17 of 20 Wild Chimpanzee Urine Samples Tested Positive for Alcohol Metabolites

For years, Robert Dudley's "drunken monkey" hypothesis has rested on an inferential foundation: chimpanzees eat fruit, fruit ferments naturally, therefore chimps consume alcohol. The logic is sound, and measurements of ethanol concentration in the fruits themselves have supported it - a 2025 study by Dudley's student Aleksey Maro found that the fruits chimps eat at Uganda's Ngogo site in Kibale National Park could provide roughly 14 grams of ethanol per day, the equivalent of about two standard drinks. But inferred consumption is not measured consumption.

Maro's eleven-day fieldwork trip to Ngogo in August 2025 was designed to close that gap. The results, published in Biology Letters, are direct: 17 of 20 urine samples from 19 individual chimpanzees tested positive for ethyl glucuronide, a metabolic byproduct of ethanol that appears in urine after alcohol consumption and is used clinically to detect drinking in abstinence-monitoring programs.

The logistics of collecting chimpanzee urine in a rainforest

There is no breathalyzer for chimpanzees. Urinalysis requires, as the name suggests, urine - which in a forest where subjects move freely through the canopy is not straightforwardly obtained. Maro developed and refined collection methods with the help of Sharifah Namaganda, a Ugandan graduate student at the University of Michigan with prior experience at Ngogo.

The team created improvised collectors from forked branches covered with plastic bags, producing shallow bowls that could be positioned under feeding chimps. Longer handles proved important for obvious reasons. Maro also discovered that urine collected from leaves on the forest floor beneath feeding trees was a more reliable and less eventful method. He observed that chimps tend to urinate just before leaving a feeding spot - a behavioral cue that helped time collection attempts. He also noted that chimps prefer to straddle small logs when urinating on the move, a detail that is simultaneously charming and useful for predicting sample locations.

Working only with individually identified chimps - verified with the help of Ngogo staff who know each animal by sight - Maro collected samples with documented provenance. That identification step was methodologically important: it allowed the team to determine which sex and age classes were showing positive results.

The positive results and what they mean

The commercial immunoassay strips used in the study are the same type used to test humans in occupational settings where alcohol abstinence is required, such as commercial aviation and heavy machinery operation. Two different sensitivity thresholds were applied: 300 nanograms per milliliter and 500 nanograms per milliliter of ethyl glucuronide.

At the 300 ng/ml threshold, 17 of 20 samples were positive. Of 11 samples tested at the 500 ng/ml threshold, 10 were positive. In humans, 500 ng/ml of ethyl glucuronide is consistent with ingesting 1 to 2 standard drinks within the previous 24 hours. Given that the chimps were gorging on star apple fruit during a bumper year, and given that chimps are estimated to consume approximately 4.5 kilograms of fruit daily, even a relatively low ethanol concentration in the fruit - the star apples measured only 0.09% ethanol by weight, lower than the 0.32% average Maro measured from Ngogo fruits in 2019 - could produce the observed metabolite levels.

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

Who tested negative, and why it matters

The four negative results were not random. They came disproportionately from females in estrus and from juveniles. Dudley raises the possibility that male chimpanzees may preferentially access or hoard the most fermented, highest-ethanol fruits, leaving lower-ethanol options to females and young animals. If true, the pattern would imply that alcohol ingestion in chimpanzee societies is not just incidental to eating ripe fruit but may involve active competition for the highest-alcohol items.

Testing that hypothesis would require both continuous observation of individual feeding behavior and ethanol measurements of the specific fruit items each chimp consumes - a substantial methodological challenge in a dense rainforest canopy.

What the drunken monkey hypothesis still needs to prove

The new study establishes physiological evidence of regular ethanol consumption, but Dudley is clear that one central prediction of the hypothesis remains unverified: that chimpanzees actively select fruits with higher ethanol content over lower-ethanol alternatives. "That hasn't really been demonstrated for any taxon in the wild," he said. "So that would be the next future direction on this - to definitively prove the universal hypothesis of attraction to alcohol."

The broader evolutionary argument - that primate attraction to fermented fruit preconditioned hominins for the domestication of alcohol - remains a hypothesis rather than an established fact, though the new data bring its empirical foundation closer to what the hypothesis requires. Immunoassay strips have proven practical in field conditions; Dudley plans to use them with fruit bat populations in Madagascar, where fermented fruit consumption would also be expected but has not been directly measured.