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 named for their special milk-producing structures called mammary glands. A female mammal may have two (like humans) or as many as 18 mammary glands. Each gland is full of small spaces lined with milk-secreting cells that all drain into a nipple. Newborn mammals have an innate sucking reflex that stimulates the mother's mammary glands to release milk. Milk is a superfood for young mammals, containing a nourishing mix of fats, proteins, vitamins, minerals, and even immune-system chemicals. The first milk (colostrum) contains antibodies from the mother to protect against disease. As the offspring grows, milk changes to maintain the right balance of nutrients. Production of milk (lactation) makes most female mammals temporarily unable to get pregnant, resulting in natural gaps between offspring. The spacing of births benefits both the offspring and the mother in terms of survival. Mother mammals stop nursing (wean) their babies after several days or years, depending on the species and environmental conditions.