Monkeys' love for boozy fruit may explain why humans enjoy alcohol

Ripe, naturally fermented fruit may have given our primate ancestors a buzz.
Derya Ozdemir

Our species' attraction to alcohol is not so unique in the animal world.

This was first proposed as the so-called “drunken monkey” hypothesis by Robert Dudley, a professor of Integrative Biology at the University of California.

Our attraction to alcohol began millions of years ago, according to Dudley, when our ape and monkey ancestors found out that the scent of alcohol led them to ripe, fermenting, and extremely nutritious fruits. This would give them an evolutionary advantage, as they could eat energizing fruits before other animals can even get to them.

Now, a new study from California State University, Northridge, adds to the evidence that the drunken monkey hypothesis may indeed be correct.

A direct test of the drunken monkey hypothesis

Dudley wrote in his book "The Drunken Monkey: Why We Drink and Abuse Alcohol" that some fruits known to be eaten by primates have a naturally high alcohol content of up to 7 percent. However, he didn't have any evidence showing that monkeys or apes preferred and ate fermented fruits, or that the alcohol in the fruit was digested.

The new research, which has been published in the journal Royal Society Open Science, was carried out at a field site in Panama, Barro Colorado Island. To measure the ingestion of alcohol-heavy fruits eaten by primates for the first time, the researchers gathered fruit consumed and discarded by black-handed spider monkeys, or Ateles geoffroyi.

The researchers found that the alcohol concentration in the fruits was typically between one and two percent by volume. Then, they collected urine from the monkeys and saw that it contained secondary metabolites of alcohol. This implies that the animals were actually using the alcohol for energy, offering first-of-a-kind evidence that the monkeys actually metabolize the ethanol within it.

Alcoholism from an evolutionary perspective

"For the first time, we have been able to show, without a shadow of a doubt, that wild primates, with no human interference, consume fruit-containing ethanol," said Christina Campbell, a CUSN professor of anthropology who led the study. "This is just one study, and more need to be done, but it looks like there may be some truth to that 'drunken monkey' hypothesis—that the proclivity of humans to consume alcohol stems from a deep-rooted affinity of frugivorous (fruit-eating) primates for naturally-occurring ethanol within ripe fruit."

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However, Dudley stated that he doubts that the monkeys experience the inebriating effects of alcohol that humans tend to appreciate. Instead, they would seek out fermented fruit due to its increased calorie concentration, which amounts to more energy.

Still, what benefits this provides to animals need to be further investigated, as better understanding the roots of our behaviors by looking at our ancestors could help society better understand and deal with the adverse consequences of alcohol abuse. After all, the present availability of inebriants has turned our evolutionary preferences into a major public health problem, as three million deaths every year, which represent 5.3 percent of all deaths, result from harmful use of alcohol.

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