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

At the heart of the study lies detailed observational data tracking the feeding behavior of African great apes across multiple populations and habitats. Contrary to prior assumptions that primates generally avoid fermenting fruit due to alcohol’s potentially toxic effects, the researchers found consistent evidence that chimpanzees, gorillas, and bonobos frequently consume fruits that have fallen and fermented beneath the canopy. Importantly, orangutans did not display this behavior with any regularity, an interspecies difference that aligns closely with genetic discrepancies in the capacity to metabolize ethanol. Such findings provide a direct behavioral correlate to genetic adaptations observed in these species.

Ethanol, a naturally occurring byproduct of ripe and overripe fruits, accumulates as sugars ferment in warm, humid environments like tropical forests. While previous ecological studies have documented trace amounts of this compound in fruit consumed by various animals, the quantification of scrumping behavior required innovative ethological approaches. The research team employed extensive field observations combined with biochemical assays to ascertain not just the presence of ethanol in consumed fruits but also the frequency and social contexts of its ingestion by multiple ape species. This multi-disciplinary methodology has allowed the unveiling of fermented fruit consumption as an integral, evolutionarily relevant dietary source.

One of the key conceptual breakthroughs in this study lies in the linguistic and cultural framing of the feeding behavior. The team revived and repurposed the archaic English term ‘scrumping,’ generally understood as the act of stealing or gathering windfallen fruit. Intriguingly, this term traces its etymology to the Middle Low German noun ‘schrimpen,’ historically used to describe overripe or fermented fruit. This etymological connection serves as an elegant metaphor for the interplay between observed primate behavior and human cultural practices surrounding fermented fruit and alcohol consumption. It underscores how language and biology can reflect and amplify evolutionary insights.

Beyond simple feeding habits, scrumping appears to be tightly interwoven with the social fabric of these primate communities. Earlier studies have suggested that chimpanzees particularly engage in communal feeding on spoiled or fermented fruits, suggesting a social component to alcohol consumption that may parallel human sociocultural dynamics involving communal drinking. By dissecting the frequency and social contexts of scrumping, the current research opens compelling avenues to explore how early hominids might have developed complex social rituals around fermented substances, forming a potential primordial ritualistic or bonding behavior antecedent to human feasting.

Genetic analyses complement the behavioral data, showing that African great apes harbor a mutation conferring markedly enhanced enzymatic efficiency in metabolizing ethanol—an ability over 40 times greater than that found in orangutans and many other primates. This enzymatic superiority suggests that the evolutionary pressures imposed by scrumping shaped this physiological adaptation, enabling these apes to exploit a resource fraught with both nutritional opportunities and risks. By directly linking behavioral ecology and molecular genetics, the study bridges micro and macroevolutionary perspectives on alcohol consumption.

The implications of these findings are far-reaching. They shed light not only on the dietary ecology of great apes but also provide critical insight into human evolutionary biology, specifically the longstanding fascination and capability of our species to metabolize and socially consume alcohol. More than an abstract biological curiosity, this research suggests that our ancestors’ early interactions with fermented fruits could have laid the biochemical and cultural groundwork for the development of early alcoholic beverages and, by extension, the cultural phenomenon of feasting and communal drinking.

Furthermore, the research revitalizes a visual and historical connection by highlighting gothic art representations of primates engaged in fruit-gathering activities beneath trees, drawn centuries before modern behavioral ecology emerged. This “life imitating art imitating life” narrative offers an intriguing cultural dimension, suggesting that observations of such behaviors have long permeated human consciousness, albeit implicitly, through artistic expression. This interdisciplinary intersection enriches our interpretation of primate behavior within a broader human context.

The research methodology itself, grounded in comprehensive and longitudinal field observations spanning multiple ape communities and habitats, ensures robustness and replicability. The team’s innovative integration of behavioral data with genetic information marks a new frontier in primatology, one where ecological variables and molecular mechanisms can be simultaneously considered to paint a fuller picture of evolutionary trajectories. This is particularly significant given the challenges inherent in observing and quantifying sporadic behaviors like scrumping in wild populations.

One of the central challenges highlighted by co-author Nathaniel Dominy, Charles Hansen Professor of Anthropology at Dartmouth, was the absence of an existing term to describe this precise feeding behavior on fermented, fallen fruit. The unavailability of a linguistic framework initially contributed to the behavior being underreported and understudied. By coining and systematically employing ‘scrumping,’ the researchers not only fill a semantic gap but also create a conceptual tool that facilitates more focused future research on this overlooked ecological niche.

Looking ahead, co-lead author Professor Catherine Hobaiter from the University of St Andrews notes that the next critical step involves investigating the role of scrumping in mediating social relationships among non-human apes. Given that human alcohol consumption is deeply enmeshed with social bonding and ritualistic practices, understanding if and how similar social dynamics operate in great apes could offer profound insights into the evolutionary origins of human sociocultural behaviors tied to alcohol. Such research promises to open new behavioral and neurobiological inquiries.

Finally, the study’s narrative poignantly reminds us that our contemporary social customs—whether enjoying a shared cold pint of scrumpy or engaging in festive communal drinking—may echo behaviors that emerged in our ape ancestors as many as 10 million years ago. This perspective imbues modern cultural practices with deep evolutionary significance, rooting seemingly mundane activities within an ancient biological and social continuum. In these revelations lies a testament to the power of interdisciplinary science in uncovering hidden connections between biology, culture, and evolutionary history.

Subject of Research: Animals
Article Title: Fermented Fruits: scrumping, sharing, and the origin of feasting
News Publication Date: 31-Jul-2025
Web References:

• https://academic.oup.com/bioscience
• http://dx.doi.org/10.1093/biosci/biaf102
• https://www.sciencedirect.com/science/article/pii/S0960982225002817
  Image Credits: Credit: Catherine Hobaiter
  Keywords: Evolutionary biology