Crocodiles can apparently keep an eye open while asleep to guard their pack and watch for predators and potential prey.
A new study, published in The Journal of Experimental Biology, has revealed that crocodiles have a ‘to-die-for’ talent. They have the ability to switch to a state called unihemispheric sleep whereby half of the brain is turned off while the other stays in an alert state to spot physical changes in their surroundings. This is the first time it has been discovered that crocodiles leave one eye open when they display this type of behaviour.
“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,” said lead researcher Michael Kelly from La Trobe University in Melbourne. “What we think of as ‘normal’ sleep may be more novel than we think.”
The scientists observed this behaviour when they filmed juvenile crocodiles living in an enclosure. For most of the time the animals spent sleeping, they had both eyes closed. However, they would open an eye while sleeping in cases when a new crocodile would be placed in their enclosure; they were also cautious of the human researchers themselves.
“They definitely monitored the human when they were in the room,” said one of the researchers, John Lesku, in a statement to 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 – suggesting that they were keeping an eye out for potential threats.”
Apparently, based on other findings, sleeping with both eyes closed might actually be an odd behaviour in the sense that it evolved later.
“We tend to think of our sleep as the norm: a behavioural shut-down that is a whole-brain affair,” said Lesku. “And yet if birds sleep unihemispherically, and if crocodiles and other reptiles that engage in unilateral eye closure – if it turns out that they are also sleeping unihemispherically, then suddenly our sleep becomes unusual.”