Crocodiles May Be Watching You While They Sleep

Researchers say crocodiles do indeed sleep with one eye open
By Michael Harthorne,  Newser Staff
Posted Oct 22, 2015 5:00 PM CDT
Crocodiles May Be Watching You While They Sleep
Researchers say juvenile crocodiles sleep with one eye open to keep a close watch on predators, prey, and humans.   (AP Photo/Desmond Boylan)

Bad news for Captain Hook: Crocodiles may be keeping an eye on humans even when the crocs are sleeping, according to new research published this week in the Journal of Experimental Biology. The BBC reports researchers in Australia monitored juvenile crocodiles using infrared cameras and determined they often slept with one eye open and may only sleep with half their brain at a time. According to, this type of unihemispheric sleep has been observed in birds and aquatic mammals, such as the dolphin. "These findings are really exciting as they are the first of their kind involving crocodilians and may change the way we consider the evolution of sleep," lead researcher Michael Kelly says in a press release. "What we think of as 'normal' sleep may be more novel than we think."

By sleeping with one eye open and half their brain active, crocodiles could respond quickly to threats and prey—including humans, reports. "They definitely monitored the human when they were in the room," researcher John Lesku tells the BBC. "But even after the human left the room, the animal still kept its open eye … directed towards the location where the human had been." Researchers are planning more tests to confirm their findings in crocodiles and other reptiles; specifically, more research is needed to determine whether one half of the crocodilian brains is actually awake as the other half sleeps. Depending on what they find, Lesku says our version of whole-brain sleep—which we think of as normal—could actually be an "evolutionary oddity." (A "tipsy" Australian hunter successfully fought off a very awake crocodile.)

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