Wild chimpanzees may get mildly intoxicated from alcoholic fruit

Recent observations reveal a fascinating habit among wild chimpanzees: they are believed to consume an equivalent of two glasses of wine per day, adjusted for their smaller body size.

The consumption of fermented fruits has been a known behavior among several primates, including chimpanzees, which could provide insight into human affinity for alcohol. This intriguing idea is encapsulated in the drunken monkey hypothesis.

Research led by Aleksey Maro from the University of California, Berkeley, sought to quantify ethanol consumption in chimpanzees by collecting and analyzing fruits from two key habitats: eastern chimpanzees (Pan troglodytes schweinfurthii) in Uganda’s Ngogo region and western chimpanzees (Pan troglodytes verus) in Ivory Coast’s Taï National Park.

The research team monitored chimpanzees through the use of camera traps, identifying their feeding preferences before collecting the target fruits. In particular, they focused on recently fallen fruits to ensure accuracy in their findings.

The results showed that eastern chimpanzees had a preference for ripe fig fruits (Ficus mucuso), while their western counterparts sought out the Guinea plum (Parinari excelsa) and fruits from the bitterbark tree (Sacoglottis gabonensis).

The research identified an alcohol concentration ranging from 0.3 to 0.4 percent in these fruits. Maro noted that chimpanzees consume about 10 percent of their body weight in fruit pulp daily, which translates to an estimated 14 grams of pure ethanol from foraging at both study sites.

To put this into perspective, a standard glass of wine (approximately 125 milliliters with 12.5 percent alcohol content) contains around 12 grams of ethanol. Considering that an average adult chimpanzee weighs approximately 40 kilograms, Maro explains, “This adjustment indicates chimpanzees are, in effect, consuming the equivalent of two glasses of wine daily.”

Although millions of years separate human evolution from that of chimpanzees, Maro suggests that our ancestors likely experienced similar exposure to fermented fruits. “Our findings imply that our ancestors were similarly chronically exposed to dietary alcohol,” he explains. This aligns with the drunken monkey hypothesis, proposing that such exposure may have influenced the evolutionary ties between alcohol consumption and the reward system related to fruit sugars.

Maro’s innovative method of collecting chimpanzee urine samples while shielding himself with an umbrella is a testament to the lengths researchers will go to understand the alcohol levels present in these animals. He is currently undertaking further analysis of these samples.

Expert Miguel Llorente of the University of Girona, who did not partake in the research, notes that this study is pivotal as it provides the first quantitative assessments of ethanol intake among wild chimpanzees. He asserts, “It adds substantial weight to the theory that alcohol exposure has ancient evolutionary roots in primate diets and potentially shaped human evolution.”

Nevertheless, Llorente does caution that equating chimpanzee alcohol consumption — which is incidental — to human voluntary ingestion and its associated addiction challenges is a significant leap.