Crocodiles sleep with one eye open and half their brain awake

Always watching. Always ready.
Always watching. Always ready.
Image: Reuters/Carlos Barria
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Crocodiles that sleep with one eye open have long been a legend in Australia. Now, a team of researchers from Australia’s La Trobe University and Max Planck Institute for Ornithology can finally back up local myth with hard evidence: Crocodiles sleep with half of their brain at a time, which lets them always keep one eye out—literally.

Their study, published Oct. 1 in the Journal of Experimental Biology, observed captive crocodiles in infrared light over dozens of hours.

While measuring the reptiles’ responses and alertness, researchers realized that they commonly engaged in ”bilateral eye closure,” or closing one eye for long periods while the other remained open. This indicates that crocodiles have the capacity for unihemispheric sleep—a rare biological feat that only a handful of birds and aquatic mammals had previously demonstrated.

By sleeping with one-half of the brain at a time, crocodiles can quickly respond to passing prey, or threats.

“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,” John Lesku, senior author of the study, told the BBC.

In addition to tracking threats, the animals tended to keep one eye open during sleep whenever young crocs were around. Unihemispheric sleep, it seems, makes animals stellar predators and excellent parents.