Different species of mammals have evolved to live in nearly all terrestrial and aquatic habitats on the planet. Mammals inhabit every terrestrial biome, from deserts to tropical rainforests to polar icecaps. Many species are arboreal, spending most or all of their time in the forest canopy. One group (Chiroptera) have even evolved powered flight, which represents only the third time that this ability has evolved in vertebrates (the other two groups being Aves and extinct Pterosaurs).
Many mammals are partially aquatic, living near lakes, streams, or the coastlines of oceans (e.g., Phocidae, Otariidae, Odobenidae, Lutrinae, Ondatra, and many others). Whales and dolphins (Cetacea) are fully aquatic, and can be found in all oceans of the world, and some rivers. Whales can be found in polar, temperate, and tropical waters, both near shore and in the open ocean, and from the water's surface to depths of over 1 kilometer. (Nowak, 1991; Reichholf, 1990a; Vaughan, Ryan, and Czaplewski, 2000)