As if crocodiles weren’t terrifying enough, it now seems that you can’t even catch a break when the reptiles are napping. It’s been found that they are able to undergo what’s known as unihemispheric sleep, where only one side of their brain shuts down for sleep at a time, while the other half remains active. Not only that, but they apparently do so while keeping one eye open to watch out for potential predators… or prey.

Many legends from ancient Australian indigenous people claimed that crocodiles have this ability, and now science has been able to confirm it.

Many birds and reptiles are already known to practice unihemispheric sleep, and it's even been found in some marine mammals, such as dolphins, porpoises and sea lions. It allows the animals to catch some Z’s while simultaneously watching out for any potential threat, or in the case of the dolphins allowing them to keep swimming. But until now, the only evidence for crocodiles exhibiting this behavior was anecdotal. A new paper, published in the Journal of Experimental Biology, details how these fearsome predators stay alert, even when sleeping.

To find this out, the researchers filmed juvenile saltwater crocodiles for 24 hours, recording them dozing and observing how the animals responded to the presence of both other young crocodiles and humans during the reptiles' sleep. They found that the crocs were far more likely to sleep with one eye open when humans were around, consistent with the theory that they were doing it – at least partly – to watch out for threats. Not only that, but the sleeping crocs even followed the humans around with the open eye, and when the person then left, it remained staring at the last place the croc had seen the person.

The researchers limited themselves to younger crocodiles, mainly for safety reasons. “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,” explains Michael Kelly, lead researcher of the study, in a statement. “What we think of as 'normal' sleep may be more novel than we think.” This is because now that it’s known crocodiles practice unihemispheric sleep, along with other reptiles, birds and some marine mammals, it suddenly seems like the all-consuming slumber of land mammals is actually quite rare.

The next step is to confirm once and for all that this is indeed what the napping crocs are getting up to. To do this, the researchers plan to take electrophysiological recordings of the crocs' brain waves as they sleep, requiring the team to somehow stick electrodes on the heads of the crocodiles. Then, they will be able to track the differences in brain waves, and see if one hemisphere of the brain is awake while the other is sleeping.

Image credit: Juvenile saltwater crocodiles. Steve Lovegrove/Shutterstock

Top image in text: Dunlin birds sleeping with one eye open. Dave Inman/Flickr CC BY-NC-ND 2.0