A new study published in the journal Biology Letters indicates that chimpanzees have high levels of alcohol byproducts in their urine, likely due to their regular consumption of fermented fruits. This discovery lends support to a contentious theory regarding the evolutionary origins of human alcohol consumption.
In 2014, University of California, Berkeley biologist Robert Dudley published the book The Drunken Monkey: Why We Drink and Abuse Alcohol, proposing the 'drunken monkey hypothesis.' This theory suggests that the human affinity for alcohol dates back around 18 million years, to the era of the great apes, with social behaviors and food-sharing evolving to locate fruit from afar. Initially, many scientists disputed this hypothesis, contending that primates such as chimpanzees do not typically consume fermented fruits or nectar.
However, in the past two decades, instances of primates partaking in fermented fruits have become more documented. Earlier this year, researchers reported seeing wild chimpanzees share fermented African breadfruit, with alcohol content confirmed through portable breathalyzer tests. These tests showed that 90 percent of the fallen fruit contained ethanol, the ripest fruits reaching up to 0.61 percent alcohol by volume.
Last September, Dudley co-authored another paper analyzing the ethanol content in fruits favored by chimpanzees in the Ivory Coast and Uganda. The study found chimpanzees consume about 14 grams of alcohol daily, equating to one standard alcoholic drink in the US. When adjusted for their body mass, this intake amounts to nearly two alcoholic drinks per day for chimps.
To further examine these findings, researchers needed to test chimpanzee urine for alcohol metabolites, expanding on a 2022 study on spider monkeys. Collecting these samples proved challenging, a task undertaken by UCB graduate student Aleksey Maro, who spent the summer in Ngogo. Maro, protected by an umbrella while sleeping in trees, gathered urine samples with the help of Ugandan graduate student Sharifah Namaganda. Namaganda demonstrated how to craft collection bowls from plastic bags on forked twigs, and Maro also collected samples from puddles on the forest floor.