Sleep, for animals, can be a dicey proposition. While rest and renewal are necessary, snoozing can leave a creature vulnerable to threats. Some birds and aquatic mammals have adapted the ability to keep half their brain on alert while the other half slumbers, a phenomenon known as unihemispheric sleep -- in essence, sleeping with one eye open.
A new study from researchers out of Australia's La Trobe University and Germany's Max Planck Institute for Ornithology shows that crocodiles may well be taking a page from that book.
The researchers studied juvenile crocodiles (adults would be a bit too dangerous to work with) in a special holding aquarium, with cameras trained on the reptiles for 24 hours at a time.
They tested for use of vigilant, one-eyed sleeping behavior by sending, in turn, another crocodile and a human into the holding area.
Most of the time, particularly at night, the crocodiles slept with both eyes closed. But in both cases -- human or crocodile interloper -- the young test crocs indeed opened one eye and trained it on their new neighbor, appearing otherwise to slumber.
Dolphins, some seals, manatees, and walruses are among those animals that have evolved unihemispheric sleep, in which half the brain, using an electroencephalogram (EEG) test, appears to be sleeping, while the other half shows brain wave activity that looks more like an animal that's awake. The eye corresponding to the awake half can remain open. Some birds species do it too.
But crocodiles have never been studied closely for the behavior.
"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 the study's lead researcher Michael Kelly, of La Trobe University, in a press release.
The scientists still need to confirm with an EEG that true unihemispheric sleep is happening in the crocodiles. If that proves to be the case, the scientists say, then our human method of total-brain sleep might begin to look a whole lot less, well, normal.
"If ultimately crocodilians and other reptiles that have been observed with only one eye closed are likewise sleeping unihemispherically," said study co-author John Lesku, "then our whole-brain (or bihemispheric) sleep becomes the evolutionary oddity."
Findings about the sleepy crocodiles have just been published in The Journal of Experimental Biology.