Penguins are birds in the family Spheniscidae in the monotypic order Sphenisciformes. They inhabit high-productivity marine habitats, almost exclusively in the southern hemisphere; the only species to occur north of the equator is the Galapagos penguin. The only group of birds other than the ratites to be entirely flightless, penguins are extremely adapted to their aquatic lifestyle, with a streamlined shape that minimizes drag, countershaded dark-and-white plumage, dense bones, powerful flippers, and insulating feathers that allow them to withstand very low temperatures on land and in water.