In the waters surrounding the Antarctic continent, sea leopards (Hydrurga leptonyx) are the main predators of penguins. They often wait near the coast. When a penguin passes, a sea leopard opens his mouth immediately and grabs the penguin with its sharp teeth. Also some whales (for example killer whales, Orcinus orca) sometimes eat penguins when they open their mouths and a penguin gets into it accidentally.
On land, skuas (genus Stercorarius) and southern giant petrels (Macronectes giganteus) are predators of penguins. They prefer penguin chicks that are not guarded by their parents or ill and weak adult penguins. Some skuas and southern giant petrels also feed on carcasses of dead penguins and seals. This is advantageous because the spread of diseases due to rotten flesh is prevented.
Southern giant petrels are also able to catch penguins when they raise their head out of the water. Especially younger and unexperienced penguins are easy prey in this situation.
Formerly, humans threatened penguins severely. Sailors ate penguins and their eggs. Besides, the fat of penguins was used as fuel for little ships. But sailors preferred sea mammals actually because penguin fat was not efficient enough. Later, killing of penguins and sea mammals was forbidden by law.
In habitats were penguins and humans live close to each other, pets and introduced wild animals have a negative effect on penguins. For example, cats, dogs and foxes spoil penguin nests or destroy them.