Animal behaviorists have always known elephants sleep very little but a new study which tracked dozing wild African savannah elephants showed they stay awake for more time than any other animal. The elephants tracked by researchers at University of Witswatersrand, South Africa, slept on average only two hours each day.
Paul Manger, a research professor at the University of Witwatersrand, has been studying sleep in non-human animals for more than two decades. Thanks to his work we now know how much sleep animals like dolphins, whales, cats or antelopes get. As a nice trivia, Manger was the first to study how the platypus sleeps and found it gets more rapid eye movement (REM) sleep -- the phase of sleep when we dream -- than any other studied animal. Dreamy platypuses --now that's a popular science book I'd love to read.
The shortest sleepers in the animal kingdom
Concerning elephants, scientists have previously established they sleep between three and seven hours a day. Studying sleep in captive and wild animals is a whole different ball game, though. A captive animal is fed by keepers and doesn't need to worry about predators. For instance, studies on captive sloths showed these sleep for 16 hours and people ran with that seeing how they already have a reputation for, you know, being sloths. A follow-up study of wild sloths, however, showed these slept for far less -- about 10 hours each day or right on par with yours truly.
To study wild elephant sleeping habits, Manger and colleague Nadine Gravett strapped actiwatches to the trunks of the animals. These are like Fitbit bracelets only instead of monitoring sports performance, these are used to assess a subject's sleep/wake pattern and activity in response to therapy. The researchers could deduce the animals were asleep if the actiwatch showed the trunk was still for more than five minutes. That's how the duo eventually found wild elephants sleep mostly standing and in four to five bursts throughout the day summing no more than two hours.
In mammals, REM sleep happens only when the muscles are relaxed. Besides facilitating awesome dreams, the REM stage is thought to be critical for memory consolidation and rejuvenating the body. But the present findings suggest elephants get little or no REM sleep seeing how they doze for only two hours, and even then while standing most of the time and in short bursts. Despite this, elephants seem very functional and intelligent to boast which raises questions about the role of REM sleep -- we might be overestimating its importance.
"If it is true that the elephant only experiences REM sleep when recumbent, then the findings of the current study indicate that the elephant will only enter REM sleep on every third or fourth day on average. As described above, both wild elephants only experienced recumbent sleep for 10 out of the 35 days of recording, limiting their potential opportunities to enter REM sleep if this only occurs during recumbency. This would then indicate a very different way of obtaining the necessary amount of REM sleep in the elephant compared to that seen in other mammals studied to date, with REM sleep not occurring each day," the reserachers concluded in their paper.
There are a couple of caveats we should mention. For one, the study only included two subjects. Secondly, both elephants are matriarchs meaning they devote most of their waking hours to rearing young calves and protecting them from predators like savannah lions. It's possible less solicited elephants sleep more.
Some might also be surprised to learn that such big animals like elephants need so little sleep. You should know, however, that science establishes that, as a rule of thumb, the smaller the animal the more sleep it needs and vice-versa. Small bats, chipmunks, and opossums all sleep for 15 hours or more per day. In contrast, the domestic horse sleeps for only three hours a day, which makes it the next shortest sleep behind that of elephants. That's because larger animals need to stay awake more to find food.
“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.”
Findings appeared in the journal PLOS ONE.