The South Polar or Antarctic skua, Stercorarius maccormicki, is an aggressive, heavily built, brownish gull about the size of a farmyard hen. Skuas roam the world's oceans but are found mainly around the islands and coasts of Antarctica during the breeding season.
Skuas are strongly territorial and mate for life. Pairs return to breed in the area of their birth, often close to a penguin or petrel colony. Two eggs are laid in a scoop on the ground. Chick mortality is high (>70% in one 8-year study), due largely to the first-hatched chick driving its younger sibling from the nest. Despite this, adult survival is high, with many living for more than 20 years.
Skuas feed mainly on surface fish, but will take full advantage of the food available at penguin colonies, scavenging dead animals and eating the krill spilt at chick-feeding time. Skuas are clever predators of penguin eggs and chicks; sometimes a pair will work together to distract a brooding Adélie so that an egg or chick can be stolen. Unguarded, lost or sickly chicks are quickly identified by observant skuas. Harried away from the protection of the colony these chicks are finally killed and eaten.
Leopard seals, Hydrurga leptonyx, are largely solitary animals, with sinuous, 3-m-long bodies and powerful heads. They are generally found throughout the Southern Ocean in the pack ice areas of Antarctica, although younger individuals may wander further afield. Weighing up to 350 kg, leopard seals are fierce predators of fish, squid, penguins, and the pups of other species of seal. Their lobed molar teeth are also adapted for straining krill from the water.
A favoured food is Adélie penguins, which are gripped by the feet and belly, and vigorously beaten against the water to remove the skin. They are then dismembered and eaten, with the skin and feathers being discarded. Leopard seals are often to be found patrolling the shores next to penguin colonies to catch incoming or outgoing birds. Queues of wary, agitated penguins are often reluctant to enter the water until they know the coast is clear. Leopard seals have been known to rise swiftly towards thin ice, fracturing it and knocking penguins into the water.
Killer whales or orcas are widespread, black and white, toothed whales with relatively large dorsal fins. Up to 7 m long, they can weigh as much as 8 tonnes. Found either singly or in groups of up to 30 individuals, orcas are common in the Southern Ocean where the population may be as high as 200 000.
Unlike most toothed whales, such as sperm whales and dolphins, which feed on fish and squid, orcas prefer warm-blooded prey like penguins, seals and other smaller cetaceans. Orcas have been seen attacking fin and minke whales larger than themselves, tearing mouthfuls of flesh, especially the lips and tongue, from their living bodies. Recent research in Antarctica points to the possibility that discrete pods of orca that specialise in hunting one particular prey type may in fact be different species.