Chimpanzees and Alcohol: What Fermented Fruit Reveals About Humans

In the African forest, a discreet dietary habit has caught scientists’ attention and revived longstanding questions about alcohol consumption.

Researchers have found that wild chimpanzees consume a meaningful amount of ethanol every day from very ripe or fermented fruit. This seemingly ordinary behaviour, repeated over millions of years, may help explain why people in so many cultures are so drawn to alcoholic drinks.

Overripe fruit that contains alcohol

When alcohol is mentioned, the usual images are obvious: a cold beer, a glass of wine or a weekend cocktail. Yet ethanol, the only type of alcohol routinely consumed by humans, is not confined to bottles. It also occurs in small quantities in everyday foods.

Sourdough bread, kombucha and even very ripe fruit release ethanol as they ferment. These amounts are generally low and do not produce any feeling of intoxication. Nevertheless, the chemical process is the same as in alcoholic drinks: microorganisms convert sugars into alcohol and carbon dioxide.

In the wild, spontaneous fermentation has devoted consumers: chimpanzees. In African forests, they have long eaten fruit that has fallen to the ground, often when it is already at an advanced stage of ripeness. These are precisely the fruits with the highest ethanol concentrations.

“For chimpanzees, the sweet smell and taste of fermented fruit seem to act as a constant invitation to eat.”

The equivalent of half a pint of beer a day

A study published on 17 September 2025 in the journal Science Advances examined the ethanol content of fruit eaten by chimpanzees in two tropical forest locations: Kibale National Park in Uganda and Taï National Park in Côte d’Ivoire.

Scientists measured the amount of ethanol in fruit lying on the ground and compared those figures with the daily volume of food consumed by the animals. The findings are striking: chimpanzees can eat around 4.5 kg of fermented fruit each day.

That quantity of food contains an average of 14 grams of pure ethanol. In human terms, international media reported that this is roughly equivalent to half a pint of 5% lager.

“Yes: a chimpanzee in the forest consumes, on an ordinary day, something comparable to a medium glass of beer.”

According to Robert Dudley, a biologist at the University of California, Berkeley, even low alcohol concentrations in fruit produce a considerable cumulative dose because these animals eat ripe fruit amounting to 5% to 10% of their body weight every day.

Why chimpanzees do not appear “drunk”

Despite their daily ethanol intake, chimpanzees observed in the wild showed no obvious signs of intoxication. There was no stumbling, unusual aggression or extreme drowsiness associated with alcohol.

Researchers estimate that, for a chimpanzee to seem clearly “drunk”, it would need to consume an extraordinary quantity of fruit, enough for its abdomen to become visibly swollen. This points to a significant physiological tolerance of alcohol.

Other species have also been observed actively seeking alcoholic drinks. Slow lorises, small nocturnal primates from South and South-East Asia, are known to lick liquids with a high alcohol content when they find an available source, such as leftover human drinks.

Possible benefits of eating fermented fruit

A preference for very ripe fruit does not appear to be random. Fruit at this stage:

• tends to contain more available sugar and therefore more calories;
• gives off a strong scent, helping animals locate it in dense forest;
• falls to the ground more often, making it easier to access;
• may signal a concentrated energy resource that requires less effort to find.

In this context, ethanol may act as a chemical marker for highly energy-rich food. Animals attracted by that smell and flavour would therefore have a better chance of finding calorie-dense fruit and surviving in competitive environments.

What chimpanzees reveal about our relationship with alcohol

Based on these findings, the study’s authors raise a provocative possibility: part of the human attraction to alcoholic drinks may have deep evolutionary roots shared with other primates.

According to the research team, our interest in alcohol may be a “biological echo” of a past in which following the smell of ethanol provided access to nutritious fruit.

The concept is straightforward. Over millions of years, primates that sought fruit with a slight alcohol content may have gained a small advantage. They found more calorie-rich food and obtained extra energy to survive, reproduce and care for their young. This sensory preference may have been subtly inherited by humans.

The difference is that we no longer need to climb trees or search the forest floor. We only need to open the fridge. Industry concentrates ethanol into bottles, cans and cocktails, increasing both access and dosage while separating it from the original context of fermented fruit.

From fruit to bottle: a change of context

When a chimpanzee consumes ethanol, it takes it in alongside fruit pulp, fibre, water and nutrients. The situation is different for humans. Alcoholic drinks provide alcohol in a much more concentrated form, with rapid digestion and faster absorption.

This distinction helps explain why a behaviour that may have been adaptive in the forest can become a risk for harmful use, dependence and illnesses linked to excessive alcohol consumption in modern societies.

Concepts that help explain the study

Two terms frequently arise in this discussion: ethanol and fermentation.

Ethanol is the type of alcohol found in beer, wine and spirits. At low doses, the human body can metabolise it relatively efficiently. In excess, it places strain on the liver, brain, heart and other organs.

Fermentation, meanwhile, is the process in which yeasts and some bacteria turn sugars into alcohol and gas. It occurs both in beer vats and in a cluster of fruit left on a tree or on the forest floor.

In chimpanzee studies, researchers measure ethanol concentrations in fruit to estimate how much alcohol is consumed per kilogram of food. They then calculate approximate comparisons with human drinks, such as the half-pint-of-beer comparison.

What this story says about our habits today

The evidence on chimpanzees is not an excuse for uncontrolled alcohol consumption, but it does provide context for why so many people feel an almost instinctive pull towards alcoholic drinks despite knowing their risks.

One practical use of this knowledge is to notice situations in which drinking seems “automatic”: social gatherings, stress relief and celebrations. Part of this impulse may be connected to ancient brain mechanisms that seek reward and calories, now operating in an environment where ethanol is abundant and highly concentrated.

Scientists are also considering whether this kind of research could inform public health policy. If attraction to alcohol has a deep biological component, prevention strategies could focus less on individual blame and more on redesigning environments: fewer constant prompts, more enjoyable alcohol-free alternatives and clear information about safe limits.

From the forests of Kibale to bars in major cities, the link between fermented fruit and sparkling glasses is long, but perhaps less tenuous than it appears. Understanding how chimpanzees deal with alcohol every day sheds light on our own choices - and on how an evolutionary trait that once seemed beneficial can become a public-health challenge when the context changes radically.