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Crocodiles can sleep with one eye open

There are a few animals, a select group, that have the ability to sleep with one eye open. This list of semi-gazers includes dolphins and some types of bird. The way the process works with crocodiles, scientists from La Trobe University suggest, is that the animals sleep with one brain hemisphere active and the other in sleep mode. Over time the crocodile switches between brain hemispheres, so that the eye which remains open alters. In other words — half brain sleeping or “sleeping unihemispherically.”

It is hoped, through future research involving electrophysiological recordings, to confirm exactly what is happening. The scientists point out that because humans sleep by shutting down the entire brain this is not necessarily so, at all times, for crocodiles.

This wariness on the part of the crocodile remains, however, unusual behavior. Most times a crocodile will sleep with both eyes closed. The scientists found out about the occasional one-eye sleep through filming thousands of hours of slumbering reptilians. The behavior was observed when another crocodile was introduced near to a sleeping crocodile or if a human came close. The researchers noted that the opening of an eye is “consistent with a vigilance function.”

Speaking with BBC Science, lead researcher John Lesku noted: “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 new discovery has been reported to the the Journal of Experimental Biology, in a paper titled “Unihemispheric sleep in crocodilians?”

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Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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