In 2014, the biologist at the University of California at Berkeley, Robert Dudley, wrote a book in which he proposed that our attraction to drinking arose millions of years ago, when our ape and monkey ancestors discovered that the smell of alcohol led them to ripe, fermented and nutritious fruit. A new study supports Dudley’s hypothesis, which he once called the “drunken monkey” hypothesis .
The current study has been led by primatologist Christina Campbell, from California State University Northridge (CSUN), and her graduate student Victoria Weaver has collaborated on it. What they did was collect the fruit that the black-handed spider monkeys ( Ateles geoffroyi ) ate and discarded in Panama. They found that the alcohol concentration in the fruit was usually between 1% and 2% by volume. This is a byproduct of the natural fermentation of yeasts that feed on the sugar in the ripening fruit.
In addition, the researchers collected and analyzed urine from these free-living monkeys. What they saw was that it contained secondary metabolites of alcohol. This result demonstrates that animals use alcohol as a source of energy .
“For the first time, we have been able to show beyond doubt that wild primates, without human interference, consume fruit containing ethanol,” said Campbell. “This is just one study, and more needs to be done, but it seems there may be some truth to that ‘drunken monkey’ hypothesis: that humans’ proclivity to consume alcohol stems from a deep-seated affinity for frugivorous primates ( that eat fruit) because of the ethanol found naturally in ripe fruit.
Robert Dudley laid out the evidence for his theory about why we humans like to drink alcohol eight years ago in the book The Drunken Ape: Why We Drink and Abuse Alcohol . Measurements showed that some fruits known to primates have a natural alcohol content of up to 7%. But at that time there was no data showing that monkeys or apes preferentially sought out and ate fermented fruit, or digested the alcohol in the fruit.
For the new study, CSUN researchers teamed up with Dudley and UC Berkeley graduate student Aleksey Maro to analyze the alcohol content of the fruits. Maro is also conducting a study on the alcohol content in the fruit-based diet of chimpanzees in Uganda and the Ivory Coast.
“It (the study) is a direct test of the drunken monkey hypothesis,” Robert Dudley said. “First of all, there’s ethanol in the food they eat, and they eat a lot of fruit . Then, second, they’re metabolizing the alcohol: the secondary metabolites, ethyl glucuronide and ethyl sulfate, come out in the urine . What they don’t What we know is how much they’re eating and what the effects are behaviorally and physiologically. But it’s confirmatory.”
The study, which appears this month in the journal Royal Society Open Science , was carried out in a field location, Panama’s Barro Colorado Island, where Dudley has often done research and where he began to think about the role of ethanol in the diet of animals and how it could influence our enjoyment and abuse of alcohol.
The researchers found that the fruit that the spider monkeys habitually sniffed and nibbled had alcohol concentrations of between 1% and 2%, about half the concentration of low-alcohol beers. The fruit has also been used for millennia by indigenous populations throughout Central and South America to make chicha, a fermented alcoholic beverage.
“It’s likely that the monkeys ate the ethanol-laced fruit for the calories ,” Campbell said. “You would get more calories from fermented fruit than from unfermented fruit. Higher calories mean more energy.”
As for whether animals feel the intoxicating effects of alcohol that we humans do, Dudley has his doubts. “They probably don’t get drunk, because their guts fill up before they reach levels of drunkenness,” he said. “But it’s providing them with some physiological benefit. Maybe, too, there’s an antimicrobial benefit within the food they’re eating, or yeast activity and microbes may be predigesting the fruit. You can’t rule that out,” he said. biologist.
According to Campbell, the need for monkeys to consume a lot of calories may have similarly influenced the decisions of human ancestors when choosing which fruit to eat. “Human ancestors may also have preferentially selected the ethanol-laden fruit for consumption, given that it has more calories,” he said. “The psychoactive and hedonic effects of ethanol can also cause increased consumption rates and calorie gain.”
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Reference:
Campbell et al. 2022. Dietary ethanol ingestion by free-ranging spider monkeys (Ateles geoffroyi). Royal Society Open Science. DOI: 10.1098/rsos.211729
