Great apes often eat fermented, alcoholic fruits. However, it was previously unclear whether they absorb significant amounts of alcohol. That’s why biologists investigated this in more detail using urine samples from wild chimpanzees in Uganda. And indeed: Significantly increased levels of the alcohol breakdown product ethyl glucuronide were found in the great apes’ urine. The concentration was similar to that of a person after one or two hard drinks. This supports the “drunken monkey hypothesis,” the team said.
The consumption of alcohol has a long tradition among us humans and occurs in almost all cultures. Thousands of years ago, people made alcoholic drinks from fermented grains or fermented fruits. This raises the question of where this love of alcohol – and susceptibility to alcohol addiction – comes from in us and our ancestors. Is this purely a cultural thing? Or maybe there are biological roots behind it because alcohol consumption brought evolutionary benefits?
Urine testing for wild chimpanzees
Observations of our closest relatives provide evidence of animal alcohol consumption. Biologists have observed several times, especially in great apes, that they often eat overripe and half-fermented fruits and seem to deliberately seek them out. Researchers have even documented “alcohol parties” among wild chimpanzees in Guinea-Bissau: several chimpanzees of all ages and genders shared fermented breadfruit weighing up to 30 kilograms. Although these fruits only had an alcohol content of around 0.61 percent, it was unclear how much and how often the great apes ate these fermented fruits.
To clarify this, Aleksey Maro from the University of California at Berkeley and his colleagues traveled to Kibale National Park in Uganda and collected urine samples from wild chimpanzees. They analyzed these samples using a common rapid test for the content of ethyl glucuronide, a direct breakdown product of the alcohol ethanol. In humans, this substance can be detected for up to 80 hours after alcohol consumption and is used, among other things, to monitor withdrawal in alcoholics.
As much alcohol as after one or two hard drinks
The tests detected ethyl glucuronide in the urine samples of 17 of the 19 chimpanzees. The alcohol degradation product content in these samples was over 300 nanograms per milliliter. Ten of the urine samples contained more than 500 nanograms per milliliter of ethyl glucuronide. “These are relatively high values and our results are measured rather conservatively,” explains senior author Robert Dudley from the University of California. “These ethyl glucuronide levels are well above the thresholds considered clinically or forensically relevant in humans.” For comparison: A level of 500 nanograms per milliliter of ethyl glucuronide in our urine is typical if we have consumed one or two glasses of hard drinks in the last 24 hours.
(Video: UC Berkeley)
In the case of chimpanzees, this alcohol came from fermented fruits of the forest tree Chrysophyllum albidum, also known as the white star apple. As Maro and his colleagues determined, these fruits eaten by the great apes only contained around 0.09 percent alcohol by weight. In order to produce such high levels of ethyl glucuronide, the chimpanzees must have consumed several kilograms of these very sugary fruits. “Our results demonstrate that male and female chimpanzees consume substantial amounts of alcohol with these calorie-containing fruits during the fruit ripening period,” the team writes.
Evidence for “drunken monkeys”
According to the researchers, this shows that wild great apes also consume physiologically relevant amounts of alcohol, at least at times. “If there were previously any doubts about the ‘drunk monkey’ hypothesis and about the fact that there is enough alcohol in the habitat of these animals, we have now dispelled them,” says Maro. What is still unclear, however, is whether the chimpanzees consume this alcohol on the side because they prefer particularly sweet and therefore overripe fruits or whether they deliberately select fruits that are already half-fermented. “The latter has not yet been clearly proven for any wild animal species,” says Maro. “Future research could start here.”
Source: University of California – Berkeley; Specialist article: Biology Letters, doi: 10.1098/rsbl.2025.0740