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Animals can be loosely grouped into two groups based upon how body temperature is maintained:

1. Ectotherms are animals that obtain body heat from their environment. Since their temperatures often vary with the temperature of their environment, they are sometimes referred to as poikilotherms (“changing temperature”). Examples include most invertebrates, amphibians, reptiles, and fish. Because many of these animals may feel cold to the touch they are called “cold-blooded” animals, but many land-dwelling ectotherms can exceed ambient temperatures by basking in the sun.

2. Endotherms are animals that generate their own body heat. They are also referred to as homeotherms because they maintain a constant internal temperature or as “warmblooded” because their temperature is relatively warm compared to ectotherms. Animals regulate their body temperatures by employing the following mechanisms:

1. Cooling by evaporation. Many animals lose heat by sweating. Since changing from a liquid to gaseous state requires energy (an endergonic reaction), body heat is removed when water vaporizes. Evaporative heat loss also occurs from the respiratory tract, a cooling process employed when animals pant.

2. Warming by metabolism. Muscle contraction and other metabolic activities generate heat. For example, shivering warms animals from the heat generated by muscle contractions.

3. Adjusting surface area to regulate temperature. The extremities of bodies (arms, hands, feet, ears) add considerable surface area to the body. By changing the volume of blood that flows to these areas by vasodilation or vasoconstriction (increasing or decreasing the diameter of blood vessels), heat can be lost or conserved. In hot environments, for example, elephants and jackrabbits increase blood flow to their large ears to reduce body temperature. In contrast, animals in cold environments reduce blood flow to their ears, hands, and feet to conserve heat. In addition, when blood moves through vessels toward an extremity, it flows adjacent to blood moving away from that extremity. In this example of countercurrent exchange, heat conduction from the warm blood to the returning cold blood is redirected to internal parts of the body before reaching the extremity. In addition, all animals have various behavioral, physiological, or anatomical adaptations that increase their ability to survive in a particular environment. To survive cold temperatures, for example, some animals hibernate, while others have hair, feathers, or blubber. Some animals avoid heat by merely moving from sun to shade, while others restrict their activity to nights.