The groundbreaking discovery that chimpanzees in their natural African habitats regularly consume alcohol from fermented fruits has been revealed through the first-ever direct measurements of ethanol content in their wild diet. Researchers at the University of California, Berkeley, have found that these primates could be ingesting the equivalent of more than two standard alcoholic drinks daily, a finding that has significant implications for our understanding of primate evolution, human origins, and the deep-seated attraction to alcohol.
This extensive study, published in the esteemed journal Science Advances, meticulously analyzed the ethanol levels in 21 different fruit species consumed by chimpanzees at two long-term research sites: Ngogo in Uganda and Taï in Ivory Coast. The findings suggest that alcohol is not merely an occasional dietary component but a routine element of the chimpanzee menu, a pattern likely mirrored in the diets of our distant human ancestors.
A Daily Dose of Alcohol: Quantifying Chimpanzee Consumption
Aleksey Maro, a graduate student in UC Berkeley’s Department of Integrative Biology and the study’s lead author, explained the significance of the findings. "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," Maro stated. He further elaborated that when adjusted for body mass – considering chimpanzees weigh approximately 40 kilograms compared to a typical human at 70 kilograms – this intake escalates to nearly two standard drinks.
It is crucial to note that a "standard drink" in the U.S. is defined as containing 14 grams of ethanol, irrespective of the consumer’s body size, a benchmark used for comparison. In contrast, much of Europe defines a standard drink as 10 grams of ethanol.
The research team’s methodology involved meticulous fieldwork conducted between 2019 and the following summer. Maro undertook two field seasons at Ngogo in Uganda’s Kibale National Park and one at Taï National Park in Ivory Coast. Ngogo, renowned for hosting Africa’s largest known chimpanzee community, provided an ideal setting. There, chimpanzees are observed climbing trees to harvest fruits, with a particular preference for several fig varieties. Maro and his colleagues collected freshly fallen fruits from beneath trees where chimpanzees had recently fed, ensuring the samples were intact and representative of recent consumption. At Taï, where fruit is more frequently consumed after falling, similar collection methods were employed.
Each fruit sample was immediately sealed in an airtight container to preserve its state. Researchers meticulously recorded details such as species, size, color, and softness. Upon returning to their base camp, the fruits were frozen to halt any further ripening or fermentation processes.
To quantify the alcohol content, Maro employed three distinct analytical techniques during his field expeditions: a semiconductor-based sensor, analogous to a breathalyzer; a portable gas chromatograph; and a chemical assay. All three methods consistently yielded comparable ethanol readings. Prior to fieldwork, Maro rigorously validated each technique in Professor Robert Dudley’s laboratory at UC Berkeley, employing a standardized protocol easily reproducible under challenging field conditions. During his field expeditions, Maro was capable of processing approximately 20 samples within a 12-hour day.
Two of the employed methods involved thawing the fruit, carefully removing the peel and seeds, blending the pulp, and then allowing it to rest in a sealed container for several hours. This resting period facilitated the evaporation of alcohol into the air above the pulp, creating a "headspace" that was then analyzed for ethanol concentration. The third method involved extracting liquid from the pulp and utilizing color-changing chemicals that react specifically to ethanol.
The "Drunken Monkey" Hypothesis Gains Traction
This research provides compelling empirical support for the "drunken monkey" hypothesis, first proposed by Professor Robert Dudley more than two decades ago. Dudley hypothesized that humans’ enduring attraction to alcohol has deep evolutionary roots, stemming from ancient foraging habits of our primate ancestors. His hypothesis, detailed in his 2014 book, The Drunken Monkey: Why We Drink and Abuse Alcohol, initially faced skepticism, particularly from primatologists who questioned the prevalence of fermented fruits and nectar in wild primate diets.
However, accumulating observational and experimental evidence has steadily bolstered Dudley’s theory. Field researchers have increasingly reported instances of monkeys and apes consuming fermented fruits. Notably, a recent observation documented chimpanzees in Guinea-Bissau engaging in this behavior. Furthermore, captive animal studies have demonstrated a clear preference for alcohol. For example, a 2016 study by Dartmouth University researchers found that captive aye-ayes and slow lorises actively chose nectar with higher alcohol levels and repeatedly returned to containers that had held the most alcoholic mixtures. More recently, in 2022, Dudley collaborated with researchers in Panama to confirm that wild spider monkeys consume fermented fruit and subsequently excrete alcohol metabolites in their urine, providing physiological evidence of alcohol intake.
Alcohol as a Routine Dietary Component
The average alcohol content across the 21 analyzed fruit species was found to be 0.26% by weight. When these figures were weighted according to the frequency with which chimpanzees consume each fruit species, the daily average intake of ethanol registered at 0.32% by weight in Ngogo and 0.31% in Taï.
Significantly, the fruits that chimps consume most frequently at each site also happened to be the most alcohol-rich. At Ngogo, this was a fig species, Ficus musuco, while at Taï, it was the plum-like fruit of the evergreen tree Parinari excelsa. Maro observed that groups of male chimpanzees often congregate high in the canopy of F. musuco trees to consume these fruits before embarking on territorial patrols. The fruits of P. excelsa are also a known favorite among elephants, animals that are famously drawn to alcohol.
Professor Dudley emphasized the robustness of the research: "I think the strength of Aleksey’s approach is that it used multiple methods. 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."
Primatologists estimate that chimpanzees typically consume around 10 pounds (4.5 kilograms) of fruit daily, with fruit accounting for approximately three-quarters of their total food intake. By estimating the contribution of each fruit species to the overall diet at each research site, the Berkeley team was able to calculate the average daily ethanol intake from their 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," stated Professor Dudley. He added a crucial caveat: "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."
Evolutionary Implications and Human Origins
Despite this steady, low-level exposure to ethanol, chimpanzees do not exhibit visible signs of intoxication. Maro noted that for a chimpanzee to become visibly drunk, it would need to consume an amount of fruit so large that its stomach would become painfully distended. This consistent, albeit sub-intoxicating, intake of alcohol suggests that the last common ancestor of humans and chimpanzees, our closest living relatives, likely encountered alcohol daily from fermenting fruits. This is a stark contrast to the diets of captive chimpanzees and many modern humans, where such nutrient sources are largely absent.
"Chimpanzees consume a similar amount of alcohol to what we might if we ate fermented food daily," Maro observed. "Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees." This perspective suggests that our preference for alcohol is not a recent development but rather an ancient evolutionary inheritance.
The Broader Context: Alcohol in the Animal Kingdom
The presence of alcohol in the diets of fruit-eating animals is not limited to primates. Earlier this year, Dudley and his colleagues at Berkeley analyzed feathers from 17 bird species and detected alcohol metabolites in 10 of them. This finding indicates that their diverse diets – encompassing nectar, grain, insects, and even other vertebrates – contained significant amounts of ethanol.
"The consumption of ethanol is not limited to primates," Professor Dudley commented. "It’s more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals."
Several theories attempt to explain why animals might seek out ethanol. One possibility is that its distinct smell acts as a beacon, guiding them to foods that are richer in sugar and, consequently, provide more energy. Another hypothesis suggests that alcohol may enhance the rewarding experience of eating, akin to the pleasure derived from sipping wine with a meal. A third intriguing prospect is that sharing fruit containing alcohol could foster social bonding within primate groups or across different species.
Professor Dudley concluded by emphasizing the need for further research: "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."
Future Research Directions
The current study lays a critical foundation for future investigations. The next phase of Maro’s research, initiated the summer following the fieldwork, involves returning to Ngogo to collect urine samples from sleeping chimpanzees. This challenging endeavor, requiring the use of an umbrella to collect samples from trees, aims to test for alcohol metabolites using kits similar to those employed in some U.S. workplaces. Alongside Laura Clifton Byrne, an undergraduate at San Francisco State University, Maro also plans to directly observe foraging chimpanzees, collect freshly dislodged fruits from beneath the canopy, and immediately measure their alcohol content in the field.
This ongoing research promises to further unravel the intricate relationship between primates, alcohol, and evolution, offering profound insights into our own species’ complex history with alcohol.
The study’s co-authors include Aaron Sandel from the University of Texas, Austin; Bi Z. A. Blaiore and Roman Wittig from the Taï Chimpanzee Project; and John Mitani from the University of Michigan, Ann Arbor, who is also one of the founders of the Ngogo Chimpanzee Project. The research was supported by funding from UC Berkeley.
