Bird-Brains Stay Alert While Ducks Sleep
Camping on the bank of the Yukon River, I once saw three ducks floating downstream. Drifting with the current of the big river and spinning in circles when they hit an eddy, they looked like wooden decoys. When a rock falling into the water scared them, the ducks started swimming, then flew off.
I realized then the ducks had been napping as they bobbed down the river, but it's a good bet they weren't sleeping too soundly. Researchers have found that ducks and other birds sometimes sleep with one eye open. Niels Rattenborg, a sleep researcher at Indiana State University in Terre Haute, recently did a study in which he and coworkers filmed a row of mallards sleeping. The birds on the both ends of the row--those that would be most vulnerable to predators--tended to keep their exposed eyes open while they slept. Mallards with ducks on both sides of them kept both eyes shut or didn't have a preference for which eye they kept open.
While sleeping with one eye open, one hemisphere of the mallard's brain is awake while the other is sleeping, Rattenborg said. The awake half allowed birds to keep an eye open for predators. Birds that sleep at the end of the line engage in this single-hemisphere sleep more often than ducks positioned in the middle. In Rattenborg's study, birds stationed at ends of a line kept their outside eyes open 86 percent of the time.
Rattenborg also has seen this behavior in other birds. He noticed penguins sleeping side-by-side in a zoo with their exposed eyes open; he once saw a cockatiel sleeping next to a mirror with the eye away from the mirror open, as if its reflection were another cockatiel on its safe side.
Birds aren't the only animals known to keep one half of the brain active while sleeping. Sea mammals, such as dolphins, whales, seals and manatees, also engage in half sleep, presumably to allow them to surface for air while sleeping.
Why do animals need to sleep, anyway? That basic question still eludes researchers. Rats deprived of sleep for two weeks will die, Rattenborg said, and humans don't perform well when we don't get enough sleep. He thinks sleep does something for the brain, citing three hypotheses: sleep may allow the brain to clear itself of damaging chemicals that build up when we're awake; the brain may "rewire" important neurological pathways while we sleep; and energy stores within the brain that are depleted when we're awake may be recharged when we sleep.
Ducks and other birds that spend their nights with only half their brain sleeping probably aren't getting the sleep benefits of birds in the middle of the flock, Rattenborg said. He suspects that wild birds shuffle their positions so that the vigilant ones that have slept with one eye open eventually push themselves into the middle in order to get more complete rest.
Rattenborg's study on ducks has a few parallels to human sleep dysfunctions. Sleepwalking may be caused because a part of the brain is waking during the night. Fatigue may be caused by sections of the brain falling asleep before the body does. Rattenborg is now working with pigeons to search for more insights into the mystery of sleep.
Maybe the pigeons will help get his ducks in a row.