Elephants were known to sleep as long as six hours a day in captivity, but Paul Manger of the University of the Witwatersrand, South Africa, and colleagues wanted to know how long they slept in the wild. They fitted two free-roaming African elephant matriarchs in Chobe National Park, Botswana, with Actiwatch devices to track their activity and gyroscopes to track their sleeping position.

They found that the elephants slept an average of two hours out of every day, usually broken into several episodes, and regularly stayed awake 48 hours straight. At the other end of the spectrum, mountain beavers can sleep more than 14 hours a day.

Why does sleep time vary with siz​e? “Larger animals need more food to keep their bodies fueled,” Manger explains. “The elephant, which can eat several hundred kilograms of low-quality food each day, devotes a lot of time to eating, leaving less time for sleep.”

Interestingly, the researchers also found that the elephants often slept standing up, lying down only every few nights. This could limit their potential for daily REM (rapid eye movement) sleep. In mammals, REM sleep occurs only when muscles are relaxed and, Manger notes, “When muscle tone is lost, it is very difficult to stand while asleep.”

This finding raises questions about the role of REM sleep, which is thought to be key to laying down memories in mammals. Other outliers for this sleep state include cetaceans, which never have REM sleep, and platypuses, which spend up to eight hours a day in REM sleep.

“Do platypuses have the best memories of all mammals, elephants a sporadic memory, and cetaceans a terrible memory?”
"Vậy thì loài rái mỏ vịt có trí nhớ tốt nhất trong các loài động vật hữu nhũ, voi có trí nhớ rời rạc, còn động vật hữu nhũ ở biển thì trí nhớ của chúng thật là tệ ư? "