Wild Chimpanzees May Be Consuming Two Alcoholic Drinks a Day

Alcohol might seem like a uniquely human invention. But in tropical forests, it’s been forming on its own for millions of years. As fruit ripens in Uganda’s rainforest, sugars break down and ethanol forms naturally. When chimpanzees eat that fruit, they may be taking in the equivalent of two drinks a day.

Now, a study published in Biology Letters shows that wild chimps are not just consuming the fermented fruit, they’re metabolizing the alcohol. Urine samples collected from chimpanzees in Kibale National Park revealed clear biochemical evidence of ethanol exposure, strengthening what biologists call the “drunken monkey hypothesis.”

“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,” said graduate researcher Aleksey Maro in a press release.

Wild Chimpanzees and Alcohol’s Evolutionary Origins

The “drunken monkey hypothesis," proposed by biologist Robert Dudley, suggests that our attraction to alcohol may be ancient. As fruit ripens and releases ethanol, the scent can signal high sugar content. Primates drawn to that smell may have had an advantage in finding calorie-dense food. That evolutionary pull could still shape how humans respond to alcohol today.

Chimpanzees are great test subjects for the hypothesis. They consume enormous amounts of fruit, sometimes more than 10 pounds a day, meaning even modest ethanol concentrations could add up.

Earlier research measured alcohol levels in fruit and estimated how much chimps were likely consuming based on how much they eat. What researchers hadn’t shown was physiological proof that the alcohol was being absorbed and metabolized. The new urine evidence confirmed this.

“Food and alcohol evolutionarily are, as it turns out, very much connected, especially in the lives of chimpanzees,” Maro said in the release.

Read More: Bonobo Kanzi Plays Pretend Like a Child, Showing Ape’s Capacity to Imagine

Alcohol Metabolism in Wild Chimpanzees

In total, the team analyzed 20 urine samples from 19 Western chimpanzees. They tested for ethyl glucuronide, a compound that appears after the body breaks down alcohol.

The strips they used are the same kind employed in human alcohol screening. To collect the samples, researchers spent days beneath feeding chimps, watching for signs that an animal was about to urinate. They gathered urine from leaves, small puddles, and even plastic bags tied to branches.

Of the 20 samples, 17 registered above the lower detection threshold. Nearly half exceeded levels comparable to what would be expected in a human after light drinking within the previous day. Juveniles and females were more likely to test negative, raising the possibility that dominant males may be hogging the more fermented fruit.

The researchers note that if ethanol is a natural byproduct of ripening fruit, then any animal that depends heavily on fruit should test positive.

“The hypothesis is, of course, that it has to be positive. The question is how much,” said Dudley. “With these strips you can do sampling through the year with their diet shifts and climate shifts. It's not just primates. It's anything else that's eating fruit.”

How Chimpanzees May Help Explain Human Alcohol Use

What researchers haven’t yet shown is whether chimpanzees actively seek out fruit with higher ethanol levels, or how long-term exposure might shape their behavior. Future studies will look at influences on aggression, social dynamics, or even the timing of female fertility.

If fruit-eating animals across ecosystems are consistently ingesting small amounts of alcohol, then ethanol may be less of a quirk of human culture and more of a shared biological inheritance.

“It all comes back to the human side: Have we evolved predisposed to the consumption of alcohol, based on this ancestral lineage? And how did that predispose us to the domestication of alcohol via brewer’s yeast,” Maro said in the press release.

Read More: Macaque Monkeys Could Unlock the Brain Secrets Behind Motivation — and Inspire New Mental Health Treatments

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a recent study published in Biology Letters: Urinary concentrations of a direct ethanol metabolite indicate substantial ingestion of fermenting fruit by chimpanzees