King penguins can distinguish between dangerous and benign sounds, even when they are asleep, new research says. King penguins are the second largest species, after emperor penguins. They are 70 to 100 cm tall and weigh 11 to 16 kg. 

King penguins who become exhausted after long diving sessions and sleep on the beach constantly keep an ear out for incoming threats from large predators like orcas and giant petrels. 

For the study, the team played different sounds to the sleeping penguins to observe their reactions.

"When we played single tones to sleeping penguins, they woke up with little reaction. However, playing them the calls of orcas or skuas caused them to wake up and flee," said Tessa Abigail van Walsum, doctoral student at University of Roehampton, in Britain.

The ability of these birds to respond differently upon waking up suggests that they might sleep with just one half of their brain, while keeping a close watch with the other half similar to some migratory birds - essentially "keeping an eye open," said the paper appearing in the Society For Experimental Biology.

Penguins also had strong reactions to some non-predator sounds like those of elephant seals. 

"The sounds of approaching elephant seals rang big alarm bells for the penguins,” van Walsum added.

On the other hand, playing them the sound of unfamiliar predators, such as a dog's growl, got little reaction when they awoke, the researchers said.

“This research helps us to understand the survival strategies of king penguins in their natural habitats,” explained van Walsum.