Above ground, grasslands are one of the most productive habitats for mammal life. The most successful mammals in this environment are ruminants—hoofed species such as buffaloes and antelope—which have a highly specialized digestive system that has evolved to break down cellulose, a tough substance that forms the walls of plant cells. In a landscape that offers few places to hide, many of these grazing mammals protect themselves by forming large herds, and use speed to escape their enemies. In some grassland areas, such as the plains of East Africa, herds of grazers carry out yearly migrations, arriving at fresh grazing areas just after rain has triggered new growth.

Unlike grassland mammals, those that live in forests are well concealed, rarely band together for safety, and are mostly nocturnal, or active at night. Arboreal species, ones that spend most of their lives in trees, include sloths, most primates, squirrels and their relatives, and a number of marsupials including opossums and tree kangaroos. Squirrels use their tails to balance as they scamper along branches, but some arboreal mammals have prehensile tails that can wrap around branches. In many South American monkeys, these tails are so strong that they can support the animal's entire weight. Forest mammals also include climbing carnivores such as martens, and species that spend most or all of their lives on the ground. In temperate regions (areas with cold winters and warm summers) the largest of these ground-based forest dwellers are bears, deer, and wild pigs, but in tropical regions they also include elephants, okapis, and tapirs.

In mountains, tundra, and deserts, mammals have to overcome hostile conditions if they are to survive. Rodents have successfully colonized all three of these habitats, because their small size enables them to avoid extreme conditions by hiding away underground or in burrows beneath the snow. Larger mammals do not have this option. Instead, they cope with cold with long fur and a layer of insulating body fat. Some mammals, such as ground squirrels, survive cold winters when food is scarce by entering a sleeplike dormant state called hibernation and awaken when food is more abundant. Larger mammals survive desert heat with a number of adaptations, including sweat glands that produce perspiration to cool the body. To combat arid conditions, many desert mammals have a sophisticated kidney function that produces a concentrated urine, so that less water is removed from the body. At high altitudes, mammals face the additional problem of shortage of oxygen. In the Andes mountains of South America, guanacos, llamas, and alpacas have successfully overcome this problem by having more oxygen-carrying red blood cells than most mammals, and by having a special form of hemoglobin, the oxygen-carrying pigment in blood, which binds oxygen at very low pressures. This adaptation allows them to run effortlessly at altitudes of up to 4900 m (16,000 ft).

Some mammals, such as otters and river dolphins, have adapted to life in freshwater habitats, but the great majority of the world's aquatic mammals live in the ocean. Seals remain close to coasts or to floating ice, but whales and dolphins are truly pelagic, meaning that they wander far out into open water. Most of these marine mammals live in areas where food is abundant, but where water temperatures are low. They survive the cold in two different ways. Some, such as sea otters and fur seals have a double coat of fur, with extremely dense underfur hairs that are so closely packed that the skin never gets wet. By contrast, whales and dolphins have very sparse hair, and keep warm with a thick layer of fat called blubber.

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