Researchers at the University of California, Berkeley have conducted the first direct measurements of ethanol content in fruits consumed by wild chimpanzees in Africa. Their findings indicate that these animals could ingest more than two standard alcoholic drinks per day through their natural diet.

The study, led by UC Berkeley graduate student Aleksey Maro and Professor Robert Dudley from the Department of Integrative Biology, analyzed 21 species of fruit collected from two field sites: Ngogo in Uganda and Taï National Park in Ivory Coast. The average alcohol content found was 0.26% by weight. According to primatologists, chimpanzees at these locations eat about 10 pounds (4.5 kilograms) of fruit daily, making up approximately three-quarters of their diet.

“Across all sites, male and female chimpanzees are consuming about 14 grams of pure ethanol per day in their diet, which is the equivalent to one standard American drink,” said Maro. “When you adjust for body mass, because chimps weigh about 40 kilos versus a typical human at 70 kilos, it goes up to nearly two drinks.”

A standard drink in the United States contains 14 grams of ethanol regardless of body size; European standards are typically lower at 10 grams.

Professor Dudley explained that even though concentrations are low, the high volume of fruit consumed results in a substantial daily intake: “The chimps are eating 5 to 10% of their body weight a day in ripe fruit, so even low concentrations yield a high daily total — a substantial dosage of alcohol. If the chimps are randomly sampling ripe fruit as did Aleksey, then that’s going to be their average consumption rate, independent of any preference for ethanol. But if they are preferring riper and/or more sugar-rich fruits, then this is a conservative lower limit for the likely rate of ethanol ingestion.”

Maro noted that chimpanzees show no visible signs of intoxication from this level of exposure: “Fruit consumption occurs throughout the day and the chimps show no overt signs of intoxication... Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees.”

Dudley’s earlier hypothesis—known as the "drunken monkey" theory—proposed that humans’ taste for alcohol is inherited from primate ancestors who regularly ate fermented fruit. This idea initially met skepticism but has gained support as additional evidence emerged showing various primates consume fermented foods both in captivity and in nature.

Further studies referenced by Dudley include research on other mammals and birds showing regular dietary exposure to ethanol through naturally fermenting food sources. He said: “The consumption of ethanol is not limited to primates... It’s more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals.” Dudley also suggested possible evolutionary reasons for seeking out ethanol-rich foods may include higher energy returns or social bonding benefits.

To measure alcohol levels accurately under field conditions, Maro used multiple methods including portable gas chromatography and chemical tests after collecting undamaged fruits recently eaten by local chimp populations. Weighted averages based on observed feeding behavior showed similar results between sites: 0.32% alcohol content at Ngogo and 0.31% at Taï.

According to Dudley: “I think the strength of Aleksey’s approach is that it used multiple methods... This dataset has not existed before, and it has been a contentious issue.”

The researchers plan further work analyzing urine samples from wild chimpanzees for metabolites indicating recent alcohol consumption.

Co-authors on this study include Aaron Sandel (University of Texas), Bi Z.A. Blaiore and Roman Wittig (Taï Chimpanzee Project), John Mitani (University of Michigan), with funding provided by UC Berkeley.

The full paper appears September 17th in Science Advances.