Chimpanzees use leafy cups to chug alcohol

A chimpanzee in Guinea drinks uses a leaf as a cup to drink fermented sap. Photo by Gaku Ohashi

Chimpanzees are quickly becoming the food connoisseurs of the jungle.

Last week, the world learned that chimps can cook, or at least comprehend some parts of the process. Now, a study shows that these primates create cups out of leaves to drink alcohol seeping from palm trees. The study, published today in the Royal Society Open Science, is the first to quantify drinking behaviors in chimpanzees.

This chimp behavior is more sophisticated than eating fermented fruits — a popular pastime in the animal kingdom. Unlike Malaysian treeshrews that consistently sip boozy nectar or Australian lorikeet lushes, these chimpanzees use tools to imbibe and only binge opportunistically.

The inadvertent bartenders in this study were villagers from Bossu, Guinea who like to collect alcoholic sap from raffia palm trees. They harvesters tap holes into the crowns of the trees and let the liquid drip into plastic containers. Once the basin is full, they cover it with palm leaves to keep out dirt, while the sap quickly ferments ethanol, the chemical otherwise known as alcohol.

Twenty years ago, scientists noticed that chimps visited the sap traps, and they began intermittently videotaping the trips. The team soon observed that the chimps drank the sap by turning the left-behind palm leaves into cups. The primates would fold or bend a leaf until it held enough liquid to tip into their mouths.

From 1995 to 2012, the team observed 20 drinking sessions among 13 chimps. A typical binge lasted 10 minutes, though the longest one went for half an hour. Also, the apes weren’t opposed to drinking once and then returning later in the same day for another round. After the chimps finished, the researchers measured the alcohol content in the sap basins, which ranged from 3.1 to 6.9 percent alcohol by volume (ABV) — as much as some stout beers.

Both adults and juveniles — younger than 12 years old — would partake, and on average, they would dip their cups around nine times per minute during a drinking session. Prior research on how chimpanzees drink water has estimated that a folded leaf might hold up to 1.7 ounces of liquid. That’s a little more than the contents of a single shot glass in the U.S. The amount ingested by a single ape ranged from less an ounce to nearly three ounces.

Both sexes of chimpanzees visited the booze basins, though males were responsible for 34 of the 51 times that the researchers documented this leaf-drinking behavior over 17 years. (One adult male accounted for 14 drinks on his own).

The team writes that future research should examine if the animals “get drunk” or if chimps outside of Bossou consume alcohol too, given the behavior could merely be a result of living near humans that collect fermented saps. Human-mediated boozing has been seen before with green monkeys on St Kitts, which like to steal cocktails from tourists. Plus a 2004 survey of primatologist showed that wild apes
rarely eat fallen fruits that are rotting and fermenting.

But if future work reveals that seeking alcohol is a general trend among chimps or other apes like gorillas, then such observations might provide validity to the “drunken monkey hypothesis,” the authors write. Ten million years ago, the common ancestor of humans and apes acquired a single mutation that allows our cells to process alcohol forty times faster than animals without this genetic change. Fermented foods carry more calories, meaning bigger brains and bodies. The result may have been that “natural selection favored primates with an attraction to ethanol,” the authors write.

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