In the dense forests of Uganda and Côte d’Ivoire, wild chimpanzees spend their days foraging high among the canopy for ripe, sugar-rich fruits. But what appears to be a simple search for food may actually hide a surprising fact: many of the fruits they eat are naturally fermented, and the apes end up ingesting measurable amounts of alcohol every day — roughly the equivalent of a couple of human drinks. A new study published on 17 September 2025 argues this chronic, low-level alcohol exposure could trace back to the diets of our earliest primate ancestors — long before humans brewed wine or distilled spirits.
The research, titled “Ethanol ingestion via frugivory in wild chimpanzees,” was led by graduate student Aleksey Maro from the Department of Integrative Biology at University of California, Berkeley. Supporting authors include Robert Dudley as senior author along with Aaron A. Sandel, Bi Z. A. Blaiore, Roman M. Wittig, and John C. Mitani. The paper appeared in the peer-reviewed journal Science Advances.
Maro and colleagues analyzed 21 species of fruit regularly consumed by chimpanzees at two long-term study sites: the Ngogo community in Uganda’s Kibale National Park and the Taï Chimpanzee Project in Côte d’Ivoire. The team measured the ethanol content of ripe-but-fallen fruit pulp and found the fruits contained, on average, 0.31-0.32% ethanol by weight.
Wild Chimps Consume Daily Ethanol from Fruit
Given that chimpanzees consume roughly 4.5 kilograms of fruit daily — which accounts for about three-quarters of their total diet — the researchers estimated a daily ethanol intake of approximately 14 grams. For a human, this equals roughly one standard drink; when adjusted for the smaller body mass of a chimpanzee (about 40 kg), the dose corresponds to nearly two standard human drinks per day.
“This finding suggests that chimpanzees consume a similar amount of alcohol to what we might ingest if we ate fermented food daily,” Maro said. “Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees.”
Despite this intake, the chimpanzees observed did not show any overt signs of intoxication. The alcohol was consumed gradually throughout the day, dispersed across many hours of foraging, which likely prevented any momentary “buzz.” “A chimp would have to eat so much fruit that its stomach would bloat to feel intoxicated,” Maro explained.
Senior author Dudley added perspective on the broader implications. He emphasized that if wild chimpanzees indeed ingest alcohol routinely through fruit, then ethanol exposure may have been a regular part of the diet of the common ancestor of humans and chimpanzees.
“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,” Dudley said.
Drunken Monkey Hypothesis
The study supports the long-standing drunken monkey hypothesis — the idea that humans’ craving for alcohol has deep evolutionary roots, grounded in dietary exposure to ethanol long before the invention of fermented or distilled beverages.
The findings carry implications beyond primatology. If early primates had routine exposure to dietary alcohol through fermented fruit, it would help explain why certain metabolic pathways — like enzymes for ethanol breakdown — evolved long before humans learned to brew. It also invites a reassessment of how alcohol consumption should be understood, not as a uniquely human cultural invention, but perhaps as an ancient ecological legacy.
Still, the researchers caution against overinterpreting the results. They do not yet know whether chimpanzees actively select for more fermented — and thus more alcoholic — fruit, or simply eat what’s available. As Maro noted, the current estimate may be conservative.
Future work, the team says, will examine whether chimps show a preference for more ethanol-rich fruit, perhaps guided by aroma, and whether these dietary habits influence behavior such as social bonding, territorial patrols, or reproduction. For now, the study provides compelling evidence that alcohol consumption — in a natural, low-dose form — is not a modern human invention but a thread woven into the primate past. (Wage Erlangga)
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