Penguins are flightless birds that are highly adapted for the marine environment. They are excellent swimmers and can dive to great depths (emperor penguins can dive to over 500 metres). Their shape enables extreme agility underwater; the feet and tail act as a rudder while the flippers act as propellers. They also have a waterproof coat of short, overlapping feathers and a well-developed layer of fat for insulation.
Penguins feed on small fish and krill, catching these one at a time. In turn, penguins become food for other marine animals, namely leopard seals and killer whales. On land their main predators are skuas and sheathbills (carnivorous birds that take both eggs and chicks).
All penguins are restricted to the Southern Hemisphere, but the greatest concentrations are on Antarctic coasts and sub-Antarctic islands. There are 18 species of penguins, four of which live in Antarctica. Adélie and emperor penguins breed on the shores of the Antarctic continent and nearby islands while chinstraps breed on islands around Antarctica and gentoos are found on islands ranging from the Antarctic to the sub-Antarctic. Another four species (king, royal, southern rockhopper and macaroni) live on the sub-Antarctic islands.