid with the soil. Its heart and brain functions cease, and the eyes of the frog turn milky white. It appears to be as solid as an ice cube, but when the temperature warms, the frog comes back to life as it thaws. The frog's brain and heart kick back in to jump-start the rest of its body, and eventually, it is able to hop away.
Research shows that starches that the frog consumes just before the stasis period are converted to glucose, or blood sugar. This makes it more difficult for the fluid in the cells of the frog to crystallize, so it acts like a kind of biological antifreeze. The wood frog's cells never fully freeze, so it is able to thaw without damage.
Other species of frogs survive months of drought by burying themselves and entering a state known as estivation, or aestivation, then surfacing when the rains come. Although they lie completely dormant for months, these 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.