Chimpanzees in the forests of Uganda and Côte d’Ivoire like to gorge on fruit. They feast on wild figs and plum-like delicacies with such enthusiasm that they may unknowingly be consuming a fair bit of alcohol in the process as well.
A new study published in Science Advances offers the first direct estimates of how much ethanol (alcohol) chimpanzees ingest in the wild through their fruit-heavy diets. The surprisingly high numbers seem to confirm a bold hypothesis that our human taste for alcohol may have begun in the canopy.
“The chimps are eating 5 to 10% of their body weight a day in ripe fruit, so even low concentrations yield a substantial dosage of alcohol,” said Robert Dudley, a professor of integrative biology at the University of California, Berkeley, and the senior author of the study, in a press release.
The Drunken Monkey
For decades, Dudley has championed what he calls the “drunken monkey hypothesis“. It’s a bold idea that the human inclination toward alcohol stems from a deep evolutionary past, when our primate ancestors used the smell and taste of ethanol as signals to locate ripe, energy-rich fruit.
Until now, the theory has been mostly speculative. But this new research, co-led by Aleksey Maro, a graduate student at UC Berkeley, brings hard data to the table.
Maro and colleagues tracked wild chimpanzees in two habitats: Ngogo in Uganda and Taï in Côte d’Ivoire. These regions span the range of chimpanzee subspecies, offering a broad look at their diets. To measure ethanol levels, they used multiple methods, including a portable gas chromatograph, a color-change chemical test, and a semiconductor-based sensor similar to a breathalyzer.
Their findings suggest that the fruit chimpanzees eat contains, on average, about 0.31–0.32% ethanol by weight. That’s hardly a heavy drink. It’s comparable to kombucha, and over 10 times less than your average beer. But with chimps consuming over 4.5 kilograms (about 10 pounds) of fruit daily, the alcohol adds up.
At both sites, chimps were estimated to ingest about 14 grams of ethanol per day. That’s the equivalent of 1.4 U.S. standard drinks. Adjusted for body mass, it’s even more: a chimpanzee’s intake translates to the human equivalent of 2.2 to 2.6 drinks per day.
Are the chimps drunk?
This is where things start to get interesting. The researchers didn’t observe any obvious signs of inebriation. That’s likely because chimpanzees consume fruit gradually throughout the day, spacing out their ethanol exposure. For inebriation, they would have to consume a lot of fruit in a shorter period of time.
Kimberley Hockings, a conservation scientist at the University of Exeter who was not involved in the study, told CNN that the data suggests that ethanol isn’t an absolute deterrent to chimpanzee feeding, but that it’s less clear whether the chimps are actively drawn to alcohol or just after the sugar.
These results hint that alcohol may not just be an incidental byproduct of ripe fruit, but a factor woven into primate evolution.
Fermentation is as old as fruit itself. Ethanol is produced when yeasts (particularly those in the Saccharomycetaceae family) digest sugars in low-oxygen environments. Yeasts and fleshy fruit-bearing plants co-evolved over 100 million years ago, creating a microbial-plant-primate triangle of mutualism. As Dudley puts it in the paper, yeasts may serve as the “silent third partner” in the relationship between fruit and fruit-eaters.
That biological relationship left its mark on the genome.
For instance, humans and great apes share an enhanced version of the ADH4 gene, which helps metabolize ethanol with far greater efficiency than our earlier primate ancestors. The same mutation is found in chimpanzees and gorillas. This likely offered a digestive advantage for ancestors who encountered alcohol-laced fruit regularly in their diets.
“We inherited the taste for alcohol,” said Dudley in the CBS interview. “Even though our diets have diversified… that bias to consume quickly when this molecule is present could still be a powerful force.”
Evolution’s Cocktail
Though the idea that chimpanzees drink is not a new one (prior studies captured wild chimps sipping fermented palm sap, or bonding over boozy breadfruit), this is the first time researchers have chemically measured the alcohol content in the actual fruits chimps eat and linked it to their daily intake.
The study’s meticulous design bolsters its claims. It spanned multiple years, fruit species, regions, and lab methods. The authors accounted for seasonal variations, fruit ripeness, and feeding preferences, weighting the ethanol calculations based on how much time chimps spend eating each fruit type.
Importantly, the fruits with the highest ethanol content—such as Ficus mucuso in Uganda and Parinari excelsa in Côte d’Ivoire—were also among the chimps’ favorites. That correlation may be circumstantial, or it could hint at a primitive preference still lurking in their evolutionary circuitry.
But if the chimps aren’t drinking to get drunk, what role does ethanol actually play?
“It just points to the need for additional federal funding for research into alcohol attraction and abuse by modern humans,” Dudley added. “It likely has a deep evolutionary background.”
The story doesn’t end with chimpanzees.
From spider monkeys in Panama to elephants raiding marula trees, many animals consume fermented fruit. Even fruit flies hone in on alcohol vapor with uncanny speed. Evolution seems to have primed many species to find alcohol—not because it gets them drunk, but because it gets them fed.
In that sense, alcohol may not be just a vice. It may once have been a survival signal.
The “drunken monkey” hypothesis suggests that early hominins, like today’s chimpanzees, may have benefited from following the faint scent of ethanol to calorie-rich meals. If true, then modern humans—grappling with alcohol misuse on one end and wellness kombucha on the other—are simply living with the evolutionary residue of that ancient craving.
