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.

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.