Elephants Never Forget… Or Sleep

It is fairly well established in humans that a good night’s sleep is important for long term memory formation. Deep sleep seems to be a part of the mechanism by which we process the days events and data and lay down the appropriate memories. People who regularly don’t sleep properly are prone to anxiety, irritability, depression, attention deficit and have difficulty remembering. A new study looking at the sleep patterns of wild African elephants, then, has rather thrown a spanner in the works of this theory.

Elephants are proverbially memorious but hard evidence of what we know about them backs up the folk knowledge. A herd of wild African elephants, led by its matriarch, will have an extremely large territory to patrol over the course of a year. As the dry season bites a herd will walk many tens of kilometres per day to get to a watering hole that it may not have visited for several years.

The best theories about sleep that we currently have would suggest that to have such faithful memories elephants would need to sleep well to consolidate them. But a new open access article, published in PLOS ONE, last week suggests that African elephants only sleep for two hours per day; a shorter duration than any other mammal yet recorded.

The data was collected by putting an ‘actiwatch’, essentially a FitBit type thing, onto the trunks of two matriarchs in Northern Botswana. As the trunk of an elephant is extremely mobile during waking it was assumed that if the trunk became inactive for five minutes or more then the animal was asleep.

Both elephants turned out to be polyphasic sleepers, meaning they don’t get all of their sleep in one go, they grab it in fits and starts through the night. Interestingly, they generally slept standing up and only laid down to sleep every 3-4 days. This is significant as it would imply that they only achieve REM sleep once or twice per week.

In humans REM sleep has been shown to be important in many functions, not all of them intuitive. Most of your tissue repair mechanisms, bone strengthening and immune system strengthening takes place during REM sleep. Also, however, this is the period of sleep that is crucial for making and maintaining memories.

The researchers also noted that there were occasions over the 5 week observation period where the elephants would be disturbed for whatever reason and completely miss a night’s sleep. This did not result in an increase in napping or of overall sleep in the next day or two; they are apparently able to just shrug it off. The amount of sleep they got also appeared to be unrelated to the amount of physical activity carried out during the day.

The fact that an animal with such a good memory gets so little REM sleep may force a rethink of our theories of sleep architecture and what its function is in mammals. More data would certainly help in interpretation. This study was of only two animals, both matriarchs. If the study could be repeated with more animals, of different ages, sexes and statuses then perhaps we could learn more.

That elephants have great memories is not in question, but we certainly can’t put it down to them slumbering their days away.

Image courtesy of Mohammad Mahdi Karim
Title image courtesy of Charles Sharp

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