Summary
Chimpanzees in the wild are consuming alcohol equivalent to nearly two drinks a day, according to a new study published in Science Advances. The research supports Robert Dudley’s "drunken monkey" hypothesis, which proposes that humans’ attraction to alcohol is rooted in our evolutionary past with great apes. The study measured ethanol levels in fruits favored by chimps in Ivory Coast and Uganda, finding that the animals are ingesting significant amounts of alcohol daily.
Chimps Consume Alcohol Equivalent of Nearly Two Drinks a Day
The latest evidence supporting Dudley’s hypothesis comes from a new paper co-authored by Robert Dudley and Aleksey Maro, a graduate student at UC Berkeley. The researchers measured ethanol levels in fruits favored by chimps in Ivory Coast and Uganda, finding that the animals are consuming 14 grams of alcohol every day, equivalent to a standard alcoholic drink in the US. Adjusting for the chimps’ lower body mass, the authors concluded that the chimps are consuming nearly two drinks per day.
The study involved collecting fallen fruit pulp samples from both sites, packing them in airtight containers, and freezing them back at base camp to keep the fruit from ripening further. The researchers then quantified the ethanol concentrations using a breathalyzer, a portable gas chromatograph, and chemical testing. The Uganda fruit contained 0.32 percent ethanol, while the Ivory Coast fruit contained 0.31 percent ethanol.
It’s worth noting that this is not the first evidence of primates consuming fermented food or nectar with measurable alcoholic content. Earlier this year, researchers reported observing wild chimpanzees engaging in what appears to be sharing fermented African breadfruit with measurable alcoholic content. The observational data was the first evidence of the sharing of alcoholic foods among nonhuman great apes in the wild.
The authors suggest that the chimps’ preference for riper fruit is a key factor in their consumption of alcohol. "If the chimps are randomly sampling ripe fruit, then that’s going to be their average consumption rate, independent of any preference for ethanol," Dudley said. However, if they prefer riper and/or more sugar-rich fruits, then this is a conservative lower limit for the likely rate of ethanol ingestion.
The Drunken Monkey Hypothesis
Dudley’s "drunken monkey" hypothesis proposes that humans’ attraction to alcohol goes back about 18 million years, to the origin of the great apes. According to Dudley, social communication and sharing food evolved to better identify the presence of fruit from a distance. This would have led to a preference for ripe, sweet fruits, which often contain higher levels of ethanol.
While it’s highly unlikely that chimps would get drunk, the study suggests that they are chronically exposed to dietary alcohol. "Our findings imply that our ancestors were similarly chronically exposed to dietary alcohol," co-author Aleksey Maro said. The researchers propose that this exposure caused our species to evolve an association between alcohol consumption and the reward of finding fruit sugars.
Implications for Human Alcohol Consumption
The study has significant implications for our understanding of human alcohol consumption. "What we’re realizing from this work is that our relationship with alcohol goes deep back into evolutionary time, probably about 30 million years," University of St. Andrews primatologist Catherine Hobaiter said. "Maybe for chimpanzees, this is a great way to create social bonds, to hang out together on the forest floor, eating those fallen fruits."
The study also highlights the importance of considering our ancestors’ diets when understanding human behavior and preferences. "If we can understand how our ancestors used to live, that might give us clues about why humans do what they do today," Hobaiter added.
Future Research Directions
The next step is to sample the chimps’ urine to see if it contains any alcohol metabolites, as was found in a 2022 study on spider monkeys. This will further refine estimates for how much ethanol-laden fruit the chimps eat every day. Maro spent this summer in Ngogo, sleeping in trees—protected from the constant streams by an umbrella—to collect urine samples.
The study’s findings have significant implications for our understanding of human evolution and behavior. By studying chimpanzees’ consumption of alcohol, researchers can gain insights into how our ancestors adapted to their environment and how these adaptations shape our behavior today.
Conclusion
Chimpanzees in the wild are consuming alcohol equivalent to nearly two drinks a day, according to a new study published in Science Advances. The research supports Robert Dudley’s "drunken monkey" hypothesis, which proposes that humans’ attraction to alcohol is rooted in our evolutionary past with great apes. The study has significant implications for our understanding of human evolution and behavior, highlighting the importance of considering our ancestors’ diets when understanding human preferences and behaviors.