Elephants Never Forget, but They Also Almost Never Sleep
Wild African elephants survive on just two hours a night -- less than any other mammal we've ever studied.
A new study tracking wild African elephants revealed that they sleep an average of just two hours each day, less than any other mammal we know of. They also routinely go up to two days without sleep, and now that we know about this elephant-sized case of insomnia, we might have to re-examine our assumptions about the role of REM sleep — not just in elephants, but in humans.
A team of researchers in South Africa outfitted two free-roaming female elephants with what are essentially very large Fitbits — an “actiwatch” implanted in the trunk to track sleep and a gyroscope-equipped collar to track what position the elephants were in — and monitored them for 35 days. Their findings were published Wednesday in the journal PLOS ONE.
“They’ll stand and sleep every day, apart from the days when they’re, you know, being chased by lions,” explains co-author Paul Manger. “But one of the weird things we found is that elephants only lie down to sleep every three or four days.”
This is interesting right off the bat, because researchers believe that the time when animals lie down is the time they likely go into rapid eye movement (REM) sleep. A lot of people are convinced that REM is what helps us aggregate our memories from the day into long-term storage. If you cornered a friend and demanded they list three things about elephants, there is an excellent chance that after “big” and “gray” they would tell you an elephant “never forgets.” According to folklore, elephants famously have the best long-term memory in the animal kindgom — yet they’re only hitting REM maybe twice a week.
The benefits of REM sleep are evangelized endlessly by doctors and wellness books and your Aunt Who Does Pilates, but elephants have excellent long-term memories even though they rarely get a good snooze. Even more astounding, perhaps, is the fact that whales and dolphins actually don’t reach REM sleep at all. The importance of REM to memory consolidation - or lack thereof - might vary wildly across different species of mammals, humans included.
As for why elephants need so little sleep overall? Their restlessness correlates strongly to their size — to slightly oversimplify, the larger the mammal, the less they sleep, at least when it comes to prey animals. The predatory lions that share the elephants’ territory will essentially gorge themselves once a week and spend the rest of their time digesting on their sides, dead to the world in a long post-meal nap. Grazers who nibble throughout the day, on the other hand, have less time (and security) for sleeping. Still, two hours is a singularly low average. Smaller mammals Manger has studied in the past, like Arabian oryx, get four or five hours; Wildebeest, maybe three.
So how does the modern eight-hour tradition among most human cultures fit in? It’s telling here that mammals observed in their natural habitat sleep less than do their counterparts in zoos. Elephants in captivity will sleep four to six hours a day — they’re getting their food delivered, and it’s richer than what they’d get foraging to begin with, leaving them with more time to sleep. Plus, nothing’s chasing them. Financially secure humans in most First World cultures have quite rich diets of their own, and are mostly liberated from foraging and running from lions — two of the four cornerstones of existence along with reproducing and sleeping. Members of hunter-gatherer tribes Manger has observed in Namibia — who must concern themselves with all four — sleep an average of six and a half hours each day.
“So we’re in a kind of captive, domesticated lifestyle ourselves,” says Manger.
The science of human sleep is a murky realm, but one that’s always of great interest to researchers and academics across the board - because it’s interesting, and because everyone always wants to know how to sleep better.
“Hopefully this’ll drum up some money for us.”
The current study provides details of sleep (or inactivity) in two wild, free-roaming African elephant matriarchs studied in their natural habitat with remote monitoring using an actiwatch subcutaneously implanted in the trunk, a standard elephant collar equipped with a GPS system and gyroscope, and a portable weather station. We found that these two elephants were polyphasic sleepers, had an average daily total sleep time of 2 h, mostly between 02:00 and 06:00, and displayed the shortest daily sleep time of any mammal recorded to date. Moreover, these two elephants exhibited both standing and recumbent sleep, but only exhibited recumbent sleep every third or fourth day, potentially limiting their ability to enter REM sleep on a daily basis. In addition, we observed on five occasions that the elephants went without sleep for up to 46 h and traversed around 30 km in 10 h, possibly due to disturbances such as potential predation or poaching events, or a bull elephant in musth. They exhibited no form of sleep rebound following a night without sleep. Environmental conditions, especially ambient air temperature and relative humidity, analysed as wet-bulb globe temperature, reliably predict sleep onset and offset times. The elephants selected novel sleep sites each night and the amount of activity between sleep periods did not affect the amount of sleep. A number of similarities and differences to studies of elephant sleep in captivity are noted, and specific factors shaping sleep architecture in elephants, on various temporal scales, are discussed.