Wild chimpanzees who live in western African forests eat naturally fermented fruits every day. And without wanting it, they take on a quantity of alcohol similar to that of two human cocktails.
This was demonstrated by a team from the University of Berkeley, which analyzed the alcoholic content of figs, plums and other fruits collected in the places where primates feed. According to the study, published on Science Advancesthe chimpanzees ingest 14 grams of ethanol per day on average. It is the same quantity present in an American standard drink, but considering that a human being weighs on average 70 kilos and a chimpanzee 40, the effect on their body is equivalent to almost two drinks.
21 types of fruit between Uganda and Ivory Coast analyzed
The research was conducted between 2019 and 2025 in two areas: the Kibal National Park, in Uganda, and the Taï National Park, in Ivory Coast. The scientists collected just fallen fruits under the trees frequented by chimpanzees and analyzed them in the laboratory, testing 21 different species.
The average of ethanol was 0.26% by weight, but with significant variations: some varieties, such as the fig Ficus Musco or the fruit of the tree Parinari Excelsacontained higher alcohol levels. They are also among the most consumed, making the daily alcoholic intake more consistent.
On average, chimpanzees eat about 4.5 kg of fruit per day, which represents about 75% of their diet. Fruit that, if too ripe, begins to ferment naturally. The result is a constant and daily exposure to alcohol, even if at low doses.
“Hypothesis of the drunk monkey”
Despite the numbers, chimpanzees do not show signs of intoxication. To really feel its effects, they should ingest quantities of fruit that causes swelling and digestive difficulties. However, this does not exclude a long -term biological effect.
Robert Dudley, one of the authors of the study, has been working on a hypothesis known as “drunk monkey” for years: according to this theory, human interest in alcohol could have evolutionary origins, linked to the food behavior of primates. Animals may have developed a preference for the most sugary and fermented fruits, because they are more energetic and easy to identify thanks to the smell of alcohol.
Similar behavior has also been observed in other animals. Some birds and mammals-such as spider monkeys or Aye-Aye in captivity-show interest in food that contain ethanol. African elephants also seem attracted to the fermented fruits of Parinari Excelsathe same loved by chimpanzees.
To measure alcohol levels, the researchers used three different techniques: a portable ethylometer, a fog and a chemical test. The samples were collected every day and worked in field conditions, adapting the methods to the reality of the context.
In the summer of 2025, the researcher Aleksey Maro returned to Uganda to collect chimpanzee urine samples, to check if the hired ethanol was metabolized. To do this, he had to place himself under the trees where the primates sleep, using an umbrella as protection.
The long -term goal is to understand if chimpanzees voluntarily choose the fruits with more alcohol. And if, as some scholars hypothesize, this behavior may have favored the selection of some food preferences in human beings, today much more complex – and often problematic – than those of our primary relatives.
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