Animals

These Animals Can Sleep with an Eye Open: Photos

Thanks to a neat evolutionary gimmick called unihemispheric sleep, some birds and aquatic mammals can remain wary even while snoozing.

This week we learned that crocodiles appear to sometimes sleep with one eye open, though that finding, neurologically speaking, needs to be confirmed with an EEG. The evolutionary trick is called unihemispheric sleep, where, essentially half an animal's brain sleeps while the other half remains awake, with the ability to keep an eye open for anything that looks like trouble. Crocs aren't the only creatures to exhibit the behavior, however. Some aquatic mammals and birds manage the feat as well. Let's take a look at a sampling of them.

Heads Up: Crocodiles Climb Trees

Bottlenose dolphins know how to shut half their brain down and keep the other at the ready while sleeping. The awake part of the brain handles breathing and surfacing.

Creatures That Live On Dolphins And Whales: Photos

The cute, stub-headed beluga whale has exhibited unihemispheric sleep.

Whales Hunting Under Northern Lights: Video

Porpoises breathe consciously and can switch off sides of their brain for sleep, which lets them stay aware of their breathing and surroundings, as well as any trouble that may be lurking.

Porpoises Turn Sound Beams Into 'Flashlights' To Hunt

The South American sea lion knows a thing or two about sleeping with half a brain.

VIDEO: Sea Lions Can Keep A Beat

The second-smallest of manatees, the amazonian variety, is another aquatic mammal that employs unihemispheric sleep. Not that the manatee in this picture feels like sleeping!

Make Way For Manatees Month: Photos

Even birds of prey, like this peregrine falcon, need to keep a watchful, half-asleep eye on things.

Enormous Wingspans Carry These Birds Aloft: Photos

If it's in an area where predators may be near, the common blackbird can still get some rest while remaining alert.

Birds' Nests In Bizarre Urban Places: Photos

The common blackbird has nothing on the white-crowned sparrow in that department. These ground foragers are ever vigilant.

World's Oldest Known Bird: Photos

Here's one more in our sampling of animals that can sleep with half a brain at the ready: the humble mallard, or wild duck.

Funky Ducks Thrive At NYC's Central Park: Photos