Chimpanzees may consume equivalent of 1 alcoholic drink a day from fermented fruit, study finds

By Issy Ronald, CNN

Chimpanzees may consume the equivalent of about one alcoholic drink per day due to the ethanol contained within the fruit they eat, according to a new study.

Understanding this behavior of our closest living relatives could help explain humans’ consumption of alcohol, the report authors suggest.

Research indicating that chimpanzees also consume alcohol has long circulated around the scientific community, but this study, published Wednesday in the journal Science Advances, is the first attempt to quantify precisely how much they consume.

After observing two groups of chimpanzees, one in Ivory Coast and one in Uganda, and sampling the fruit they ate, the team of researchers estimated that the animals consumed approximately 14 grams (0.5 ounces) of ethanol a day.

That amount is equivalent to about 1.4 alcoholic drinks or, once adjusted for the smaller weight of the chimpanzees, it’s the equivalent of a human consuming 2.6 alcoholic drinks a day.

“The chimps are eating 5 to 10% of their body weight a day in ripe fruit, so even low concentrations yield … a substantial dosage of alcohol,” said senior study author Robert Dudley, a professor of integrative biology at the University of California, Berkeley, in a news release.

“If the chimps are randomly sampling ripe fruit as did Aleksey (Maro, lead study author), then that’s going to be their average consumption rate, independent of any preference for ethanol. But if they are preferring riper and/or more sugar-rich fruits, then this is a conservative lower limit for the likely rate of ethanol ingestion.”

Dudley is known for coining the “drunken monkey” hypothesis, which he said posits that our own predilection for alcohol is linked to humans’ “ancient tendencies as primates to seek out and consume ripe, sugar-rich and alcohol-containing fruits.”

The sweetest fruit

The researchers of the new study, based at both American and Ivorian universities, sampled ripe fruit pulp from 20 species in both Uganda and Ivory Coast, using three different methods to ascertain the alcohol content.

The study team found that the fruit contained an average of 0.31% to 0.32% of ethanol and that the chimpanzees typically ate around 4.5 kilograms (10 pounds) of fruit per day, meaning they consumed around 14 grams (0.5 ounces) of ethanol daily.

The scientists also observed that the most commonly eaten fruits at each study site — a type of fig in Uganda and a plumlike fruit in the Ivory Coast — had the highest alcohol content, indicating they are typically riper with more sugars to ferment.

The chimpanzees are “unlikely” to get drunk from consuming alcohol in this way, since they eat fruit “over the course of their day,” said conservation scientist Kimberley Hockings, an associate professor at the UK’s University of Exeter who was not involved in this study but has conducted research within this field.

“The data reinforce that ethanol is not an absolute deterrent to chimpanzee feeding, but we cannot say whether ethanol is an attractant or not,” she told CNN via email.

‘Drunken monkey’ hypothesis

Dudley’s hypothesis was initially met with skepticism, but more and more evidence has emerged to support it as primatologists, including in this study, have observed monkeys and apes eating fermented fruit.

“The consumption of ethanol is not limited to primates,” Dudley said, adding that the compound’s smell may help animals find food that contains more sugar. “It’s more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals,” he said.

“It just points to the need for additional federal funding for research into alcohol attraction and abuse by modern humans. It likely has a deep evolutionary background,” Dudley added.

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