From August 15 to 21, 2025, primatologist at the University of California at Berkeley, Aleksey Maro, got up every day, even at night, to stand under trees in Uganda and wait for them to see that there were branches above. Sometimes he used a plastic bag to collect urine. Other times, he would gather several leaves to create a kind of ditch that would channel it. And the least, they were lucky and one of the primates came down to the ground as soon as they stretched to climb a log, urinating on one side and defecating on the other. Maro’s scientific zeal, whose results have been published in , has served to demonstrate that Pan troglodyteslike humans, consume significant amounts of alcohol.
Maro, led by Professor Robert Dudley, also from Berkeley, already demonstrated last year that chimpanzees in two communities in two national parks took . To discover this, they analyzed hundreds of fruits from around twenty fruit trees of different species in the forests of Kibale and Taï (Ivory Coast), where groups of two different subspecies live, the eastern one (Pan troglodytes schweinfurthii) and the western one (P. t. true). They observed their desire to eat very ripe fruits, with an average ethanol concentration of 0.31%. But they needed to confirm the presence of alcohol in their system after eating them.
To demonstrate this, during that week in 2025, Maro, as soon as he collected the urine samples, returned to the base camp to submit them to two standard urine tests. They are the same ones that are marketed and used in detoxification programs or to confirm abstinence in certain risky professions. What these assays measure is the presence of ethylglucuronide (EtG), a metabolite that is produced when ethanol reaches the liver hidden in the blood. Unlike blood alcohol, which lasts a few hours and is influenced by other factors, such as food intake, the concentration of EtG remains in the urine for days (and much longer in nails and hair).
Of the twenty chimpanzees subjected to the blood alcohol test, 17 tested positive, exceeding the threshold established by one of the tests at 300 nanograms per milliliter. These positives were subjected to a second, more demanding test again. In this they saw that ten of the samples exceeded 500 ng/ml. In humans, a similar level occurs after moderate alcohol consumption, one to two drinks, in the previous 24 hours. According to the study authors, similar levels would be expected in a chimpanzee that had spent the morning gobbling lightly fermented fruit.
“I must say that humans and chimpanzees consume alcohol in different ways, which is important for the way they absorb it,” Maro clarifies. The EtG metabolite is produced in the liver, representing around 1% of ethanol metabolism, “so it is true that it is proportional to the amount you drink,” he adds. But he points out that both primates do not consume ethanol the same: “When alcohol is consumed as a liquid quickly within 15 minutes, it travels beyond the stomach to the small intestine without impediments.” Meanwhile, chimpanzees consume fruit throughout the day. “Alcohol is mixed with semi-solid fruit pulp, it remains in the stomach for longer and is absorbed more slowly. For this reason, I believe that chimpanzees would need more alcohol to overcome the same limit,” he adds.
For Maro, the most important thing about his work is that, for the first time, “we have found widespread physiological evidence of alcohol consumption by chimpanzees.” And he adds: “If there was any doubt about the drunken monkey hypothesis—that there is enough alcohol in the environment for animals to experience alcohol in a way analogous to that of humans—it has been dispelled,” he adds in a note. Maro is referring to an idea that Dudley, the study’s senior author, raised at the turn of the century. Years later, the professor published a book that caused more than one controversy. the book The Drunken Monkey: Why We Drink and Abuse Alcohol (something like The Drunken Monkey: Why We Drink and Abuse Alcohol) .
In the work, for example, it is recalled that primates were fundamentally frugivorous and detecting the fruits with the highest caloric value was key and here volatile substances such as ethanol were key. In a kind of symbiotic relationship, fruits send a message that they are rich in calories so that their consumers eat them and thus disperse their seeds, as, for example, has already been demonstrated. As , the current problem with alcohol would be a matter of dosage: “Compared to the relatively high availability in the Neolithic, increased with distillation [descubierta hace solo un milenio]our historical exposure to low levels of the fruit would have been too low, producing a mismatch between our evolutionary exposure and environmental availability.”
With the new work, Dudley is getting closer to confirming his hypothesis: “We have confirmed the presence of ethanol in the fruit and, with the new urine study, we show that they are metabolizing it,” Dudley highlights in an email. That is, if humans and primates have the enzyme necessary to metabolize alcohol, this implies that they inherited it from a common ancestor. And both lineages separated between five and seven million years ago. But his theory lacks proof that would be definitive: “We still don’t know if they prefer fruits with a higher concentration of ethanol, since it is not possible to measure recently consumed fruit,” he recalls. However, they are already working on indirect methods to evaluate whether there is a preference for fruits with ethanol versus those without.
