Almost two glasses per day
This latest study involved Chimpanzees populations in the Chimpanzee Ngogo (Uganda) project and a second site in Tai (Ivory Coast), where scientists estimated that animals consume between 5 and 10% of their body weight (around 40 kilos) in fruits every day, 45 kilograms. The authors collected samples of fruit dough fallen on the two sites, wrapped them in hermetic containers and frozen at the base camp to prevent the fruits from ripening further.
Then they quantified the ethanol concentrations using a blood pressure, a portable gasel chromatograph and chemical tests. Uganda’s fruits contained 0.32% ethanol, while Côte d’Ivoire’s fruit contained 0.31% ethanol, which may not seem much before considering the amount of fruit they eat. And the most frequently consumed fruits on both sites had the highest ethanol content.
If anything, it is a conservative estimate, by Dudley. “If the chimpanzees sample mature fruit at random, then it will be their average consumption rate, regardless of any preference for ethanol,” he said. “But if they prefer rich and / or more rich fruits in sugar, then it is a cautious lower limit for the likely rate of ethanol ingestion.” This is in accordance with a 2016 report that captive AYES and slow loises prefer nectar with the largest alcohol content.
“Our results imply that our ancestors were also exposed chronically to food alcohol,” the co-author Aleksey Maro, a graduate of the UC Berkeley, told New Scientist. “The hypothesis of the drunk monkey suggests that this exhibition has changed our association between alcohol consumption and the reward for the search for fruit sugars and explains human attraction for alcohol today.” A warning is that the monkeys accidentally ingest ethanol, while humans drink it deliberately.
“What we are doing with this work is that our relationship with alcohol dates back to evolving time, probably around 30 million years,” the Primatologist of St. Andrews, who was not involved in the study, told BBC News. “Maybe for chimpanzees, it’s a great way to create social ties, spend time together on the forest, eat these fallen fruits.”
The next step is to taste the chimpanzee urine to see if it contains alcohol metabolites, as was found in a 2022 study on the spider monkeys. This will further refine the estimates of the quantity of fruits loaded with ethanol that chimpanzees eat every day. Maro spent this summer in NGOGO, sleeping in trees – protected from constant rivers by an umbrella – to take urine samples.
Science Advances, 2025. DOI: 10.1126 / SCIADV.ADW1665 (About DOI).
