Almost Two Drinks a Day
A recent investigation into chimpanzee behavior at the Ngogo Chimpanzee Project in Uganda and a second location in Tai, Ivory Coast, has revealed that these primates consume a significant quantity of fruit. Scientists estimate that chimps at these sites eat between 5 to 10 percent of their body weight, which translates to approximately 40 kilograms of fruit daily—amounting to around 45 kilograms for the average chimp.
To analyze the ethanol content of the fallen fruit, researchers collected pulp samples from both locations, carefully sealing them in airtight containers before freezing to prevent further ripening. The team employed a breathalyzer, a portable gas chromatograph, and chemical testing to measure ethanol levels. The analysis showed the Uganda fruit contained 0.32 percent ethanol, while the Ivory Coast fruit registered at 0.31 percent. Although these concentrations may seem low, the high volume of fruit consumed is noteworthy. Additionally, the most commonly eaten fruits at both sites had the highest ethanol levels.
Dudley, one of the study’s authors, pointed out that these figures may actually underestimate the true level of ethanol consumption. He speculated that if the chimps are selectively choosing riper or sweeter fruits, their actual ethanol intake could be higher than reported. This aligns with a 2016 study that discovered that captive aye-ayes and slow lorises favor nectar with the highest alcohol content.
“Our findings suggest that our ancestors were likely exposed to dietary alcohol in a similar way,” commented Aleksey Maro, a graduate student at UC Berkeley and co-author of the study. Maro explained that the “drunken monkey hypothesis” implies this exposure may have contributed to an evolutionary connection between alcohol consumption and the reward mechanisms related to fruit sugars, potentially influencing human attraction to alcohol today. However, it is important to note the distinction that while apes consume ethanol inadvertently, humans do so intentionally.
University of St. Andrews primatologist Catherine Hobaiter, who was not involved in the research, emphasized that the emerging evidence indicates a longstanding relationship between primates and alcohol that may date back approximately 30 million years. She suggested that for chimpanzees, consuming these fruits could serve as a social bonding activity, allowing them to congregate on the forest floor.
The next phase of the research involves analyzing urine samples from the chimps to check for alcohol metabolites, similar to findings in a 2022 study on spider monkeys. This additional testing will enhance the accuracy of estimates concerning the amount of ethanol-rich fruit the chimpanzees consume daily. Maro dedicated his summer to the Ngogo site, sleeping in trees to gather urine samples while protecting himself from rain with an umbrella.
Science Advances, 2025. DOI: 10.1126/sciadv.adw1665 (About DOIs).
