Penguins are flightless birds which have adapted to living in the cooler waters of the Southern Hemisphere. The 17 species of penguins found today are thought to have evolved from petrel-like flying birds some 50 million years ago. Some species spend as much as 75% of their lives in the ocean, yet they all breed on land or sea-ice attached to the land.
All penguins have a very similar torpedo-shaped body form, though they vary greatly in size. Penguin wings are highly modified to form stiff paddle-like flippers used for swimming, and their feet and stubby tails combine to form a rudder. The penguin's bones are solid and heavy, which help them to remain submerged and reduce the energy needed for diving.
Penguins are able to withstand the extreme cold because insulation provided by their short, densely-packed feathers forms a waterproof coat. A thick layer of fat or blubber also serves as an energy store. These adaptations, among others, enable them to minimize heat loss in icy cold waters so they can cope with the harsh conditions of the Antarctic.