Chimpanzees consume alcohol every day and never get drunk

Fermenting fruit in African forests naturally contains low levels of ethanol. New measurements show that wild chimpanzees could easily ingest the equivalent of more than one – perhaps nearly two – standard drinks per day just by eating their usual fruit. 

There’s no sign that the chimps get tipsy. But chronic, low-level exposure looks like a routine part of life in the trees.

The research was conducted by Aleksey Maro and Professor Robert Dudley from the University of California, Berkeley. The team sampled fruits eaten by chimpanzees at two long-studied sites: Ngogo in Uganda and Taï in Côte d’Ivoire.

How much alcohol is in the fruit?

Across 21 fruit species, the average alcohol content was about 0.26% by weight. Weighing by how often chimps eat each species, the averages climbed to 0.32% at Ngogo and 0.31% at Taï. 

That sounds tiny, but chimps eat a lot of ripe fruit – roughly three quarters of their diet, and many pounds per day. Small percentages add up.

“Across all sites, male and female chimpanzees are consuming about 14 grams of pure ethanol per day in their diet, which is the equivalent to one standard American drink,” said Maro, a graduate student in the Department of Integrative Biology. 

“When you adjust for body mass, because chimps weigh about 40 kilos versus a typical human at 70 kilos, it goes up to nearly two drinks.”

Do they seek ethanol – or just sugar?

It’s not yet clear whether the chimps seek out ethanol. Riper fruit carries more sugar, which yeasts ferment into alcohol. 

Chimpanzees might be chasing sweetness and calories, with ethanol along for the ride. Or they might use alcohol’s smell as a cue to find richer fruit. Either way, the exposure is steady.

“The chimps are eating 5 to 10% of their body weight a day in ripe fruit, so even low concentrations yield a high daily total – a substantial dosage of alcohol,” said Professor Dudley. 

“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. But if they are preferring riper and/or more sugar-rich fruits, then this is a conservative lower limit for the likely rate of ethanol ingestion.”

Human attraction to alcohol 

Field observers see no obvious drunkenness. Fruit is eaten throughout the day, and a chimp would need to gorge far past comfort to get a buzz. The bigger point is evolutionary. 

“Chimpanzees consume a similar amount of alcohol to what we might if we ate fermented food daily,” Maro said. “Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees.”

Dudley proposed years ago that our taste for alcohol is ancient, shaped by fruit-eating ancestors. Reports since then have piled up. 

Wild spider monkeys consume fermented fruit. Captive aye-ayes and slow lorises prefer higher-alcohol nectar. Even birds show alcohol metabolites in feathers. 

“The consumption of ethanol is not limited to primates,” Dudley said. “It’s more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals.” 

“It just points to the need for additional federal funding for research into alcohol attraction and abuse by modern humans. It likely has a deep evolutionary background,” Dudley said.

Measuring ethanol in the field

Maro visited Ngogo twice and Taï once. He collected intact, fresh fruits from beneath trees that chimps had just foraged, and undamaged fallen fruits at Taï. 

Each sample was sealed, labeled for species, size, color, and softness, then frozen at camp to halt ripening. 

Back in makeshift labs, Maro used three independent approaches: a semiconductor device like a breathalyzer, a portable gas chromatograph, and a chemical color test. All gave comparable readings. 

For two methods, he thawed, peeled, and blended pulp, then let it sit in an airtight container and sampled the headspace gas for ethanol. For the color test, he extracted liquid pulp and used reagents that change hue in proportion to ethanol. 

“I think the strength of Aleksey’s approach is that it used multiple methods,” Dudley said.

In the U.S., a standard drink contains 14 grams of pure ethanol. Much of Europe uses 10 grams. Chimps weigh less than people – about 40 kilograms versus 70 on average – so the same grams per day translate to a higher dose per body weight.

Wild chimpanzees and alcohol

This isn’t cushy lab work. At Ngogo, male chimps often gather to feed in Ficus musuco trees before patrols. At Taï, Parinari excelsa fruit is also a favorite – elephants love it, too. 

To push beyond fruit chemistry, Maro has begun collecting urine from sleeping chimps to test for alcohol metabolites. That means standing under nests at dawn with an umbrella and catching what falls.

The team wants to know whether chimps favor fruit with higher ethanol and how exposure varies through the year. 

They’re testing fresh-dislodged fruits and banking urine for metabolite assays, using kits similar to workplace tests. The aim is simple: link chemistry, diet, and biology in the wild.

The big picture is even simpler. Wild chimps live with a constant trickle of natural alcohol. So did our ancestors. 

That everyday exposure may help explain why fermented foods and drinks feel rewarding to us today – and why understanding that pull, in both biology and culture, still matters.

The study is published in the journal Science Advances.

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