Rodents are famous for their teeth. The large, paired cutting teeth (incisors) in the front of their mouth give them a buck-toothed appearance. The incisors are self-sharpening, thanks to the way they grind together during gnawing. While a strong enamel coating prevents wear on the front, the back of each incisor is soft and wears down. The outcome is a chisel-like shape that can cut into wood, food, or predators. The incisors grow continuously, creating tooth material for sharpening throughout the life of a rodent. A rodent skull is shaped to support the muscles used in gnawing, such as an enlarged region (the pterygoid) that anchors the lower jaw muscles. A skull modification allows the lower jaw to move not only up and down, but back and forth for better grinding action. Rodents are the largest group of mammals on Earth, and their ability to gnaw is part of the reason they have been so successful.

Mammals are able to make enough internal body heat to keep their bodies at a relatively constant, high temperature. Endothermy (endo= inside; thermic= heat) makes mammals less dependent on outside temperatures, freeing them to be active in a wide range of conditions. Mammals may be active at night (nocturnal) or in the day (diurnal), in climates as extreme as the cold poles or the hot tropics. Adaptations to warm up or cool off, such as shivering and sweating, allow mammals to maintain their preferred body temperatures in the face of these extremes. Mammals also regulate body temperature with behavior: elephants flapping ears to cool off, humans wearing coats to warm up, dogs panting to cool down. The hairs making up mammal fur are raised or lowered to provide more or less insulation as needed.