海角大神

If you鈥檝e ever felt tired and overworked, be thankful you鈥檙e not an elephant. The lumbering giants may get by on at little as two hours a night, according to scientists.

The first sleep study of wild elephants, published in journal PLOS ONE on Wednesday, concluded that elephants may get , including new human parents. What鈥檚 more, they regularly skip nights and often sleep standing up, leaving little chance to dream. The research contributes to a growing understanding of the relationship between body size and sleep time, but complicates science鈥檚 quest to understand the ultimate purpose of sleep.

Scientists had previously observed captive elephants sleeping 4 to 6 hours a day, but catching some extra z鈥檚 while unemployed and with nothing to do in an enclosed area perhaps isn鈥檛 so surprising. Wild animals have to feed themselves, avoid threats, and in the case of the two matriarch subjects of this study, lead their herd.

"Sleep needs to be studied in an animal's natural environment if we are truly to understand it," said Paul Manger, a research professor in the School of Anatomical Sciences at the University of the Witwatersrand in South Africa who led the study, in an interview with Reuters.

To get a more accurate picture of wild behavior, researchers tagged two female leaders known as matriarchs with a gyroscope-equipped GPS collar to track location and position, and surgically implanted a benign Fitbit-like movement tracker into each elephant鈥檚 trunk. Embedding an electrical device to monitor the brain directly into the animals鈥� thick skulls was , so researchers used trunk movement as an approximation of wakefulness, the New Scientist reports.

鈥淲e reasoned that measuring the activity of the trunk, the most mobile and active appendage of the elephant, would be crucial, making the reasonable assumption that if the trunk is still for five minutes or more, the elephant is likely to be asleep,鈥� said Dr. Manger .

The month-long study found that the elephants slept an average of two hours each day, a full hour less than the previous mammalian record holder, the domestic horse. Those two hours generally fell in the early hours before dawn聽and were spread out over four to five short nap sessions, like Spaniards but with more siestas.

On multiple occasions, the animals walked through the night, staying awake for up to two days straight and covering distances as far as 19 miles. Researchers suspect they may have been avoiding predators such as lions or poachers. Unlike most humans, the elephants didn鈥檛 log any extra catch-up time the day after an all-nighter. 聽

When the giants did sleep, it was in a different spot each night, and often standing up. Because they only laid down for about an hour every three to four days, researchers concluded that elephants don鈥檛 enter REM sleep daily like humans. While dreaming, most mammals relax their muscles, making it difficult to stand.

However, Manger told New Scientist that birds can dream in sub-minute bursts, and whales and dolphins appear to go without REM sleep entirely. Since this investigation only studied trunk movement, it鈥檚 impossible to know for sure when and if the elephants dreamt. 鈥淲e鈥檙e not sure which of those is true yet and that鈥檚 something we鈥檇 like to follow up and discover,鈥� he said.

The short sleep time didn鈥檛 come as a surprise, though. The relationship between body size and sleep duration is still debated, but the authors used data on other captive plant-eating mammals to predict that elephants would sleep about 2.5 to 3 hours per day. It seems that larger animals tend to sleep less, perhaps because they have to spend so much time eating.

What was more surprising was the possibility that elephants don鈥檛 dream very often. The purpose of sleep remains mysterious, but one proposed function is the re-writing of memories during REM.

鈥淩EM sleep (or dreaming) is thought to be important for consolidating memories, but our findings are not consistent with this hypothesis of the function of REM sleep, as the elephant has well-documented long-term memories, but does not need REM sleep every day to form these memories,鈥� said Manger.

In the paper, the team wrote that the 鈥渃urrent study has produced a number of exciting and interesting results,鈥� but highlights a few potential biases in their data. The study recorded sleep times in only two individuals, and matriarchs at that. As any president can confirm, leaders don鈥檛 often get to sleep in. One was also nursing a 1-year-old calf. 聽

However, they also suspect that using trunk movement may have lead to an overestimate of sleep time, including those moments when the animal was motionless but hadn't yet drifted off.

Despite these drawbacks, Manger thinks the team鈥檚 results are representative.

鈥淚 think what we鈥檝e got is pretty close to the mark,鈥� he told New Scientist. 鈥淥bviously it would be nice to do a lot more animals, but there are ethical considerations and the bottom line is getting enough funding.