The first-ever measurements of the ethanol content of fruits available to chimpanzees in their native African habitat show that the animals could easily consume the equivalent of more than two standard alcoholic drinks each day, according to researchers at the University of California, Berkeley.
It's not clear whether they actively seek out fruit with high ethanol levels, which are typically riper fruit with more sugars to ferment. But the availability of ethanol in many species of fruit that they normally eat suggests that alcohol is a regular part of their diet and likely was a part of the diets of our human ancestors.
"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.
The 21 species of fruit Maro sampled at two chimp study sites - Ngogo in Uganda and Taï in Ivory Coast - had an average alcohol content of 0.26% by weight. Primatologists who have studied chimps at these sites estimate that the animals consume about 10 pounds (4.5 kilograms) of fruit per day, on average, and that fruit makes up about three-quarters of their diet. The researchers also have recorded for each site the approximate proportion of each fruit species in the chimp diet. This information allowed the Berkeley biologists to calculate an average rate of dietary ethanol consumption.
"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."
Fruit consumption occurs throughout the day and the chimps show no overt signs of intoxication, Maro said. In fact, to get a buzz on, a chimp would have to eat so much fruit its stomach would bloat. But chronic low-level exposure suggests that the common ancestors of humans and chimps - our closest living relative among the apes - were also exposed daily to alcohol from fermenting fruit, a nutrient that is missing from the diets of captive chimps and many humans today.
"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 paper about the study that appeared today (Sept. 17) in the journal Science Advances.
The 'drunken monkey' hypothesis
Dudley first began to suspect more than 20 years ago that the human appetite for alcohol was inherited from our primate ancestors, and wrote a 2014 book about his theory: The Drunken Monkey: Why We Drink and Abuse Alcohol. This "drunken monkey" hypothesis drew skepticism from many scientists - particularly those who study primates - who told him that chimps and other primates don't eat fermented fruit or nectar. These nutrients typically contain alcohol produced by yeast metabolizing sugar, just as yeast ferments sugary grape juice into wine.
Aaron Sandel
But over the years, Dudley's theory has gained an increasing number of adherents. More primatologists now report seeing monkeys and apes eating fermented fruit, a practice that was recorded earlier this year among chimps in Guinea-Bissau. Researchers also have published papers about captive primates' preferences for alcohol. Dartmouth University researchers in 2016 reported that when captive aye-ayes and slow lorises were offered nectar with varying percentages of alcohol, they finished off nectar with the highest alcohol content first - and then repeatedly revisited the empty high-alcohol containers as if they wanted more. In 2022, Dudley collaborated with researchers in Panama to document that spider monkeys consume alcohol-laden fermented fruit in the wild and express alcohol metabolites in their urine.
It's not only mammals that get a daily dose of alcohol from their diet. In a paper published earlier this year, Dudley and his Berkeley colleagues reported that the feathers from 10 of 17 bird species tested contained secondary metabolites of alcohol, indicating that their diet - nectar, grain, insects and even other vertebrates - included substantial amounts of ethanol.
"The consumption of ethanol is not limited to primates," said Dudley, who is also a research associate at the Smithsonian Tropical Research Institute in Panama. "It's more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals."
He said that one theory about why animals seek out ethanol is that its odor helps animals find food with a higher sugar content, providing greater energy returns over time. Alcohol also may increase the pleasure of eating, similar to sipping wine with dinner. It's also possible that sharing alcohol-infused fruit plays a role in social bonding among primates or other 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.
Collecting urine samples - with an umbrella as protection
Beginning in 2019, Maro made two trips to Ngogo in Uganda's Kibale National Park and one to Taï National Park in Côte d'Ivoire. At Ngogo, which hosts the largest chimpanzee social group in Africa, the chimps climb trees to pluck fruits and prefer several varieties of figs. Maro and colleagues at Ngogo collected undamaged, freshly fallen fruits from the ground under trees that had recently been foraged by chimps. At Taï, where chimps typically eat fallen fruit, the team collected undamaged and unnibbled fruit from the ground under trees.
Aleksey Maro/UC Berkeley
Each sample was packed in an airtight container, the species, size, color and softness were recorded, and, once back at base camp, frozen to prevent further ripening. To test for alcohol content, Maro used different methods on each of the three field trips: a semiconductor-based device similar to a breathalyzer, a portable gas chromatograph and a chemical test. All recorded similar alcohol percentages. He tested each method in advance in Dudley's Berkeley lab using a standard procedure that could easily be replicated in the field, where he typically processed 20 samples in a 12-hour day.
Two of the procedures required thawing the fruit, removing the rind and seeds, blending the pulp, and letting it sit in an airtight container for a couple of hours to release alcohol. Air in the box, or "headspace," was then extracted for analysis of alcohol content. A third procedure involved extracting the liquid part of the pulp and using color-changing chemicals that react with ethanol.
Weighted by the proportion of time chimps eat each type of fruit, the average alcohol content of fruit was 0.32% by weight at Ngogo and 0.31% at Taï. The most frequently consumed fruits at each site - a fig, Ficus musuco, at Ngogo, and the plum-like fruit of the evergreen Parinari excelsa at Taï - were the highest in alcohol content. Troops of male chimpanzees often gather in the canopy of F. musuco trees to consume fruit before going on boundary patrols of their community, Maro noted. And the fruit of P. excelsa is also very popular among elephants, which are known to be attracted 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."
The new study provides the foundation for further studies in chimpanzee reserves to determine how much of the fermented, alcohol-laden fruit is preferentially consumed by chimpanzees. This summer, Maro returned to Ngogo to collect urine samples from chimps sleeping in trees - a fraught endeavor requiring an umbrella - in order to analyze them for alcohol metabolites, using test kits similar to those deployed in some U.S. workplaces. He and team member Laura Clifton Byrne, an undergraduate student at San Francisco State University, also followed chimpanzees to pick fruit freshly dislodged in the canopy and analyzed 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.
