Heavy elephants are light sleepers

Do elephants dream? That’s one of the intriguing questions thrown up by a recent investigation into the sleeping patterns of the giant mammals.

As a general rule, larger mammals tend to sleep less than smaller ones, but until now no one had accurately answered the question of how much sleep the largest land-dwelling mammal of them all enjoys. The answer, it turns out, is very little.

Fieldwork by Paul Manger and colleagues from the University of the Witwatersrand, South Africa, found that adult female African elephants sleep just two hours a day – the shortest known sleep period of any mammal – and often go up to 46 hours without any sleep at all.

The study, published in PLoS One, tracked two “matriarch” elephants for a 35 days, monitoring them through an implanted actiwatch – essentially a trunk-worn Fitbit – and a GPS-equipped collar.

The data revealed that when the elephants did sleep, they did so for short periods between 2am and 6am. Most of the time they slept standing up, and “only exhibited recumbent sleep every third or fourth day, potentially limiting their ability to enter REM sleep on a daily basis.”

In humans at least, rapid-eye movement (REM) sleep is closely associated with vivid dream states. Its potential absence raises the (admittedly unanswerable) question of whether elephants experience internal imagery while dozing.

Manger’s data indicated that elephant sleep periods were not affected by the amount of energy the animals had expended beforehand. On a few occasions the two females walked up to 30 kilometres before sleeping – a journey the scientists suggest might have been necessitated by poachers, predators or bull elephants with anything but sleep on their minds.