Chimpanzees Regularly Consume Natural Alcohol from Fruit

Wild chimps ingest ethanol daily from fermented fruit, offering insights into primate diets and evolutionary links to human alcohol use.

Wild chimpanzees may be consuming more alcohol than previously thought—without showing signs of intoxication. A recent study found that ripe fruit, which makes up over 70% of their diet, often contains naturally occurring ethanol due to fermentation. Researchers estimate that chimps ingest around 14 grams of ethanol daily, roughly equivalent to two standard human drinks. This intake is spread across hours of foraging, minimizing any noticeable behavioral effects.

Fermented Fruit and Dietary Patterns

The study observed chimpanzees in Uganda’s Kibale National Park and Ivory Coast’s Tai National Park. Across both sites, researchers sampled 21 fruit species commonly eaten by the animals. Ugandan chimps spent a quarter of their feeding time on figs, while those in Ivory Coast favored a plum-like fruit with bright green flesh. Yeasts and other microbes within the fruit pulp were identified as the likely source of ethanol, fermenting sugars without visible decay.

Lead author Aleksey Maro from UC Berkeley noted that while chimps don’t appear drunk, ethanol might still influence their behavior. Factors like territorial patrol frequency and mating cycles are known to correlate with food availability. It’s plausible that ethanol contributes to these dynamics, especially when chimps consume large volumes of ripe fruit quickly. Researchers also hypothesize that chimps may use the smell of ethanol to assess fruit ripeness before eating.

Evolutionary Implications and the “Drunken Monkey” Theory

The findings support the “drunken monkey” hypothesis, which suggests that human attraction to alcohol may stem from ancestral exposure to fermented fruit. Robert Dudley, senior author of the study, argues that ethanol ingestion offered caloric benefits and survival advantages to early primates. Given that chimpanzees and bonobos are humans’ closest genetic relatives, the evolutionary link appears plausible. The study adds weight to the idea that alcohol preference may be a retained trait from shared ancestors.

Ethanol Detection in Wildlife

Beyond chimpanzees, other fruit-eating animals—including bats and elephants—have been observed consuming fermented fruit. Some species possess enzymes that efficiently metabolize ethanol, suggesting evolutionary adaptation. In certain cases, animals actively seek out overripe fruit, possibly drawn by its scent and caloric density. These behaviors continue to inform research on the biological roots of alcohol consumption across species.