Average snooze of two hours per night deepens mystery of sleep’s role
Fitbit-style tracking of two wild African elephants suggests their species could break sleep records for mammals. The elephants get by just fine on about two hours of sleep a day. Much of that shut-eye comes while standing up — the animals sleep lying down only once every three or four days, new data show.
Most of what scientists previously knew about sleeping elephants came from captive animals, says neuroethologist Paul Manger of the University of the Witwatersrand, Johannesburg. In zoos and enclosures, elephants have been recorded snoozing about three hours to almost seven over a 24-hour period.
Monitoring African elephants in the wild, however, so far reveals more extreme behavior. Data are hard to collect, but two females wearing activity recorders for about a month averaged less sleep than other recorded mammals. Especially intriguing is the elephants’ ability to skip a night’s sleep without needing extra naps later, Manger and colleagues report March 1 in PLOS ONE.
“The remarkably short amount of sleep in wild elephants is a real elephant in the room for several theories for the function of sleep,” says Niels Rattenborg of the Max Planck Institute for Ornithology in Seewiesen, Germany. Ideas that sleep restores or resets aspects of the brain for peak performance can’t explain animals that sleep only a little and don’t need catch-up rest, says Rattenborg, who wasn’t involved in the elephant study. The results also don’t fit well with the thought that animals need sleep to consolidate memories. “Elephants are usually not considered to be forgetful animals,” he says.
Before the latest elephant sleep stats, horses were the record-holders among mammals for the shortest sleep requirement at 2 hours, 53 minutes, Manger says. Donkeys weren’t far behind at 3 hours, 20 minutes. Game rangers familiar with wild African elephants, however, claimed the pachyderms virtually never slept.
To investigate, Manger and colleagues implanted activity monitors in the trunks of the matriarchs of two herds in the Chobe National Park in northern Botswana. An implant about the size of a Fitbit activity tracker shouldn’t bother an elephant trunk, Manger says, because it’s “250 pounds of muscle.” Trunks, like human hands, are important for exploring and manipulating the world, so they’re rarely still for long. The researchers assumed that a trunk monitor motionless for at least five minutes probably meant the animal was asleep. Gyroscopes in neck collars helped researchers figure out whether animals were standing up or lying down.
The trunk implants caught occasions when the matriarchs went as long as 46 hours without any form of sleep. A predator, poacher or a bull elephant loose in the neighborhood might explain the restlessness, Manger says. Animals in captivity don’t face the same dangers.
African elephants’ low sleep requirements join a growing body of results showing that wild animals don’t need as much sleep as studies of captive animals suggest, Rattenborg says. His monitoring of wild sloths has revealed they aren’t as slothful as captives. And other work finds that great frigate birds and pectoral sandpipers still perform well on less than two hours of sleep a day.
Still, it’s unclear how these findings for two females will translate to entire elephant populations. But the results do fit a trend that links larger species with shorter sleep and smaller species with longer sleep, Manger says. Some bats, for example, routinely sleep 18 hours a day. He and colleagues are toying with the idea that sleep duration might be related to a daily time budget. Bigger animals may sleep less as they need that time for tasks to sustain their size. Building and maintaining an elephant body, for example, may take more feeding time than maintaining a little bat body, Manger says.
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