There's an old saying that elephants never forget. You also can say they almost never sleep.
Scientists say a first-of-its-kind study tracking the sleep behaviour of wild elephants found the world's largest land mammal sleeps two hours per day on average, and some days not at all, and does so mostly standing up.
This represented the shortest-known sleep time of any mammal. Previous research showed captive elephants got four to six hours' sleep daily.
"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 published in the journal PLOS ONE.
The researchers monitored two free-roaming female African elephants in Botswana's Chobe National Park for 35 days. They got data to track sleep accurately from a wristwatch-sized device implanted under the skin of the trunk that was not harmful to the animals. They used a satellite-tracking collar with a gyroscope to monitor their location and sleep position.
"We do feel these two elephants are representative of the broader population," Manger said, adding he hoped future research could be done with larger numbers of wild elephants, including males.
The elephants sometimes went up to 46 hours without sleep while walking distances of about 19 miles (30km), possibly to avoid threats like lions or human poachers.
They typically slept somewhere between 2am and 6am. After a sleepless night, they had no extra sleep the next night. The maximum sleep recorded was five hours in a day. They spent just 17 per cent of their sleeping time lying down.
The next shortest sleepers among mammals may be domestic horses, which get under three hours' sleep daily. Some mammals have been shown in captivity to sleep most of the day, including the little brown bat (19 hours), opossum (18 hours) and armadillo (17 hours), Manger said.
The elephants appeared to experience rapid eye movement (REM) sleep, associated with more dreaming and body movements and loss of muscle tone, only every three to four nights.
"REM sleep is often associated with the consolidation of memories. However, we do know elephants have good memories, so this finding contradicts one central hypothesis of REM sleep function," Manger said.
Meanwhile, Africa's elephant population fell around 20 per cent between 2006 and 2015 because of a surge in ivory poaching, the International Union for Conservation of Nature (IUCN) said in a report on Sunday.
Switzerland-based IUCN is regarded as the most authoritative source on wild fauna populations, and the report's release at a UN conference on the global wildlife trade will lend a sense of urgency as some countries seek to keep the global ivory trade shut while others want to reopen it.
"This is yet another set of data clearly indicating that governments must take all necessary actions to address the crisis," said Susan Lieberman, head of international policy for the Wildlife Conservation Society.
The IUCN, which drew on a range of estimates and census data, said it now had a fairly accurate count of 415,000 elephants in Africa in the areas where extensive surveys could be taken, down from over 500,000 in 2006.
There are a number of regions where systematic surveys could not be taken and so it is difficult to say what is happening to elephants in such places. These include South Sudan, Liberia and savannah areas of Central African Republic.
Losses in some countries have been staggering. Tanzania, which relies heavily on wildlife tourism, saw a 60 per cent decline in its elephant population.
"The surge in poaching for ivory that began approximately a decade ago — the worst that Africa has experienced since the 1970s and 1980s — has been the main driver of the decline," the IUCN said.
Elephant poaching has risen to meet red-hot demand among fast-growing consumer markets in Asian economies such as China's, where ivory is a coveted commodity used in carving and ornamental accessories.
The IUCN noted that southern populations in "Namibia, South Africa and Zimbabwe are stable or increasing, and there is evidence of elephant range expansion in Botswana."
But poachers are also training their guns on southern Africa, with big declines noted in Mozambique, it said.
Namibia and Zimbabwe have submitted proposals to the UN's Convention on International Trade in Endangered Species (CITES), which is meeting in Johannesburg, seeking permission to lift a global ban on the ivory trade so they can sell stockpiles.
This is opposed by other African nations such as Kenya, which fear that illicit ivory can be laundered with clean supplies and that it could stimulate demand for a commodity which has one main source — a dead elephant.