According to a new paper published in the journal Biology Letters, chimpanzees’ urine contains high levels of alcohol byproducts, most likely because chimpanzees regularly eat fermented fruits. This is the latest evidence in support of a hotly debated theory regarding the evolutionary origins of the human love of alcohol.
As previously reported, in 2014, biologist Robert Dudley of the University of California, Berkeley (UCB) wrote a book called Drunken Monkey: Why We Drink Alcohol and Abuse It. His controversial “drunken monkey hypothesis” proposed that human attraction to alcohol predates the origin of great apes by about 18 million years, and social communication and food sharing evolved to better recognize the presence of fruit from a distance. At the time, skeptical scientists insisted that this was unlikely because chimpanzees and other primates do not eat fermented fruits or nectar.
But in the next two decades, reports of primates doing so have increased. Earlier this year, we reported that researchers had caught wild chimpanzees on camera who appeared to be sharing fermented African breadfruit with measurable alcohol content. That observational data was the first evidence of the sharing of alcoholic foods among non-human great apes in the wild. The authors measured the alcohol content of the fruits with a convenient portable breathalyzer and found that almost all of the fallen fruits (90 percent) contained some ethanol, while the ripest fruit had the highest levels – equivalent to 0.61 percent ABV (alcohol by volume).
And last September, Dudley co-authored a paper reporting the first measurements of the ethanol content of fruits preferred by chimpanzees in Ivory Coast and Uganda, which found that chimpanzees consumed 14 grams of alcohol per day, the equivalent of a standard alcoholic drink in the US. After adjusting for the chimpanzees’ lower body mass, the authors concluded that the chimpanzees were drinking about two drinks per day.
a thankless job
The next step was to sample the chimpanzees’ urine to see if it contained alcohol metabolites, as a 2022 study on spider monkeys found. This will further refine estimates of how much ethanol-rich fruit chimpanzees eat per day. That thankless task was assigned to Alexei Maro, a UCB graduate student who spent last summer sleeping in trees in Ngogo – protected from the constant streams by an umbrella – to collect urine samples. Sharifa Namaganda, a Ugandan graduate student at the University of Michigan, showed them how to make shallow bowls from plastic bags hung on thorny twigs for more efficient collection. They also collected samples from urine puddles on the forest floor.
<a href