In 2014, University of California, Berkeley biologist Robert Dudley wrote a book called The Drunken Monkey: Why We Drink and Abuse Alcohol. His controversial “drunken monkey hypothesis” proposed that the human attraction to alcohol goes back about 18 million years, to the origin of the great apes, and that social communication and sharing food evolved to better identify the presence of fruit from a distance. At the time, skeptical scientists insisted that this was unlikely because chimpanzees and other primates just don’t eat fermented fruit or nectar.

But reports of primates doing just that have grown over the ensuing two decades. Dudley co-authored a new paper published in the journal Science Advances reporting the first measurements of the ethanol content of fruits favored by chimps in Ivory Coast and Uganda, finding that the chimps are consuming 14 grams of alcohol every day, the equivalent of a standard alcoholic drink in the US. After adjusting for the chimps’ lower body mass, the authors concluded the chimps are consuming nearly two drinks per day.

Earlier this year, we reported that researchers had caught wild chimpanzees on camera engaging in what appears to be sharing fermented African breadfruit with measurable alcoholic content. That observational data was the first evidence of the sharing of alcoholic foods among nonhuman great apes in the wild. They recorded 10 instances of selective fruit sharing among 17 chimps, with the animals exhibiting a marked preference for riper fruit. Between April and July 2022, the authors measured the alcohol content of the fruit with a handy portable breathalyzer and found almost all of the fallen fruit (90 percent) contained some ethanol, with the ripest containing the highest levels—the equivalent of 0.61 percent ABV (alcohol by volume).

That’s comparatively low to alcoholic drinks typically consumed by humans, but then again, fruit accounts for as much as 60 to 80 percent of the chimps’ diet, so the amount of ethanol consumed could add up quickly. It’s highly unlikely the chimps would get drunk, however. It wouldn’t confer any evolutionary advantage, and, according to the authors, there is evidence in the common ancestor of African apes of a molecular mechanism that enhances the ability to metabolize alcohol.