Whether paranoid or realistic about the threats surrounding them, crocodiles and other animals are able to sleep with one eye open.
Published in the Journal of Experimental Biology, the new study detailed how crocodiles and various other aquatic and avian animals manager this, and whether it should be considered strange or not.
"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," study lead researcher Michael Kelly, of La Trobe University (LTU) in Melbourne, Australia, said in a press release. "What we think of as 'normal' sleep may be more novel than we think."
Unilateral eye closure (UEC) occurs during unihemispheric sleep, which is how a being would sleep with one eye open. One half of the brain stays awake and the eye that corresponds to this side remains open and active.
"It is thought that [UEC] reflects a way of maintaining group cohesiveness in a highly social animal. It could also be that, in a fairly boring aquarium, they simply keep their open eye on the most interesting thing - each other," John Lesku, a research fellow at LTU, told Live Science.
The researchers observed juvenile saltwater crocodiles in 24-hour periods and measured how often they either had both eyes open, both eyes closed, or just one closed. They concluded that animals generally prefer both their eyes being open or closed.
Discovery News has an image gallery detailing other animals with this ability.
"The value of the research is that we think of our own sleep as 'normal' - a behavioral shutdown that is a whole-brain affair," Lesku said in the release. "And yet, some birds and aquatic mammals sleep unihemispherically with one eye open. If ultimately crocodilians and other reptiles that have been observed with only one eye closed are likewise sleeping unihemispherically then our whole-brain (or bihemispheric) sleep becomes the evolutionary oddity."
