Male chimpanzees eating the plum-like fruit of the evergreen Parinari excelsa tree at Taï National Park in the Ivory Coast in 2021. Credit: Aleksey Maro/UC Berkeley
The first study to survey the ethanol content of fruits eaten by chimpanzees suggests the primates could easily be consuming the equivalent of more than 2 standard alcoholic drinks a day.
While the researchers couldn’t determine whether the chimps pick these fruits because they are ripe with more sugars, or for their higher ethanol levels, the results do suggest that alcohol is a regular part of their diet, and likely for early humans too.
Ethanol is the active ingredient in beer, wine and other alcoholic drinks. It is produced in the fruits through the process of fermentation, where microorganisms turn sugars into alcohol.
“Across all sites, male and female chimpanzees are consuming about 14g of pure ethanol per day in their diet, which is the equivalent to 1 standard American drink,” says the study’s leading researcher Aleksey Maro, from the University of California, Berkeley in the US.
In Australia, a standard drink contains 10g of ethanol.
“When you adjust for body mass, because chimps weigh about 40kg versus a typical human at 70kg, it goes up to nearly 2 drinks,” says Maro.
The study focused on 21 species of fruit from 2 different chimp sites, Ngogo in Uganda and Taï in the Ivory Coast. The chimps consume around 4.5 kilograms of fruit per day, which makes up 3 quarters of their diet.
The researchers collected fallen fruit and stored it in air-tight containers to prevent further ripening. They collected samples on 3 different occasions and used a different method to test alcohol percentages each time: a portable gas chromatograph, a chemical test and a semiconductor-based device somewhat like a breathalyser.
All methods recorded similar alcohol percentages.
A chimpanzee eating figs at Ngogo in Uganda’s Kibale National Park in 2018. Credit: Aleksey Maro/UC Berkeley
The results, published in Science Advances, found that the fruit had an average alcohol content of 0.26% by weight.
“The chimps are eating 5 to 10% of their body weight a day in ripe fruit, so even low concentrations yield a high daily total – a substantial dosage of alcohol,” says co-author Robert Dudley, a professor of integrative biology at UC Berkeley.
The team observed that most of the fruit consumption occurred during the day and that the chimps showed no overt signs of intoxication other than stomach bloating.
“Chimpanzees consume a similar amount of alcohol to what we might if we ate fermented food daily,” Maro said.
“Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees.”
Dudley first hypothesised that humans inherited their alcoholic taste buds from primate ancestors, in what he calls the ‘drunken monkey’ theory, more than 20 years ago.
While the idea initially received a lot of criticism, over time, it has gained more followers. Just this year, another study reported chimps in Guinea-Bissau share fermented fruit.
Dudley and his Berkeley colleagues also published a study this year showing that it’s not just primates getting on the sauce. The feathers of 10 out of 17 bird species they tested indicated they likely had substantial amounts of ethanol in their diet.
”The consumption of ethanol is not limited to primates,” says Dudley. “It’s more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals.”
Dudley suggests that animals may pick ethanol-rich food due to its distinct smell, as it helps animals find food with higher sugar content which gives them greater energy. But he says it’s also possible that sharing alcoholic fruit plays a social role among primates like it does in humans.
“It just points to the need for additional federal funding for research into alcohol attraction and abuse by modern humans,” says Dudley.
“It likely has a deep evolutionary background.”