(CN) -- By eating fermented fruit, wild chimpanzees may be regularly consuming the equivalent of more than two alcoholic drinks per day, according to new research from the University of California, Berkeley. This practice could explain humans' affinity for alcoholic beverages, scientists say.
First proposed over two decades ago by Robert Dudley, a professor of integrative biology at UC Berkeley, the "drunken monkey" hypothesis suggested humans' taste for alcohol stems from the ancestral primate behavior of consuming fermented fruit.
Dudley's 2014 book, The Drunken Monkey: Why We Drink and Abuse Alcohol, argued that yeast fermentation in overripe fruit naturally produced alcohol, which was regularly consumed by primate ancestors. Skeptics pushed back, insisting wild primates don't eat fermented fruit or nectar.
But Dudley's claims had merit, as wild monkey and apes have been recorded eating fermented fruit -- primatologists even reported chimpanzees doing so in West Africa's Guinea-Bissau earlier this year -- and lab studies suggest captive monkeys may prefer nectar higher in alcohol. A 2016 Dartmouth University paper documented captive aye-ayes and slow lorises preferring nectar high in alcohol and repeatedly revisiting empty high-alcohol containers as if they wanted more, to researchers' surprise.
In 2022, Dudley and colleagues in Panama confirmed that spider monkeys eat fermented fruit in the wild and express alcohol metabolites in their urine, further supporting the hypothesis.
Inspired by the scientific observations, UC Berkeley graduate student Aleksey Maro undertook a multiyear field study in Uganda's Kibale National Park and Côte d'Ivoire's Taï National Park. Both sites are host to large, well-documented chimpanzee communities and social structures.
Maro and colleagues collected freshly fallen, undamaged fruit previously foraged by chimps. The samples were frozen and categorized by features such as species, size and color before undergoing various alcohol content tests -- one involved a classic semiconductor breathalyzer, while other tests involved complex processes more commonly seen in a chemist's lab, like a gas chromatograph.
One variety of fig and another variety of plum seemed to be the most popular amongst the chimpanzees, eaten at a much higher frequency than other fruits available in the forest. These fruits also had the highest alcohol levels of the 21 fruit species sampled.
"Across all sites, male and female chimpanzees are consuming about 14 grams of pure ethanol per day in their diet, which is the equivalent to one standard American drink," said Maro, noting that when body mass is adjusted, the average increases to nearly two drinks. "Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees."
Primatologists estimate that chimpanzees consume around 10 pounds of fruit daily, making up nearly 75% of their diet. This means daily alcohol intake is still significant, even if ethanol levels are too low to cause significant intoxication.
The study, published Wednesday in Science Advances, supports the idea that alcohol was a regular part of our ancestors' diets -- possibly explaining modern humans' attraction to it.
"One of the reasons this has been a tempting target but no one's gone after it is because it's so hard to do in a field site where there are wild primates eating known fruits," said Dudley, who is a senior author of the paper alongside first author Maro. "This dataset has not existed before, and it has been a contentious issue."
Alcohol consumption isn't just limited to primates, Dudley explained. Almost all fruit-eating animals, including elephants and birds, likely metabolize alcohol as part of their diets.
The smell of alcohol may help animals locate sugar-rich, and therefore energy-rich, food, Dudley suggested. Alcohol could also enhance the eating experience or promote social bonding in animals.
Looking into our past and evolutionary background can be key in explaining and fighting modern issues such as alcoholism, Dudley explained. The research "points to the need for additional federal funding for research into alcohol attraction and abuse by modern humans," said Dudley.