An urban legend about sleeping crocodiles has been proven true, when researchers discovered that the fearsome reptiles really do sleep with one eye wide open.
The study, published in the Journal of Experimental Biology, was conducted by Australian scientists, led by Dr. John Lesku, from La Trobe University in Melbourne.
Several juvenile saltwater crocodiles, around 40 to 50 cm long, were filmed with infrared cameras while sleeping in an aquarium, and afterwards researchers analyzed the footage.
The aim was to see how the young crocodiles would behave while asleep, and if the proximity of others would influence their habits in any way.
It was proven that crocodiles actually rest while keeping just one eye closed, especially when there are other people or crocodiles present.
“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”, explained senior author John Lesku.
It appears that the same behavior was also encountered among adult Caimans and Nile crocodiles, analyzed by German scientists.
Displaying such unusual sleeping patterns indicates that just half of the animals’ brain is actually asleep, while the other half is still alert.
This would make them similar to several other species of aquatic mammals (walruses, dolphins), reptiles and birds, which are also “unihemispheric” sleepers.
Moreover, it might imply that in fact this type of evolutionary trait may date farther back in history than previously imagined.
Probably there was a common ancestor of reptiles and birds which used half-brain sleeping, and another distant predecessor of aquatic mammals that also had this type of rest.
It may be that the habit is even more ancient, being characteristic of another older species, from which mammals, reptiles and birds all evolved.
We might actually be among the few species that sleep while shutting down the brain in its entirety, making our previously considered “normal” routine far from the usual.
Sleeping with one eye open can be an effective defense mechanism, because it keeps animals aware of potential predators, so that they can make a safe retreat or guard themselves successfully.
For example, birds display this type of behavior, being especially prone to “unihemispheric sleep” while being faced with a threatening situation.
On the other hand, marine mammals sleep with one eye open in order to ensure that they aren’t separated from their group.
Now, researchers want to prove that indeed these sleeping patterns are in close connection with brain activity.
As a result, they will carry out a follow-up study where they will use elecrophysiological analysis in order to determine what exactly happens with the reptile’s cerebral hemispheres when they are asleep.
The recordings monitoring the crocodile’s brain activity will be prepared by the Australian team of experts, in collaboration with German colleagues, who will assist them in placing electrodes on the heads of several Nile crocodiles.
The scientists’ current supposition is that indeed these animals are also unihemispheric sleepers, and that they choose this type of strategy in order to keep an eye out for predators (literally), while remaining closer to their bask of fellow reptiles.
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