In the day when the air is hot, a lizard avoids the sun and stays in the shade. At night when the air is cold, a lizard goes into its ground burrow. Endotherms also use behavior to regulate body temperature Besides doing what lizards do, endotherms also build nests, huddle, and select clothing. When body temperature rises, extra heat from the body core is brought to the skin by the blood and the heat is lost to the environment which cools the body. When body temperature drops, the blood vessels in the skin constrict and decrease blood flow and heat transport to the skin which reduces heat loss the environment.
Some ectotherms produce heat Flying insects use their flight muscles to produce heat.
Some fish elevate body temperature by conserving metabolic heat Cold blood from the gills is transported into muscles that are also transporting warm blood from the muscles to the heart. Since the cold blood is in close contact with the warm blood, heat is exchanged from the warm to the cold blood. This called a countercurrent heat exchanger . Endotherms actively increase heat production or heat loss Below a critical temperature, an endotherm s metabolic rate increases because the animal must produce more heat to maintain a constant body temperature. Shivering heat production 2.