Chimps get buzzed on fermented tree sap

There’s a fabulously named drunken monkey hypothesis that posits that humans’ love of alcohol comes from our primate ancestors. Those primates, the theory goes, got an evolutionary advantage from being able to eat lots of fermenting fruit and the ethanol that forms in the process.

One of the big problems with this hypothesis is that there isn’t any evidence of contemporary primate relatives that consume alcohol. Green monkeys in St. Kitts steal cocktails from tourists. And at least one captive chimp had to be sent to rehab after getting addicted to nicotine and alcohol. But that’s about it. Apes, our closest relatives, rarely eat fruit off the ground where it ferments.

There are hints in our DNA that our craving for alcohol may have a long history, though. The ability to metabolize ethanol apparently arose 10 million years ago in a common ancestor of chimps, bonobos, gorillas and humans.

And now Kimberly Hockings of Oxford Brookes University in England and colleagues report a case of ethanol consumption in wild chimps, a first. But the animals aren’t eating fermented fruit — they’re stealing fermenting palm sap from humans. And they invented a tool to help them do it.

In Bossou, Guinea, local people set out plastic containers to trap sap dripping from raffia palm trees, covering their containers with palm leaves to avoid contamination. Inside the containers, the sap quickly ferments into a beverage with an alcoholic content equivalent to a weak beer. People drink the concoction usually within a day without any sort of further processing.

The containers aren’t guarded, and the leaves, it turns out, aren’t any good at keeping out curious chimpanzees. And the researchers actually spotted the chimps using the leafy covers or leaves from nearby plants as tools to soak up the drink.

The behavior isn’t incredibly frequent, and not all chimps imbibe. In 17 years, the research team spotted 13 adult and immature chimps, half the population, having a total of 51 drinking sessions. But one male chimp accounted for 14 of those escapades. And some of the animals drank enough to display signs of inebriation, the researchers report June 8 in Royal Society Open Science.

Even when combined with the earlier genetic findings, the discovery still doesn’t prove the drunken monkey hypothesis. But, the researchers say, it does show that it’s worth continuing the search for more evidence of imbibing apes.