Unlike land mammals, dolphins sleep with only part of their brains at any time, past research has suggested. Half of their brains rest, while the other half remains "awake," and dolphins regularly switch which side is active. These findings suggest how dolphins can keep on the constant lookout for sharks, investigators added.
"After being awake for many hours or days, humans and other animals are forced to stop all activity and sleep," said researcher Brian Branstetter, a marine biologist at the National Marine Mammal Foundation in San Diego. "Dolphins do not have this restriction, and if they did, they would probably drown or become easy prey."
To see just how mindful dolphins are with just half a brain, researchers tested their ability to scan the environment. Dolphins use echolocation to map the world, a biological form of sonar where they emit clicks and listen for their echoes to probe murky, dark surroundings. Sleep Tight! Snoozing Animals Gallery.
The researchers set up a portable floating pen outfitted with eight modules, each consisting of an underwater sound projector and microphone. When a dolphin scanned any of these modules using echolocation clicks, they could respond with sounds mimicking echoes of those clicks from remote surfaces. Essentially, these modules could behave as "phantom targets" -- illusions that acoustically simulated physical objects.