For 11 days in late summer 2025, Aleksey Maro found himself in the Ugandan rainforest, doing whatever he could to collect chimpanzee urine.
"The most consistent, predictable time is in the morning, just like people, the first thing they do when they wake up is they go pee," says Maro, a PhD student in integrative biology at the University of California, Berkeley.
Now, in a study published recently in Biology Letters, Maro and his colleagues explained what those urine samples reveal: Chimpanzees appear to consume a fair amount of alcohol when eating ripe, fermenting fruit. The findings may tell us something about human evolution.
"In primates," says Maro, "it could be that when you smell alcohol, that means that's where the sugars are." In other words, the scent of fermentation might be a shortcut to getting more calorie-dense food.
Maybe it's this tendency to associate a sugary reward with alcohol consumption that explains where human attraction to inebriating substances first originated — and why we still gravitate towards it today. In biological anthropology, it's called the drunken monkey hypothesis.
That is what Maro says he was intent on figuring out: "the evolutionary origins of human attraction to alcohol."
Avoiding the splash zone
To be clear, monkeys and apes are not likely consuming enough alcohol to get drunk. But an ancient affinity may explain why humans are drawn to the stuff to the point of intoxication. As Maro puts it, this could represent "a profound mismatch between the way we live today and the way we evolved."
Maro had already shown that many of the ripe fruits that typically make up the chimp diet in Uganda and Côte d'Ivoire contain a good amount of ethanol. But he wondered if the animals were actually consuming the alcohol — hence all the urine collection.
His team used a few techniques in the field including pipetting it off of leaves. Or, more perilously, watching for behaviors indicating that urination was imminent and catching droplets streaming down from the chimps overhead into a plastic bag stretched over a forked branch.
"You need to make sure that you are not going to be splashed," says Sharifah Namaganda, a PhD candidate at the University of Michigan who assisted Maro in the field. "But plastic-bag pee is the best you can get. And it is also the sample that will make you most proud!"
That's because it's not contaminated and can be collected quickly without losing the chimps as they move.Each time the team captured a sample, Maro then quickly tested it to see if the urinating chimp had metabolized alcohol from ripe, fermenting fruit.
"Chimpanzees consume ten pounds of fruit pulp per day, on average," says Maro. The fruit his study subjects were feasting on is called African star apple. It's eaten by humans too and has sweet flesh that contains some latex.
"That's the weirdest thing," says Maro. "You chew it and as you eat the fruit, you get a little bubble gum."
A tantalizing swig
Out of 19 chimps in the study, urine samples of 17 tested positive for ethanol. And at least ten of those contained a concentration equivalent in humans to having had one or two drinks.
It's too small a number to say anything definitive, "but it is tantalizing," says Maro. "Chimpanzees are consuming alcohol. It's plausible that our ancestral diet may have had similar alcohol just baked into our everyday existence."
And that may have led to our modern attraction to alcohol — except that today we can concentrate it and consume it at much higher levels.
In fact, Maro points out that this affinity may go back even further than primates. "We know that fruit flies evolved to lay their larvae in fermenting fruit pulp," he says. "The more fermented, the more alcohol, the better for the fruit flies."
"It is a wonderful study," says Matthew Carrigan, an evolutionary biologist at the College of Central Florida who didn't participate in the research. Even if he would have liked to have seen a larger sample size, he praises the work. "It nicely supports what earlier studies have alluded to. This takes it one step further and is measuring the output."
Cat Hobaiter, a primatologist at the University of St. Andrews, who wasn't involved in the study either, says it "will open up a lot of really exciting new avenues for us to understand both chimpanzee behavior," and "potentially some of the evolutionary origins of rituals and social rites of passage that are really important in our own culture, too."
The next step, Maro says, will be to determine whether the chimps are actively seeking out the fruits containing ethanol, or just consuming it by mistake. Seeking it would suggest a preference for the boozy aroma and flavor… perhaps akin to their human relatives.
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