While humans have been drinking alcohol for thousands of years, some scientists think this history could go back millions of years. According to the drunken monkey hypothesis, humans’ taste and indeed preference for alcohol may have evolutionary roots, shaped by our primate ancestors consuming small doses of alcohol found inside ripe fruit. Our bodies may also have evolved to metabolise alcoholic compounds more effectively as a result.
In a new study in Science Advances, researchers at the University of California (UC), Berkeley, analysed ripe fruits eaten by wild chimpanzees, the closest living relatives of humans, to discover that chimpanzees could be ingesting as much alcohol as in one to two glasses of wine every day.
Ripe fruits
Chimpanzees share about 98.8% of their DNA with humans, making them an excellent model for exploring questions about human evolution. Their fruit-heavy diet is also believed to resemble that of our last common ancestor. By estimating the chimpanzees’ daily alcohol intake from these fruits, researchers can gain insights into the levels that may have been ingested by this ancestor as well.
The researchers conducted their studies in two forests in Africa: one in Ngogo in Kibale National Park in Uganda, home to one of the world’s largest known chimpanzee communities; and the other in the Taï National Park in Côte d’Ivoire.
From previous observations, the researchers had already identified the fruit varieties the chimpanzees consumed. They collected ripe fruits from these feeding sites — sometimes plucked directly from branches, at others scooped up from the ground just after they dropped — and tested them in the lab for their alcohol content. Since they also knew the approximate quantity of fruit a single chimpanzee ate, they were able to estimate the amount of alcohol the individual ingested in a single day.
The lab results confirmed that the fruits the chimpanzees consumed did contain small amounts of ethanol. The microbe yeast, which is used to brew beer and wine, is naturally present in fruits. When the fruits ripen, their sugar content increases and the yeast ferments these sugars into alcohol.
‘Nearly two drinks’
The lab results also showed that ripe fruits contained less than 1% alcohol. To compare, a pint of beer (473 ml) typically contains 4-6% alcohol by volume. However, unlike humans, a large portion of a chimpanzee’s diet consists of fruits: nearly three-quarters of their intake, amounting to around 4.5 kg per day.
“The chimpanzees are eating 5-10% of their body weight in a day in ripe fruit, so even low concentrations yield a high daily total, a substantial dosage of alcohol,” Robert Dudley, an evolutionary physiologist at UC Berkeley, and coauthor of the study, said.
Researchers calculated that the total alcohol intake from consuming fruits amounted to roughly 14 grams — equivalent to a glass of wine, or two when adjusted for a chimpanzee’s body weight.
“Across all sites, male and female chimpanzees are consuming about 14 grams of pure ethanol per day in their diet, which is equal to one standard American drink,” Alexsey Maro, a PhD researcher at UC Berkeley, and the first author of the paper. “When you adjust for body mass, because chimps weigh almost 40 kg versus a typical human at 70 kg, it goes up to nearly two drinks.”
‘An evolved attraction’
Among the 21 fruit varieties the researchers identified in the chimpanzees’ diet, the primates showed a strong preference for sticky figs (Ficus mucuso). Only 75 of these figs, which are abundant and sugary, can contain as much as 10 grams of alcohol — a significant amount when consumed in a single bout.
However, despite consuming as much as 14 g of alcohol in a day, the chimpanzees didn’t display any obvious signs of intoxication. Researchers suggested this was because they didn’t imbibe the alcohol in one go but spread it throughout the day.
The researchers also said such low-level exposure to natural alcohol may have influenced key physiological adaptations.
“Our research shows that chimpanzees in Cantanchez National Park in Guinea-Bissau share fermented fruits with each other and use leaf tools to access fermented palm wine harvested by humans, indicating an evolved attraction and tolerance to ethanol,” Kimberley Hockings, a primatologist at the University of Exeter who has been studying apes in West Africa, said. “These findings suggest that an attraction to ethanol is deeply embedded in animal and human evolutionary history.”
This interpretation is aligned with research by Matthew Carrigan, an associate professor of biology at College of Central Florida, USA. His work has concluded that about 10 million years ago, the last common ancestor of living African apes and modern humans experienced a genetic mutation that increased the rate of ethanol metabolism 40-fold.
Cue for ripeness
The UC Berkeley study also raises a few questions. Do chimpanzees actively prefer fruits with higher alcohol content, or are they simply attracted to the sweetness of ripe fruits?
Dr. Dudley, who first proposed the drunken monkey hypothesis in 2014, has theorised that alcohol in fruits could serve as a cue for ripeness — a signal of extra calories and perhaps enhanced taste that adds to the pleasure of eating.
While previous studies have documented alcohol use in chimpanzees, the UC Berkeley effort is the first to measure the alcohol content of the fruits they consume.
Tetsuro Matsuzawa, former director of the Primate Research Institute at Kyoto University, Japan, said, “These observations contribute to a growing body of evidence for the drunken monkey hypothesis. From tool-assisted palm wine drinking to the spontaneous consumption of fermented fruits, wild chimpanzees consistently show behaviours that suggest an evolved attraction to ethanol.”
Ipsita Herlekar is an independent science writer.
Published – October 22, 2025 05:30 am IST
