Analysis of urine samples from chimpanzees in Uganda’s Kibale National Park has revealed widespread alcohol consumption among the primates, confirming a long-held theory about the evolutionary roots of humanity’s attraction to ethanol. Researchers found biochemical evidence of ethanol exposure in at least 17 of 20 samples collected, indicating the apes are regularly ingesting significant amounts of alcohol from fermented fruit.
The findings, published this month, provide physiological proof that chimpanzees metabolize alcohol, bolstering what biologists call the “drunken monkey hypothesis.” This hypothesis, initially proposed by biologist Robert Dudley, suggests that primates’ fondness for fermented fruit – and the alcohol it contains – isn’t accidental, but may be linked to an evolutionary advantage in locating calorie-rich food sources. The scent of ethanol released during fruit ripening can signal high sugar content, potentially guiding primates to valuable nourishment.
Aleksey Maro, a graduate student at the University of California, Berkeley, and lead author of the study, collaborated with Sharifah Namaganda, a Ugandan graduate student at the University of Michigan, to overcome the logistical challenges of collecting urine samples from wild chimpanzees. Namaganda’s prior experience collecting samples in Kibale National Park proved crucial. The team devised a method using forked branches fitted with plastic bags to create makeshift collection bowls, carefully positioned under trees where chimpanzees were observed feeding. They too discovered that collecting urine from leaves under trees proved to be a more reliable method.
Previous research had estimated chimpanzee alcohol intake based on the ethanol levels found in the fruits they consume and the quantity of fruit eaten. These estimates suggested that chimpanzees could be consuming the equivalent of two alcoholic drinks – roughly 14 grams of alcohol – per day. The new urine analysis confirms that this estimate is accurate, according to researchers. Some studies have even suggested consumption levels as high as 2.5 alcoholic drinks daily.
Chimpanzees consume substantial amounts of fruit, sometimes exceeding 10 pounds daily. Even modest concentrations of ethanol in the fruit can therefore accumulate into a significant alcohol dose. The study’s findings suggest that this regular alcohol consumption may have played a role in shaping primate – and ultimately human – responses to alcohol. The research builds on a 2025 German documentary, “Wild on a High” (“Tiere im Rausch”), which visually documented chimpanzee behavior in the region.
Researchers are continuing to investigate the implications of these findings for understanding the evolutionary history of alcohol consumption. Further studies are planned to explore whether chimpanzees actively seek out fermented fruit, and to examine the potential effects of alcohol on their behavior and social interactions. Maro indicated that future sampling efforts might include collecting urine near chimpanzee sleeping sites, as they often urinate upon waking.
