The majority of the 17 species of penguins do not live in Antarctica but rather between latitudes 45° and 60° S, where they breed on islands. A few penguins inhabit temperate regions, and one, the Galapagos penguin lives at the Equator. Evidence from paleontology and genomic studies indicates that penguins and Procellariformes (albatrosses, shearwaters, and petrels) had a common origin. Both groups are represented by well-defined fossils dating to about 50 million years ago.

The penguins are highly specialized for their flightless aquatic existence. Their forelimb is transformed into a paddle. The feet are located much farther back than those of other birds, with the result that the bird carries itself mostly upright. The sole comprises the whole foot instead of just the toes, as in other birds.

Insulation of the bird's body is particularly important for Antarctic species that live in water that is always below 0 °C (32 °F). With a wind of 110 km (70 miles) per hour, the cooling power of seawater at −1.9 °C (28.6 °F) is equal to that of a temperature of −20 °C (−4 °F) . The skin is insulated by a layer of air trapped under the plumage, and the only bare skin in direct contact with the water is that of the feet. There is a remarkable anatomical arrangements in the lower limb, whereby closely adjacent arteries and veins form a system of heat exchange between opposing flows of blood.

penguin. (2011). Encyclopædia Britannica Ultimate Reference Suite.  Chicago: Encyclopædia Britannica.