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Australian scientists said on Thursday that they have found crocodiles sleep with one eye open to monitor their surroundings even though they're still asleep.
The researchers from Melbourne's La Trobe University, with the help of the German colleagues, captured three saltwater crocodiles to conduct the tests, and the results were published in the Journal of Experimental Biology on Thursday.
The prehistoric reptiles were placed in the university's large aquarium and filmed round the clock. Researchers found the crocodiles opened one eye when they provided artificial stimulus to the tank.
"We had a human stand still next to the tank for 10 minutes - the animals responded to that quite strongly and watched the person closely with one eye," La Trobe University's John Lesku, who led the study, told the Guardian Australia on Thursday.
"After the human left the room they kept looking at the last location of the person with the other eye closed. This suggests that crocodiles have the same kinds of brain activity that birds do."
Other animals - such as birds, dolphins, walruses, seals and reptiles - have already been found to possess what biologists call "unihemispheric sleep," which allows the animal to shut down half of its brain while the other portion stays awake.
Lesku said the researchers then introduced another form of stimulus - a young crocodile - into the tank, and discovered the older crocodiles watched over it while dozing.
The Australian researchers' colleagues at the Max Planck Institute for Ornithology in Germany have found the behavior is shared among Nile and Caiman crocodile species.
"This suggests that this is a crocodilian trait not specific to one species," Lesku said.
"It persists into adulthood, meaning that an ambush predator could be immobile yet still looking out for prey. If an animal went to the bank of a river, the crocodile could fully wake up and attack it."
Lesku said that regardless of the new findings people should always be wary of the carnivorous creatures.
"I would certainly never approach a crocodile whether its eyes are open or closed. A good rule is to never approach them," he said.
