Wild Chimpanzees Caught Drinking Alcohol, Urine Tests Show

UC Berkeley researchers find evidence of widespread alcohol consumption in chimps

Published on Feb. 25, 2026

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Researchers from UC Berkeley have found evidence that wild chimpanzees in Uganda's Kibale National Park regularly consume significant amounts of alcohol from fermenting fruit. By collecting and analyzing urine samples from the chimps, the team detected high levels of ethyl glucuronide, a byproduct of alcohol metabolism, proving the chimps ingest substantial quantities of ethanol in their diet.

Why it matters

The findings provide strong support for the "drunken monkey" hypothesis, which posits that the ancestors of humans and other primates evolved a predisposition to seek out and consume alcohol. This could have implications for understanding the origins of human alcohol use and abuse.

The details

To collect the urine samples, graduate student Aleksey Maro worked with a Ugandan researcher to develop specialized collection devices using forked branches and plastic bags. They gathered samples from under trees where chimps fed, as well as from puddles on the forest floor. Of the 20 samples collected, 17 tested positive for ethyl glucuronide, indicating significant alcohol consumption. The team also analyzed the alcohol content of the chimps' favored "star apple" fruit, finding it contained 0.09% ethanol by weight.

  • Maro collected the urine samples during an 11-day trip to Ngogo in Uganda's Kibale National Park in August 2025.
  • The research findings are set to be published in the journal Biology Letters next week.

The players

Aleksey Maro

A UC Berkeley graduate student who led the urine sample collection efforts.

Robert Dudley

A UC Berkeley professor of integrative biology and Maro's advisor on the project.

Sharifah Namaganda

A Ugandan graduate student at the University of Michigan who has experience collecting urine samples for previous projects at Ngogo.

Laura Byrne

A researcher from San Francisco State University who is a co-author on the paper.

Pan troglodytes

The Western chimpanzee species that was the focus of the urine sample collection.

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What they’re saying

“We find widespread physiological evidence of the consumption of alcohol by chimpanzees. If there's any doubt about the drunken monkey hypothesis — that there's enough alcohol in the environment for animals to experience alcohol in a way analogous to humans — it's been cleared up.”

— Aleksey Maro, Graduate student (Biology Letters)

“The levels are high, and this is a conservative estimate given the time course of exposure through the day. In nanograms per milliliter, these are coming in way above some of the clinically relevant and forensically relevant human thresholds.”

— Robert Dudley, Professor of integrative biology (Biology Letters)

What’s next

Dudley and Maro plan to conduct further studies to assess the effects of dietary alcohol on the physiology and behavior of chimpanzees, including whether consumption of fermented fruit affects aggression or the timing of female fertility.

The takeaway

This study provides strong evidence supporting the 'drunken monkey' hypothesis that the ancestors of humans and other primates evolved a predisposition to seek out and consume alcohol. The findings could shed light on the origins of human alcohol use and abuse.