Monkeys routinely consume fruit containing alcohol, shedding light on our own taste for booze: Study supports ‘drunken monkey’ hypothesis: humans inherited love of alcohol from primate ancestors

For 25 years, UC Berkeley’s biologist Robert Dudley has been fascinated by people’s love of alcohol. In 2014, he wrote a book that suggested that our attraction to spirits arose millions of years ago, when our monkeys and apes’ ancestors discovered that the smell of alcohol led them to ripe, fermenting and nutritious fruit.

A new study now supports this idea, which Dudley calls the hypothesis of “stuffed monkey”.

The study was led by primatologist Christina Campbell of California State University, Northridge (CSUN), and her doctoral student Victoria Weaver, who collected fruit eaten and discarded by black-handed spider monkeys (Ateles geoffroyi) in Panama. They found that the alcohol concentration in the fruit was usually between 1 and 2% by volume, a by-product of natural fermentation of yeast that eats sugar in ripe fruit.

In addition, the researchers collected urine from these free-range monkeys and found that the urine contained secondary metabolites of alcohol. This result shows that the animals actually used alcohol for energy – it did not just pass through their bodies.

“For the first time, we have been able to show, without a shadow of a doubt, that wild primates, without human intervention, consume fruit-containing ethanol,” said Campbell, a CUSN professor of anthropology who received his doctorate. . in anthropology from Berkeley 2000. “This is just a study, and more needs to be done, but it seems that there may be some truth in that ‘drunk monkey’ hypothesis – that human propensity to consume alcohol stems from a deep-rooted affinity of frugivorous (fruit-eating) primates for naturally occurring ethanol in ripe fruit. “

Dudley presented evidence for his idea eight years ago in the book, The Drunken Monkey: Why we drink and abuse alcohol. Measurements showed that some fruits that are known to be eaten by primates have a naturally high alcohol content of up to 7%. But at that time he had no data that showed that monkeys or monkeys preferred to seek out and eat fermented fruits, or that they melted the alcohol in the fruit.

For the recently reported study, CSUN researchers collaborated with Dudley and UC Berkeley student Aleksey Maro to analyze the alcohol content of the fruits. Maro is conducting a parallel study on the alcohol content of the fruit-based diet for chimpanzees in Uganda and Côte d’Ivoire.

“It (the study) is a direct test of the hypothetical monkey hypothesis,” said Dudley, a professor of integrative biology at UC Berkeley. “Part one, there is ethanol in the food they eat, and they eat a lot of fruit. Then, part two, they actually metabolize alcohol – secondary metabolites, ethyl glucuronide and ethyl sulfate come out in the urine. What we do not know is how much of what they eat and what the effects are behaviorally and physiologically. But it is affirmative. “

The study, which was published this month in the journal Royal Society Open Sciencewas conducted at a field site, Barro Colorado Island in Panama, where Dudley has frequently researched and where he first began to think about the role of ethanol in animal diets and how it can play into our enjoyment and abuse of alcohol.

The researchers found that the fruit that spider monkeys sniffed and took a bite of routinely had alcohol concentrations of between 1% and 2%, about half the concentration of low-alcohol brews. The ripe fruits they gathered were from the job tree, Spondias mombin, and was an important component of the spider diet. But the fruit has also been used for millennia by indigenous peoples throughout Central and South America to make chicha, a fermented alcoholic beverage.

The researchers also collected urine from six spider monkeys. Five of the samples contained secondary metabolites of ethanol.

“The monkeys probably ate the fruit with ethanol for the calories,” Campbell said. “They would get more calories from fermented fruit than they would get from unfermented fruit. The higher the calories, the more energy.”

Dudley said he doubts the monkeys feel the intoxicating effects of alcohol that humans appreciate.

“They probably do not get full because their intestines fill up before they reach intoxicating levels,” he said. “But it gives a certain physiological advantage. Maybe there is also an antimicrobial advantage in the food they consume, or the activity of the yeast and the microbes can digest the fruit. You can not rule it out.”

The need for the monkeys’ high calorie intake may similarly have influenced the decision of human ancestors when choosing which fruit to eat, Campbell said.

“Human ancestors may also have preferentially selected ethanol-containing fruit for consumption, given that it has more calories,” she said. “Psychoactive and hedonic effects of ethanol can similarly result in increased consumption and calorie gain.”

Today, the availability of alcohol in liquid form, without the gut-filling pulp from fermenting fruit, means that it is easy to overdo it. The idea that humans’ natural affinity for alcohol is inherited from our primates can help society deal with the negative consequences of alcohol abuse.

“Excessive alcohol consumption, as with diabetes and obesity, can then be seen conceptually as a disease of malnutrition,” Campbell said.

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