'Drunken Monkey' Hypothesis: These Spider Monkeys Eat Alcohol-Containing Fruits For Energy

The scientists also collected urine from these free-ranging monkeys, and found that the urine contained secondary metabolites of alcohol.

New Delhi: Human beings' fetish for alcohol is said to have arisen millions of years ago, when their ape and monkey ancestors discovered that the scent of alcohol led them to ripe, fermenting and nutritious fruit. In 2014, University of California Berkeley biologist Robert Dudley wrote a book titled, The Drunken Monkey: Why We Drink and Abuse Alcohol, in which he described the theory behind humans' love of alcohol. Dudley calls this idea the "drunken monkey" hypothesis. 

Now, an international team of researchers have described the "drunken monkey" hypothesis in a new study published in the journal Royal Society Open Science.

Black-Handed Spider Monkeys Consume Alcohol-Containing Fruits 

The study was led by researchers at California State University, Northridge (CUSN), who collected fruit eaten and discarded by black-handed spider monkeys (Ateles geoffroyi) in Panama. The alcohol concentration in the fruit was typically between one to two per cent by volume, a by-product of natural fermentation by yeasts that eat sugar in ripening fruit, the researchers found.

Urine Contained Secondary Metabolites Of Alcohol

The scientists also collected urine from these free-ranging monkeys, and found that the urine contained secondary metabolites of alcohol. Secondary metabolites are small, organic molecules produced by an organism that are not essential for their growth, development, and reproduction. 

Monkeys Utilising Alcohol For Energy

The findings suggest that the animals were actually utilising the alcohol for energy, and it was not just passing through their bodies. 

In a statement issued by University of California, Berkeley, primatologist Christina Campbell of CSUN said that for the first time, researchers have been able to show, without a shadow of doubt, that wild primates, with no human interference, consume fruit-containing ethanol. She further said that this is just one study, and more need to be done, but it looks like there may 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-occuring ethanol with ripe fruit." 

According to Dudley's book, measurements show that some fruits known to be eaten by primates have a naturally high alcohol content of up to seven per cent. However, at that time, the biologist did not have data showing that apes or monkeys preferentially sought out and ate fermented fruits.

The CUSN researchers teamed up with Dudney and Aleksey Maro, a student at University of California, Berkeley, to analyse the alcohol content in the fruits.

Monkeys Are Metabolising Alcohol

Dudley said that the study is a direct test of the drunken monkey hypothesis. First, the study found that there is ethanol in the food they are eating, and they are eating a lot of fruit. Second, the monkeys are actually metabolising alcohol. Secondary metabolites, ethyl glucuronide and ethyl sulphate are coming out in the urine of the monkeys, Dudley said. 

He said what researchers do not know is how much of it they are eating and what the effects are behaviourally and physiologically. 

According to the study, the fruit that spider monkeys sniffed and took a bite out of routinely had alcohol concentrations of between one per cent and two per cent, about half the concentration of low-alcohol brews. 

Monkeys Collected Ripe Fruits From Which Tree?

The monkeys collected ripe fruits from the jobo tree, or Spondias mombin. The fruits were a major component of the spider monkey diet. 

Indigenous human populations throughout Central and South America had used the fruit for millennia to make chicha, a fermented alcoholic beverage, the study said.

The urine from six spider monkeys was collected as part of the study. Of these, five samples contained secondary metabolites of ethanol.

Campbell said the monkeys were likely eating the fruit with ethanol for the calories, and that they would get more calories from fermented fruit than they would from unfermented fruit. The higher calories mean more energy, the researcher explained.

Why Are The Monkeys Not Inebriated?

Dudley said the monkeys are probably not getting drunk, because their guts are filling before they reach inebriating (exhilarated by alcohol) levels. 

He added that it is providing some physiological benefit. 

Antimicrobial Benefits Of Consuming Fermented Fruits

Dudley explained that there could be an antimicrobial benefit within the food that they are consuming, or the activity of the yeast and the microbes may be pre digesting the fruit.

Campbell said that human ancestors' decisions when choosing which fruit to eat may have been similarly influenced by the need for the monkeys' high caloric intake.

She said that human ancestors may also have preferentially selected ethanol-laden fruit for consumption, given that it has more calories.

How Is The Research Important?

Campbell asserted that excessive consumption of alcohol, as with diabetes and obesity, can then be viewed conceptually as a disease of nutritional excess. 

The researchers noted in the study that the idea that humans' natural affinity for alcohol is inherited from their primate ancestors could help society deal with the adverse consequences of alcohol abuse.