The skin (part of the shell) can contract, or become thicker, to decrease the amount of heat transferred from the core to the environment. In addition, the amount of heat transferred through the shell may also be controlled by the amount of blood that flows to the skin. In cold animals, vasoconstriction (narrowing of the blood vessels) decreases blood flow to the skin and decreases the amount of heat transferred from the blood (which comes from the core) to the skin. In contrast, hot animals undergo vasodilation (widening of the blood vessels) in which blood flow to the skin is increased to allow more heat to be transferred from the blood to the shell, which includes the skin, comb, wattles, and feet.