In the dense canopies of Africa’s forests, a remarkable discovery has emerged that challenges long-held assumptions about primate diets and the evolutionary roots of human alcohol consumption. Researchers at the University of California, Berkeley, have uncovered compelling evidence that wild chimpanzees routinely ingest ethanol through their natural diet, an insight that could illuminate the ancient origins of humanity’s complex relationship with alcohol. This groundbreaking study, soon to be published in Science Advances, demonstrates for the first time quantified ethanol levels in the fruits that chimpanzees consume daily, revealing that these primates may ingest the equivalent of nearly two standard alcoholic drinks per day when adjusting for body weight.
Scientists have long speculated about the presence of naturally fermented fruit in the diets of primates, but direct measurements remained elusive until Aleksey Maro and colleagues embarked on extensive fieldwork at two distinct African sites: Ngogo in Uganda’s Kibale National Park and Taï National Park in Côte d’Ivoire. Utilizing a combination of innovative methodologies—including portable gas chromatography, semiconductor devices akin to breathalyzers, and colorimetric chemical assays—the team was able to accurately gauge ethanol content in freshly foraged fruits. These multiple analytical approaches, validated in a controlled laboratory setting, provided a robust dataset highlighting a consistent presence of ethanol across a variety of fruit species.
The average ethanol concentration in the fruit species sampled was approximately 0.26% by weight, a seemingly modest figure that belies the significant cumulative exposure chimpanzees face. Given that these apes consume large quantities of fruit—often between 5 to 10% of their body weight daily—the total ethanol ingested accumulates to a biologically relevant dose. This translates to about 14 grams of pure ethanol each day, equivalent to one standard U.S. alcoholic drink for an average human, or close to two drinks when accounting for the lower body mass of chimpanzees compared to humans. These findings suggest a pervasive and chronic exposure to dietary alcohol that has likely persisted for millions of years within the evolutionary lineage that humans share with chimpanzees.
Intriguingly, the study also sheds light on the “drunken monkey hypothesis,” first posed more than two decades ago by UC Berkeley professor Robert Dudley. This theory postulates that humans’ innate attraction to alcohol may be an inherited trait, deeply rooted in ancestral frugivory—a dietary habit involving the consumption of fermenting fruits laden with natural alcohol. Despite initial skepticism from primatologists, ongoing observations and additional research have increasingly supported Dudley’s proposition. For instance, a recent video captured chimpanzees actively consuming fermented fruits in Guinea-Bissau, while parallel studies on captive prosimians indicate a strong preference for nectar with higher alcohol content, demonstrating a clear interest in naturally fermented sources.
From an evolutionary perspective, the presence of ethanol in the diet of wild chimpanzees and other fruit-eating animals is far from trivial. Ethanol itself is a secondary metabolite produced as yeast ferments sugars in ripening fruit. It also serves a nuanced ecological role by signaling the presence of energy-rich, sugar-laden food sources. The olfactory detection of ethanol may help animals identify optimal feeding opportunities, granting an adaptive advantage. Additionally, the mild psychoactive effects of ethanol might enhance the palatability of food or play a subtle role in social dynamics among primates, potentially facilitating bonding through shared foraging experiences.
The methodology employed by Maro and colleagues underscores the challenges of conducting rigorous biochemical analyses in remote field settings. Collecting undamaged fruit samples from beneath feeding trees required precise timing and care to prevent contamination or further fermentation post-collection. Samples were rapidly frozen at base camps to preserve their chemical integrity until analysis. Processing varied with each field trip to accommodate different equipment: thawing the fruit, homogenizing the pulp, extracting ethanol-rich headspace air, and conducting sensitive color reactions. This multipronged approach not only ensured reliable quantification but also established a replicable protocol for future studies aiming to unravel the complexities of ethanol exposure in wild primates.
Significant variation was noted in ethanol content across different fruit species and sites. At Ngogo, the fig species Ficus musuco emerged as a particularly ethanol-rich fruit, favored by male chimpanzees who often maximize their intake prior to territorial boundary patrols. Meanwhile, at Taï, the grape-like fruit of the evergreen Parinari excelsa exhibited high alcohol concentrations and attracted a range of wildlife, including elephants, known for their affinity for fermented food. These site-specific fruit preferences and ethanol levels suggest that chimpanzees might be selectively targeting ripe, sugar-rich fruits, although definitive behavioral evidence remains to be established.
Beyond direct dietary assessment, the Berkeley team has pursued biochemical verification of ethanol exposure through the collection of chimpanzee urine samples using innovative non-invasive techniques—an exhaustive process that required careful timing, patience, and even umbrellas to protect samples from contamination. Early results indicate the presence of alcohol metabolites, confirming that dietary ethanol is indeed metabolized by wild chimpanzees. Such physiological markers complement the fruit analyses and robustly document that these primates chronically process dietary alcohol without evident signs of intoxication or adverse health effects.
This pioneering research opens new avenues for understanding the evolutionary interplay between diet, metabolism, and behavior in primates. It challenges the traditional view that alcohol consumption is a uniquely human cultural phenomenon by framing it instead as an inherited biological trait with deep evolutionary roots. The consistent exposure of our closest living relatives to low doses of alcohol from natural sources implies that ancient hominins likely faced similar environmental pressures, potentially influencing the genetic and neurological pathways governing alcohol perception and preference.
Moreover, the implications extend beyond primatology into broader fields such as evolutionary biology, neuroethology, and addiction research. Recognizing that ethanol ingestion is a widespread and natural aspect of frugivorous diets among diverse animal taxa—from primates to nectar-feeding birds—provides a novel ecological context for studying the adaptive significance of alcohol. It also prompts reconsideration of how modern human alcohol use and misuse might be framed within this evolutionary backdrop, emphasizing the role of ancestral dietary patterns in shaping contemporary physiological and behavioral responses.
The quest to unravel this complex relationship continues, with future studies aimed at clarifying whether chimpanzees intentionally seek out fermented fruits or consume ethanol incidentally while foraging indiscriminately. The Berkeley team is also investigating the potential social and nutritional benefits of ethanol ingestion among chimpanzee communities, as well as exploring corresponding genetic adaptations. Ultimately, these interdisciplinary efforts promise to advance our understanding of alcohol’s role in natural ecosystems and human evolution alike.
As this research garners attention, it underscores the value of field-based biochemical investigations combined with observational primatology. Such integrative approaches are essential for addressing contentious questions about animal behavior and evolutionary biology that laboratory studies alone cannot answer. By documenting ethanol ingestion in wild chimpanzees, UC Berkeley scientists have illuminated a hidden facet of primate ecology and opened a window into the ancient origins of one of humanity’s most culturally and socially influential substances.
Subject of Research: Animals
Article Title: Ethanol ingestion via frugivory in wild chimpanzees
News Publication Date: 17-Sep-2025
Web References: http://dx.doi.org/10.1126/sciadv.adw1665
References: Science Advances, DOI 10.1126/sciadv.adw1665
Image Credits: Aleksey Maro/UC Berkeley
Keywords: ethanol ingestion, chimpanzees, frugivory, fermented fruit, evolutionary biology, drunken monkey hypothesis, primate diet, alcohol metabolism, natural fermentation, behavioral ecology, UC Berkeley, ape evolution
