Monkeys often eat fruit containing alcohol, which illustrates our fondness for alcohol

A new study of black-handed spider monkeys in Panama shows they seek out and eat fruit ripe enough to be fermented and containing up to 2% ethanol. The results shed light on the theory that the human propensity to drink alcohol may have its roots in our ancient ancestors’ propensity to consume fermenting but nutritious fruit. (Photo by Victoria Weaver/CSUN)

For 25 years, UC Berkeley biologist Robert Dudley has been fascinated by people’s love of alcohol. In 2014 he wrote a book in which he suggested that our fondness for alcohol originated millions of years ago when our apes and simian 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 “drunk monkey” hypothesis.

The study was led by primatologist Christina Campbell of California State University, Northridge (CSUN) and her graduate 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 typically between 1% and 2% by volume, a byproduct of natural fermentation by yeasts that eat sugars in ripening fruit.

In addition, the researchers collected urine from these free-ranging monkeys and found that the urine contained secondary metabolites of alcohol. This result shows that the animals were actually using the alcohol for energy – it wasn’t just flowing through their bodies.

“For the first time, we have been able to show beyond doubt that wild primates consume fruit-based ethanol without human intervention,” said Campbell, a CUSN professor of anthropology who received her Ph.D. in Anthropologie of Berkeley in 2000. “This is just one study and more needs to be done, but it looks as if there may be some truth to this ‘drunken monkey’ hypothesis – that the inclination of humans 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 in the book eight years ago, The Drunken Monkey: Why We Drink and Abuse Alcohol. Measurements have shown that some fruits known to be eaten by primates have naturally high alcohol levels of up to 7%. But at the time, he had no data showing that monkeys or apes preferentially sought out and ate fermented fruits, or that they digested the alcohol in the fruits.

In this 2014 video, Robert Dudley explains his “drunk monkey” hypothesis and his reasons for writing a book about the primate origins of our love of alcohol. (Video by Roxanne Makasdjian and Stephen McNalley)

For the newly published study, CSUN researchers teamed up with Dudley and Aleksey Maro, a UC Berkeley graduate student, to analyze the alcohol content in the fruit. Maro is conducting a parallel study on the alcohol content in the fruit-based diet of chimpanzees in Uganda and Ivory Coast.

“It (the study) is a direct test of the drunk monkey hypothesis,” said Dudley, professor of integrative biology at UC Berkeley. “First part, 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 are excreted in the urine. What we don’t know is how much of it they eat and what effects it has on behavior and physiology. But it’s affirmative.”

The study, which appeared in the journal this month Open Science of the Royal Societywas conducted at a field site on the island of Barro Colorado in Panama, where Dudley has done extensive research and where he first began thinking about the role of ethanol in animal nutrition and how it might impact our alcohol consumption and abuse.

The researchers found that the fruit that spider monkeys sniffed and routinely bite off had alcohol concentrations between 1% and 2%, about half the concentration of low-alcohol beers. The ripe fruit they gathered came from the Jobo tree. Spondias mombin, and were a staple of the spider monkey diet. But the fruit has also been used for millennia by indigenous peoples across 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. “You would get more calories from fermented fruit than from unfermented fruit. The higher calories mean more energy.”

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

“They probably don’t get drunk because their guts fill up before they get to intoxicating levels,” he said. “But it offers some physiological benefit. There may also be an antimicrobial benefit in the food they consume, or the activity of the yeast and microbes can pre-digest the fruit. You can’t rule that out.”

Similarly, the need for the apes’ high caloric intake may have influenced human ancestors’ decisions about what fruit to eat, Campbell said.

“Human ancestors may also have preferred to choose ethanol-laden fruits for consumption because they contain more calories,” she said. “Psychoactive and hedonic effects of ethanol may similarly result in increased consumption rates and calorie gain.”

Today, the availability of alcohol in liquid form, without the gut-filling pulp of fermenting fruit, means it’s easy to overindulge. The idea that humans’ natural affinity for alcohol is inherited from our primate ancestors could help society deal with the adverse effects of alcohol abuse.

“Excessive alcohol consumption, as in diabetes and obesity, can then be viewed conceptually as a disease of nutritional excess,” Campbell said.

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