(Royal Society Open Science)

Humans everywhere have been known to drink alcohol if they can get their hands on it. And researchers have long suspected that our ability to metabolize ethanol (or alcohol) serves some evolutionary purpose. They called it the "drunken monkey hypothesis."

Yet there had been few scientific observations of primates drinking alcohol intentionally in the wild. But a new study found surprising evidence that chimpanzees in Guinea regularly consumed naturally fermented alcohol in the wild.

In the study, published in the journal Royal Society Open Science, researchers observed drinking sessions involving adult chimpanzees near Bossou, Guinea, over the course of 17 years.

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They found that the clever animals had learned to take advantage of containers humans placed in the raffia palm trees to collect its alcoholic sap. Fermented raffia sap is produced by the plants year-round and researchers estimate that it is about 3.5 percent alcohol by volume on average -- but the alcohol content can get as high as 6.9 percent. That's about as much as a serving of beer.

Humans collect the sap from the trees several times a day and consume it soon after without processing it. But when they leave the tapped trees unattended, chimps are known to use the leaves that cover the containers to drink the sap instead.

Some chimps drank as much as a bottle of the palm wine in a session, according to researcher Kimberley Hockings of Oxford Brookes University and the Centre for Research in Anthropology in Portugal. In some cases, researchers observed tell-tale signs of intoxication.

"[They] displayed behavioral signs of inebriation, including falling asleep shortly after drinking," Hockings told the BBC. "On another occasion after drinking palm wine, one adult male chimpanzee seemed particularly restless."

"While other chimpanzees were making and settling into their night nests, he spent an additional hour moving from tree to tree in an agitated manner," she added. "Again pure speculation, but it's certainly something we would like to collect further data on in the future," she added.

The chimps had also (brilliantly) developed a method of drinking that utilized the leaves that villages placed over the containers as a protective covering. They dipped the leaves in the sap and squeezed it in their mouths like a sponge to extract the palm wine.

In a single session, chimps might monopolize a tree -- drinking alone -- or take turns dipping the leaf sponges in the containers.

A previous study identified a genetic mutation in humans and African apes that gave us the shared ability to metabolize alcohol about 10 million years ago.


An adult male chimpanzee uses a leaf tool to drink raffia sap from a container. (M. Nakamura/Royal Society Open Science)

The evolutionary benefit could potentially be that the alcohol provides quick and easy calories. It may also stimulate appetite prompting animals to search for more fruit. Or it could be a consequence of eating fruit in general: Overripe fruit naturally contains higher levels of ethanol.

But primates aren't really attracted to overripe fruit and rarely eat it.

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There have been reports of a wild species of primate native to Asia regularly consuming fermented nectar and of monkeys doing things like snatching cocktails from unsuspecting beach-goers in St. Kitts. But validated research confirming chronic alcohol consumption in the wild has been pretty rare.

In this case, the chimps are drinking alcohol by cleverly taking advantage of a human-made opportunity. There's no evidence that they'd be exhibiting the same behavior by tapping the palms themselves.

And according to Hockings, while it doesn't confirm that the animals were attracted to the alcohol content, it does suggest that they weren't just consuming the sap as a substitute for other forms of food.

"The chimps are ingesting palm wine throughout the year," she told USA Today. "It wasn't just when no wild food was available."

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