Wild chimpanzees in Uganda's Kibale National Park and Côte d'Ivoire's Taï National Park consume enough ethanol from naturally fermenting fruit to equal around 14 grams a day-roughly two standard alcoholic drinks for a human of similar size. That is the finding of a new study published in Science Advances, offering fresh support for the“drunken monkey” hypothesis, which argues that human attraction to alcohol has deep evolutionary roots.

Scientists analysed over 20 fruit species frequently eaten by chimps, measuring alcohol content in fruit pulp scattered beneath trees in both forests. The ethanol concentration averaged about 0.3% by weight. Given chimpanzees eat between 5% to 10% of their body weight in fruit daily, those small concentrations add up.

Neither male nor female chimps studied showed overt signs of intoxication despite this ethanol intake. Behavioural observations imply that consumption is spread gradually throughout the day, as they forage, rather than being concentrated in bursts. This pattern likely mitigates the acute effects of alcohol.

Lead author Aleksey Maro and senior author Robert Dudley noted that fruits with higher sugar content tend to ferment more readily, producing more ethanol. There is still uncertainty as to whether chimps intentionally prefer riper, more fermenting fruits, or whether they simply eat what is available.

Fruit constitutes about 70‐75% of the chimps' diet at the two study sites. The researchers collected fruit that chimps actually eat-not just fallen fruit-and measured ethanol directly. That gave credibility to estimates of ethanol intake and its likely physiological relevance.

Adjusted for body mass, chimps' ethanol intake equates to more than one drink per human daily; when size is accounted for, the equivalent becomes approximately two drinks. This degree of exposure matches what might be considered a light daily alcohol consumption in humans.

The“drunken monkey” hypothesis, first proposed in 2000, holds that many primates have long been exposed to small quantities of ethanol via fermenting fruit, and that this exposure was integral to dietary energy, smell cues, and feeding behaviour. This new research adds quantitative support to the idea, showing that ethanol exposure was not occasional but part of everyday foraging in these chimp populations.

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