Crocodiles have the ability to sleep with one eye open, scientists have discovered.
Researchers found the reptiles can use "unilateral eye closure", allowing them to keep a lookout for potential threats or prey while dozing.
Like many birds and some aquatic animals such as dolphins and walruses, the study suggested crocodiles are able to nap with one half of the brain at a time, known as unihemispheric sleep, leaving the other half alert.
The research findings were published in the Journal of Experimental Biology.
The scientists studied juvenile crocodiles round the clock using an aquarium lined with infrared cameras.
They found the crocodiles were more likely to leave one eye open when a person was present.
Dr John Lesku, who led the study, said: "They definitely monitored the human when they were in the room.
"But even after the human left the room, the animal still kept its open eye directed towards the location where the human had been - suggesting that they were keeping an eye out for potential threats."
The researchers then introduced a new young crocodile to the group and the reptiles again were found to open one eye and keep watch.
This could be because juvenile crocodiles can be vulnerable to predators and so group together to reduce any threat.
While marine animals use this ability to make sure they stay together in a group, birds use the technique to watch out for predators.
"It seems to be a bit of both, in the case of these juvenile saltwater crocodiles," said Dr Lesku of La Trobe University in Melbourne.