Elephants are the only proboscids on Earth today, relics of a group that used to have dozens of species. While the earliest proboscids were large animals (about the size of ponies), over time proboscids evolved to be huge. Not only bigger bodies, but also the evolution of long trunks and tusks added to their weight. The biggest proboscid, now extinct, was Mammuthus trogontherii, weighing in at about 9000 kilograms (about 20,000 pounds). Compare that to a modern, male African elephant at a modest 6000 kilograms. To maintain their weight, proboscid skeletons have sturdy limb bones, and feet whose toes are spread and supported by connective tissue to form a strong base. An elephant must eat several hundred kilograms of food every day to get enough calories. Their diet consists entirely of plants, which get processed by massive, ridged molars and a long intestinal system.

All mammals have hair, outgrowths of the skin, during at least some part of their development. While it evolved to insulate animals in cold climates, mammal hair has various functions. Long, stiff hairs (vibrissae, or whiskers) are found in all mammals but humans. Whether on the face, legs, or tail, vibrissae extend an animal's sense of touch. Some mammals, such as porcupines, have enlarged hairs that function as defensive spines. Defense is also achieved by raising individual hairs to make an animal look larger (such as an angry cat). Muscles in the skin make the hair stand up or lie down. Raising and lowering hair changes how much air is trapped under it, which also makes it more or less insulating. Mammals in cold climates tend to have an extra layer of insulating fur, or extra fat under their fur. Mammals in climates that change seasonally shed and replace their hair to create a thicker winter coat and a thinner summer coat.