New York Times
, October 23, 2007Link
Last month, a bird known as a bar-tailed godwit took flight from Alaska and headed south. A day later, it was still flapping its way over the Pacific. An airplane pilot would have a hard time staying awake after 24 hours of flight (the Federal Aviation Administration allows pilots to fly just eight hours in a row). But the godwit kept flying for an additional week. After eight days and 7,200 miles, it landed in New Zealand, setting a record for nonstop flight.
“If they spend so many hours flying,” said Ruth M. Benca of the University of Wisconsin, “where do they find the time to sleep?”
Bird sleep is so mysterious that scientists are considering several answers, all intriguing. The godwit may have managed to stay awake for the entire journey. Or it may have been able to sleep while flying. Or, as Dr. Benca and other scientists suspect, its brain may have been in a bizarre state of semilimbo that they do not understand.
Bird brains produce patterns of electrical activity that look strikingly like human brains during sleep, a remarkable similarity considering that birds and their brains have been on a separate evolutionary course from mammals for 300 million years. But similarities reach just so far.
The amount of sleep birds need can change drastically through the year. Birds may be able to put parts of their brains to sleep while keeping others awake. They may be able to adjust sleep in the course of minutes, even seconds. By figuring out the mysteries of bird sleep, scientists hope to understand some universal rules of sleep.
Like humans, birds typically get some sleep every day. A pigeon usually sleeps through the night, for example, and has a few naps during the day. Why birds and mammals should sleep so much has long puzzled scientists. Some researchers have even argued that sleep is something that animals do when they have nothing else on their agendas.
Many sleep experts disagree. Something about sleep is essential to human well-being. It is possible that certain types of sleep are particularly important. In the course of a night’s sleep, humans pass through distinct stages. In one stage, the eyes move rapidly behind closed lids while the brain produces electrical signals with a pattern much like that of a waking brain. It is during this so-called REM sleep that people experience dreams.
In other parts of sleep, however, many neurons produce electric signals with a nearly identical rhythm. The neurons also fire more slowly than in REM sleep, from 40 to 400 times a second. This dream-free sleep is so deep that it is hard to rouse people from it.
Several experiments suggest that slow-wave sleep, in particular, has a crucial role in human well-being. As neurons fire in synchrony, their connections change, consolidating the memories formed in the previous day. One sign of the importance of slow-wave sleep is that if people do not have enough of it, they catch up when they can, producing stronger waves.
“If you pull an all-nighter,” Dr. Benca said, “the next night your slow waves will be much larger.”
Other mammals experience REM sleep and slow-wave sleep, as well, indicating that humanlike sleep patterns existed early in the history of mammals. But beyond mammals, scientists have had a hard time finding humanlike sleep patterns. So far, they have been seen just in birds. The fact that the closest relatives of birds, like alligators and turtles, do not have our kind of REM sleep and slow-wave sleep suggests that birds, or their dinosaur ancestors, evolved humanlike sleep independently.
This parallel evolution has given scientists the opportunity to test the hypothesis that slow-wave sleep is essential. “If slow-wave sleep is a fundamental building block of sleep, then it should be true in birds as well as in mammals,” Dr. Benca said.
Niels Rattenborg of the Max Planck Institute of Ornithology in Germany tested this hypothesis by depriving pigeons of some slow-wave sleep. “We kept pigeons from taking their daytime naps,” he said. “All we did was tap their cage or move the cage floor or give them things to play with for eight hours before we turned the lights off.”
After the lights went dark, the pigeons had slow waves 27 percent stronger than on undisturbed nights. “What we found was that they actually showed response very much like that observed in mammals,” Dr. Rattenborg said. “There’s something in common in being a bird and being a mammal that results in sleeping this way.”
Dr. Rattenborg contends that birds and mammals have similar kinds of sleep because birds and mammals have much larger and more complex brains for their size than other vertebrates. In mammals, much of that expansion occurred in the front of the brain, in the neocortex. The neocortex endows mammals with sophisticated, flexible learning and decision making.
Only in recent years have scientists realized that birds have a brain region similar to the mammal neocortex. Known as the pallium, it arises from the same population of embryonic cells that produces the neocortex in mammals.
The pallium is made up of clumps of neurons, while the neocortex is organized in layers. Despite the differences, the pallium also lets birds carry out many impressive mental tasks. Some birds can remember thousands of locations where they hide food. Others fashion tools like sticks, to obtain food. Others can learn many bird songs. Pigeons can learn how to distinguish between Cubist and Impressionist paintings.
Dr. Rattenborg proposes that big, powerful brains need the same kind of slow-wave sleep to work properly, whether those brains are in birds or mammals.
“If we didn’t have birds,” he said, “people might say, ‘Well a neocortex is absolutely necessary.’ But here we have birds doing the same thing. So clearly, it’s not having the neocortex that’s essential.”
Although the parallels between sleep in birds and humans is striking, they extend just so far. A bout of slow-wave sleep in a human may last for hours. In birds, a normal period may last a few minutes, even a few seconds. “You and I can’t sleep in 10-second bouts,” Dr. Benca said.
Dr. Rattenborg has found that birds can also keep one side of their brain awake while the other sleeps. He suspects that the awake half can keep a lookout for predators while the other half sleeps.
Dr. Benca suspects that birds may be able to make smaller parts of their brains go to sleep or wake up.
“Maybe,” she said, “we need to get away from thinking of sleep as something you have to do for so many minutes, and if the whole brain isn’t doing something that looks like sleep, then sleep isn’t happening. I think their brains are doing something else.”
Part of Dr. Benca’s hunch comes from her difficulty in keeping birds awake. Working with Dr. Rattenborg and other colleagues, she tried to deprive pigeons of sleep. The researchers put pigeons on a circular platform over a tank of water. When the pigeons produced slow waves for four seconds or more, the platform began to turn slowly, so that they had to walk.
In humans and other mammals, sleep deprivation eventually causes weight loss, hunger and other symptoms. It can even lead to serious illnesses. But pigeons showed none of those changes, as Dr. Benca and her colleagues will report in a paper to be published in Physiology and Behavior.
Birds have apparently evolved an ability that many humans would envy.
“We could deprive the pigeons for weeks,” Dr. Benca said, “and they seemed to be doing fine.”
Copyright 2007 The New York Times. Reprinted with permission