Chimpanzees could be consuming the equivalent of a beer and half a day from eating alcoholic fruits, according to the first estimates of wild chimp alcohol intake.
By virtue of their fruit-filled diets alongside natural fermentation, chimps (Pan troglodytes) in Uganda and the Ivory Coast probably eat around 0.5 ounces (14 grams) of ethanol a day.
"They're definitely not getting drunk," study co-author Aleksey Maro, a primatologist at University of California Berkeley (UC Berkeley), told Live Science. "If you had two drinks split through the whole day, you wouldn't feel much either."
The idea that humans have a taste for alcohol because it first appeared naturally in our diets is called the "drunken monkey hypothesis," and was proposed by study co-author Robert Dudley, a professor of integrative biology at UC Berkeley. Dudley's hypothesis postulates that there was a period of primate evolution during which our early ancestors couldn’t metabolize fermented fruits, and so were missing out on high calorie meals.
To tap into this nutritious resource, primates evolved to be able to hold their liquor by being able to break down ethanol.
However, until now scientists have only directly observed chimps eating fruit known to contain ethanol once, reporting chimps in Guinea-Bissau eating and sharing naturally-occurring fermented African breadfruit in 2022.
"I'm as skeptical as everyone else about the drunken monkey hypothesis," Maro said, "so I've taken care to do my own sleuthing."
To figure out how regularly chimpanzees could be consuming ethanol rich fruits, Maro and his team followed two groups of chimps: The Ngogo chimpanzees in Uganda and the Taï chimpanzees in Côte d'Ivoire. They observed the chimps for three periods lasting several months between 2017 and 2021.
The researchers took samples of freshly fallen fruit whenever they encountered them eating. From this, they retrieved 254 ripe fruit samples from 15 fruit species in Ngogo and 245 samples from six species in Taï.
To estimate the alcohol content in the samples, the team either analyzed the alcohol vapor found inside the fruit or used a chemical reagent that turned yellow on contact with ethanol.
They found that the Ngogo chimps ate fruit with an average ethanol concentration of 0.32%, meaning that there was 1.1 ounces (32 grams) of ethanol in every 3.5 ounces (100 grams) of the total weight. The fruit eaten by the Taï chimps had a remarkably similar average ethanol concentration at 0.31%.
Maro and his team then used pre-existing data on chimp diets and body weights to estimate the chimpanzees' daily ethanol intake across the two populations. They found that both male and female chimps consume approximately 0.5 ounces ethanol per day, which is around 2.5 standard drinks when adjusted for their smaller body size.
Matthew Carrigan, a molecular biologist at the College of Central Florida who was not involved in the research, said that while the results aren't necessarily surprising, they are exciting because it's moving forward an "important question about human ethanol addiction and evolution."
However, Carrigan pointed out that "there's a very wide range" in the error bars, meaning that the chimps could be eating as little as 0.14 ounces (4 grams) of ethanol per day, or as much as 0.85 ounces (24 grams) — the difference between less than one drink or four.
Maro also acknowledged that the research samples could have been fruits the chimps refused to eat. The next step is to investigate if the chimps are opting to avoid or seek out individual fruits based on their degree of fermentation, Carrigan said.
Ultimately, humans and chimpanzees "evolved to get calories without getting drunk," Carrigan said. "Otherwise, if we evolved to get drunk, then we would not evolve to metabolize the ethanol faster."
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