Wild chimps consume more alcohol than anyone expected

It is still unknown whether chimpanzees deliberately choose fruits with higher ethanol levels, which tend to be riper and richer in sugar that can ferment. However, many of the fruit species they regularly eat contain measurable ethanol, indicating that alcohol is a routine part of their menu and was probably present in the diets of our human ancestors as well.

“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 UC Berkeley graduate student Aleksey Maro of 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.”

A “standard drink” in the U.S. contains 14 grams of ethanol, irrespective of the consumer’s body size, although in much of Europe the standard is 10 grams.

Measuring ethanol in wild fruits

Maro analyzed 21 different fruit species eaten by chimps at two long-term research sites, Ngogo in Uganda and Taï in Ivory Coast. On average, these fruits contained 0.26% alcohol by weight. Primatologists working at these locations estimate that chimps typically eat about 10 pounds (4.5 kilograms) of fruit per day and that fruit accounts for roughly three quarters of their total food intake. Researchers have also estimated how much each fruit species contributes to the overall diet at each site, which allowed the Berkeley team to calculate an average daily intake of ethanol from food.

“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 Robert Dudley, UC Berkeley professor of integrative biology. “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.”

Low-level alcohol exposure and evolutionary clues

According to Maro, chimps feed on fruit throughout the day and do not appear visibly drunk. To actually feel intoxicated, a chimp would need to eat so much fruit that its stomach would become painfully distended. Even so, this steady, low-level intake of ethanol implies that the last common ancestor of humans and chimpanzees, our closest living relatives among the apes, probably encountered alcohol every day from fermenting fruit. That nutrient is largely absent from the diets of captive chimps and from many modern human diets.

“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.”

Maro is first author and Dudley is senior author of a peer-reviewed paper describing these findings that is published in the journal Science Advances.

The ‘drunken monkey’ hypothesis

More than two decades ago, Dudley proposed that humans’ interest in alcohol has deep roots in primate evolution and stems from ancient foraging habits. He later expanded this idea in his 2014 book The Drunken Monkey: Why We Drink and Abuse Alcohol. His “drunken monkey” hypothesis initially met resistance from many scientists, especially primatologists, who argued that primates in the wild do not commonly eat fermented fruits or nectar. These foods contain alcohol produced when yeast digests sugars, in much the same way that yeast turns grape juice into wine.

Over time, however, observational and experimental evidence has increasingly supported Dudley’s view. More field researchers now report that monkeys and apes do in fact eat fermented fruit, including a recent observation of chimpanzees in Guinea-Bissau. Other studies, conducted with animals in captivity, have shown that some primates actively favor alcohol. In 2016, for example, Dartmouth University researchers found that captive aye-ayes and slow lorises given nectar with different alcohol levels tended to finish the most alcoholic nectar first and then repeatedly return to those empty containers. In 2022, Dudley worked with collaborators in Panama to show that wild spider monkeys consume fermented fruit containing alcohol and later excrete alcohol metabolites in their urine.

Alcohol in the diets of many animals

Mammals are not the only creatures that take in alcohol as part of normal feeding. In a study published earlier this year, Dudley and colleagues at Berkeley analyzed feathers from 17 bird species and found alcohol metabolites in 10 of them. This indicates that their diet — nectar, grain, insects and even other vertebrates — contained significant amounts of ethanol.

“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.”

He noted that one idea about why animals might seek out ethanol is that its smell helps them locate foods that are richer in sugar and thus provide more energy. Alcohol may also make eating feel more rewarding, in a way that resembles sipping wine with a meal. Another possibility is that sharing fruit containing alcohol contributes to social bonding within primate groups or among other species.

“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.

Fieldwork in African forests

Beginning in 2019, Maro conducted two field seasons at Ngogo in Uganda’s Kibale National Park and one season at Taï National Park in Côte d’Ivoire. Ngogo is home to the largest known chimpanzee community in Africa. There, chimps climb into the trees to harvest fruits and often prefer several types of figs. Maro and his colleagues collected intact, freshly fallen fruits lying beneath trees where chimps had recently been feeding. At Taï, where chimps more often eat fruit that has fallen, the team similarly gathered undamaged and unbitten fruits from the ground below fruiting trees.

Each fruit sample was sealed in an airtight container, and the team recorded details including species, size, color and softness. Back at base camp, the fruits were frozen to halt further ripening. To determine alcohol content, Maro applied three different techniques across his field trips: a semiconductor-based sensor similar to a breathalyzer, a portable gas chromatograph and a chemical assay. All three methods produced consistent alcohol readings. Before heading to the field, Maro validated each technique in Dudley’s Berkeley laboratory using a standardized protocol that could easily be reproduced under field conditions, where he often processed about 20 samples in a 12 hour day.

Two of the methods involved thawing the fruit, removing the peel and seeds, blending the pulp and then letting it sit in a sealed container for a couple of hours so that alcohol could move into the air above the pulp. This air, or “headspace,” was then sampled and analyzed for ethanol content. The third method extracted liquid from the pulp and used color changing chemicals that react to ethanol.

Alcohol rich fruits and what chimps choose to eat

When the alcohol content of the fruits was averaged and weighted according to how often chimps eat each species, the numbers came out to 0.32% by weight at Ngogo and 0.31% at Taï. The fruits that chimps consume most frequently at each site, a fig called Ficus musuco at Ngogo and the plum like fruit of the evergreen Parinari excelsa at Taï, were also the most alcohol rich. Maro noted that groups of male chimpanzees often gather high in the canopy of F. musuco trees to eat fruit before heading out on patrols along the borders of their territory. The fruits of P. excelsa are also a favorite of elephants, which are known to be drawn to alcohol.

“I think the strength of Aleksey’s approach is that it used multiple methods,” Dudley said. “One of the reasons this has been a tempting target but no one’s gone after it is because it’s so hard to do in a field site where there are wild primates eating known fruits. This dataset has not existed before, and it has been a contentious issue.”

Next steps in tracking chimpanzee alcohol exposure

The new research establishes a baseline for future projects in chimpanzee reserves that aim to determine how often chimps select fermented, alcohol containing fruits over less fermented options. During the following summer, Maro returned to Ngogo to collect urine from chimps while they slept in trees, a difficult task that required an umbrella, so that he could test for alcohol metabolites using kits similar to those used in some U.S. workplaces. Along with team member Laura Clifton Byrne, an undergraduate at San Francisco State University, he also shadowed foraging chimpanzees, retrieving freshly dislodged fruits from beneath the canopy and measuring their alcohol content.

Co-authors of the paper are Aaron Sandel of the University of Texas, Austin; Bi Z. A. Blaiore and Roman Wittig of the Taï Chimpanzee Project; and John Mitani of the University of Michigan, Ann Arbor, one of the founders of the Ngogo Chimpanzee Project. The work was funded by UC Berkeley.