There are two natural groups of whales, toothed and baleen. The toothed whales include the large sperm whales together with various smaller dolphins and porpoises, which prey on fish and squid, and killer whales which prey on fish, penguins, seals and whales. The baleen whales, such as the blue whale and humpback, have fibrous plates of baleen instead of teeth, which they use to strain plankton from seawater. The Leopard seal is also a predator of penguins and other seals. Thick layers of blubber beneath the skin of both whales and seals, act as a food reserve and insulation.
Most seals also have a layer of fur, giving additional insulation on land.
Seals can leave the water and move on terrain to breed, rest and moult. Of the six Antarctic species, four are ice habitat specialists, breeding on the sea ice in spring. Leopard and Ross seals tend to be solitary, whereas Weddell and Crabeater seals form loose breeding aggregations. Antarctic fur seals and elephant seals are both found north of the pack-ice zone and breed in dense colonies on beaches. In constantly defending these, bulls will not forage at sea, relying instead on blubber reserves laid down in the previous winter.