Major Characteristics

All mammals, such as the lions shown in Figure 43-1, have the following six major characteristics:

• Endothermy—Mammals, like birds, regulate body heat internally through metabolism and externally through insulation. A mammal's body temperature stays high and nearly constant because of adjustments in metabolic rate and regulation of heat loss through the body surface. This manner of controlling body temperature is called endothermy.

• Hair—All mammals have some hair. Most mammals are covered with a thick coat of hair, which insulates the body against heat loss. Hair is made of filaments of the protein keratin.

• Completely divided heart—Mammals have a four-chambered heart whose two ventricles are completely separated by a muscular wall. This division keeps deoxygenated blood from mixing with oxygenated blood and allows efficient pumping of blood through the circulatory system.

• Milk—Female mammals produce milk to feed their offspring. Milk is a nutritious fluid that contains fats, protein, and sugars. Milk is produced by mammary glands, which are modified sweat glands located on the thorax or abdomen.

• Single jawbone—A mammal's lower jaw is made up of a single bone. By comparison, a reptile's lower jaw is made up of several bones. Mammalian fossils are often identified by the jawbone.

• Specialized teeth—Mammals have various types of teeth modified for different functions. Teeth at the front of the jaw bite, cut, or hold prey. Teeth along the sides of the jaw crush, grind, or slice. By comparison, most reptiles' teeth are uniformly sharp and conelike throughout the mouth.

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