Chimpanzees, according to the initial estimates of wild chimp alcohol intake, are equivalent to beer and can consume half a day by eating alcoholic fruits.
Thanks to a diet filled with fruit alongside natural fermentation, Ugandan and Ivory Coast chimpanzees (pantrogloissete) probably eat about 0.5 ounces (14 grams) of ethanol per day.
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“They are definitely not drunk,” Aleksey Maro, co-author of primates at the University of California, Berkeley, told Live Science. “If you have two drinks all day, you won’t feel much.”
The idea that humans have a taste of alcohol because they first appeared naturally in our diet, called the “Drunk Monkey Hypothesis,” and was proposed by researchers of Robert Dudley, a professor of integrated biology in Berkeley, California. Dudley’s hypothesis assumes there was a period of primate evolution when early ancestors were unable to metabolize fermented fruits, and missed high-calorie diets.
To exploit this nutritious resource, primates have evolved to retain alcohol by being able to break down ethanol.
But up until now, scientists have directly observed only chimpanzees that eat fruits known to contain ethanol, and have reported that they share African panty, which was naturally fermented in 2022 with Guinea-Bissau chimpanzees.
“I’m just as skeptical as everyone else about the drunken monkey hypothesis,” Maro said.
To grasp how chimpanzees consume ethanol-rich fruits, Malo and his team followed two groups: Ngogo chimpanzees from Uganda and Tai chimpanzees from Ivory Coast. They observed chimpanzees for three periods that last several months between 2017 and 2021.
Researchers took samples of newly fallen fruits each time they encountered eating. From this, they collected 254 ripe fruit samples from 15 different Ngogo fruits and 245 samples from 6 of Thailand.
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To estimate the alcohol content of the samples, the team analyzed alcohol vapors found inside the fruit or used chemical reagents that turned yellow when contacted with ethanol.
They found that Ngogo Chimps ate fruit at an average ethanol concentration of 0.32%. This means that every 3.5 ounces (100 grams) of ethanol was 1.1 ounces (32 grams) of ethanol. Fruits eaten in Thai chimpanzees had very similar average ethanol concentrations at 0.31%.
Maro and his team then estimated chimpanzees’ daily ethanol intake across two populations using existing data on chimpanzee diet and weight. They found that both male and female cocks consume about 0.5 ounces of ethanol per day.
Matthew Currigan, a molecular biologist at the University of Central Florida, was not involved in the study, but the results aren’t necessarily surprising, but they are exciting as they advance “an important question about human ethanol addiction and evolution.”
However, Carrigan pointed out that the error bars have “a very wide range.” This means that chimpanzees can eat 0.14 ounces (4 grams) of ethanol per day, or 0.85 ounces (24 grams).
Malo also admitted that the study sample may have been fruits that chimpanzees refused to eat. The next step is to investigate whether chimpanzees choose to avoid or explore individual fruits based on the degree of fermentation, Carrigan said.
Ultimately, humans and chimpanzees “evolved to get calories without getting drunk,” Carrigan said. “If we otherwise evolve to get drunk, we won’t evolve to metabolize ethanol faster.”
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