Spider monkeys seek out fruit with alcohol, finds research into human love of drinking

Spider monkeys routinely seek out fruit ripe enough to contain alcohol, a new study suggests, lending weight to a theory that humans’ enthusiasm for drinking was inherited from our biological ancestors.

According to the “drunken monkey” hypothesis, proposed in 2014 by Berkeley biologist Professor Robert Dudley, our attraction to alcohol dates back millions of years to when primates discovered that the smell of ethanol led them to ripe, fermenting and nutritious fruit.

But while his theory was based on knowledge that some fruits eaten by primates have an alcohol content of up to 7 per cent, he was unable to provide evidence on whether monkeys and apes actually sought out fermented fruits or digested the alcohol.

Now, however, a new study in which scientists collected fruit eaten and discarded by black-handed spider monkeys in Panama suggests they do in fact prefer fruit ripe enough to contain ethanol.

Researchers found the jobo tree fruits – also used for millennia by humans to make the alcoholic drink chica – which spider monkeys sniffed and took a bite out of routinely had alcohol concentrations of between 1 and 2 per cent.

The scientists also took urine samples from six spider monkeys, five of which contained secondary metabolites of ethanol – showing that they were actually utilising the alcohol for energy rather than it merely passing through their bodies.

“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 the study’s lead author, Dr Christina Campbell, a primatologist at California State University, Northridge.

“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 [fruit-eating] primates for naturally-occurring ethanol within ripe fruit.”

Prof Dudley, who was a co-author of the study – published this month in the Royal Society Open Science journal – called it “a direct test of the ‘drunken monkey’ hypothesis”.

“Part one, there is ethanol in the food they’re eating, and they’re eating a lot of fruit. Then, part two, they’re actually metabolising alcohol,” he said. “What we don’t know is how much of it they’re eating and what the effects are behaviourally and physiologically. But it’s confirmatory.”

Prof Dudley said the spider monkeys are “probably not getting drunk, because their guts are filling before they reach inebriating levels”, but that the alcohol “is providing some physiological benefit”.

He suggested that there could be “an anti-microbial benefit within the food that they’re consuming, or the activity of the yeast and the microbes may be predigesting the fruit”, adding: “You can’t rule that out.”

Dr Campbell said the monkeys were likely choosing the alcoholic fruit because fermented fruit would provide them with more calories – and in turn more energy – than unfermented fruit.

This may have similarly influenced human ancestors’ decisions when choosing which fruit to eat, Dr Campbell said.

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

Today, of course, the availability of alcohol in liquid form, without the gut-filling pulp of fermenting fruit, means it’s easy to overindulge.

Researchers argue the contemporary overindulgence in alcohol could derive from these ancestral associations between ethanol and nutritional rewards, and could even help society deal with the adverse consequences of alcohol use.

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