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11 June 2015

Closing the gap between man and ape: like humans, chimpanzees are getting drunk

...and some are underage.

By Tosin Thompson

Tired of the harsh scheduling of monotonous daily chores, such as foraging for food and attempting to find a mate? Want some time to relax and unwind? Time to call your friends over for a soothing drinking session, which you know will inevitably turn wild as alcohol consumption increases. 

Dr Kimberley Hockings, a biological anthropologist of Oxford Brookes University might have thought this scenario would only have applied to a human 20 years ago, but over 17 years (1995-2012), Hockings and her fellow researchers have studied the chimpanzees of Bossou, south-eastern Guinea, and have witnessed evidence of their long-term and frequent ingestion of ethanol from raffia palm, aka, alcohol.

“African apes and humans share a genetic mutation that enables them to effectively metabolize ethanol. However, voluntary ethanol consumption in this evolutionary radiation is documented only in modern humans,” the authors write in their 2015 paper published in Royal Society Open Science.

Recent DNA analysis suggests we, humans, are around 96 to 99 per cent identical to our primate counterparts (we are ten times more similar to each other than mice are to rats). We formerly referred to other primates as “sub-human primates” – a derogatory term that implied they were beneath us. However, after evidence of their kinship with humans, as well as their human-like behaviours, such as hanging out in cliques, laughing, the inclination to cook, and now boozing, they’re today more correctly termed as “non-human primates”.

The sugary sap of raffia palm naturally ferments into palm wine. It has been a tradition that the locals of Bossou place containers near the crown of trees at dawn to collect palm sap throughout the day. The aroma of the wine fills the jungle. It lures the chimps to the containers that locals cover in leaves in order to prevent dust and insects from contaminating the sap. The chimps cheekily come up with makeshift wine glasses by using their mouths to make sponges from the leaves, and then dip them into the containers. Results showed that the ethanol content of palm sap collected from 16 raffia palms varied from 3.1 per cent to a 6.9 per cent, the equivalent of a strong beer.

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An adult male chimpanzee uses a leaf tool to drink raffia sap from a container: (a) He inserts his right hand holding the leaf tool into the fermented palm sap container, (b) retrieves the leaf tool that is soaked in fermented palm sap, and (c) transfers the soaked leaf tool to his mouth to drink the palm sap it carries. Photos: M Nakamura (1996)

Some of the chimps went, for lack of a better term, apeshit, over it. “(They) consumed significant quantities of ethanol (alcohol) and displayed behavioural signs of inebriation,” the paper said. “Researchers rarely collected detailed behavioural data before-versus-after exposure to ethanol, but some drinkers rested directly after imbibing fermented sap.

“These data show that ethanol does not act as a deterrent to feeding in this community of wild apes, supporting the idea that the last common ancestor of living African apes and modern humans was not averse to ingesting foods containing ethanol,” the paper adds.

The chimps observed are among a closely-observed colony, and this isn’t the first time they’ve made headline news. In 2008, one of the chimps found a way to use a stick to “fish” for ants.

Other primate species have been living it up, including Happy Jerry, the gin-swilling, pipe-smoking Mandrill, and booze-fuelled monkeys on the Caribbean island of St Kitts:

The Bossou chimps observed are the first to provide real data about how much alcohol primates consume, and when.

The researchers found that sometimes a single chimp would go to the top of the palm, but occasionally, there would be “drinking sessions” where several apes would gather in the crown of the tree. From the paper, “individuals either co-drank, with drinkers alternating dips of their leaf-sponges into the fermented palm sap, or one individual monopolised the container, whereas others waited their turn.”

Over 17 years, the researchers observed 51 drinking events, 20 of which were drinking sessions involving 13 adults and young chimps.

The results suggest there is something in the “drunken monkey” hypothesis, which states natural selection favours those who are more susceptible to alcohol because of its associations with proximate benefits, such as acting as an appetite stimulant, or a cue to finding fruit, thus increasing calories and vitamins.