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The Drunken Monkey Hypothesis

Clues from our distant ancestral past suggest that an attraction to alcohol may once have conferred an evolutionary advantage, and explain the the mystery of why we humans are often compelled to consume what is essentially a toxin.

Addicted to a toxin

The body responds to alcohol in the same way it would a dangerous toxin. The feeling of being ‘drunk’ occurs when the liver can’t process the amount you are drinking quickly enough. This leads to a cacophony of neurobiological effects such as the release of Gamma-Aminobutyric Acid – GABA, a neurotransmitter that dampens responses – and the reduction in the uptake of glutamate, a substance that is involved in the brain’s excitatory functions.

A great deal of research has gone into the effects of alcohol on the body, but what remains a mystery is just why humans are so attracted to alcohol, which is essentially a toxin, in the first place. An emerging evolutionary explanation could hold the key.

The odour of fermentation

The ‘Drunken Monkey hypothesis’ proposed by Dr Robert Dudley, from the University of California at Berkeley — and the subject of his book ‘The Drunken Monkey: Why we drink and abuse alcohol’ — seeks to unravel the mystery by taking a trip back to the times of our ancient forebears, the primates.

Of all the drugs that are enjoyed by society today, alcohol is one of only a handful which occur naturally, and has done for a very long time. Dudley claims that our attraction to alcohol has also stood the test of time — over tens of millions of years.

Alcohol, or rather ethanol, is formed during the fermentation of ripening fruits. Dudley suggest that the ability to detect odours secreted by ripening fruit would have helped primates locate food sources vital to their survival, often in rainforest areas where calories were scarce. Natural selection favoured those primates with an attraction to ethanol and their brains became hardwired to associate ethanol with nutrition.

Monkeys separated from their group were more likely to 'abuse' alcohol

Drunken macaques

Support for Dudley’s hypothesis soon came from a series of monkey studies led by Scott Chen of the National Institutes of Health Animal Centre in Maryland, USA. Chen’s studies demonstrated the potential for monkeys to engage both in voluntary alcohol consumption . . . and alcohol abuse. He found that macaque monkeys exhibit behaviours in response alcohol that intriguingly similar to human behaviours:

  • Macaques who were separated from their peers, tended to consume more alcohol on average
  • After a particularly stressful day, monkeys tended to indulge more than they did after a comparatively calm day
  • Monkeys separated from their group were more likely to ‘abuse’ alcohol, consuming about four times as much as those who remained in the social group setting.

Boozing vervets

Similar behaviours were confirmed in a study that observed the patterns of voluntary alcohol consumption in vervet monkeys. In this research, a number of fascinating individual differences were observed, such as that older monkeys imbibed lower frequencies than younger ones – a finding that has been replicated in the drinking behaviour of human teenagers.

  • Social drinkers, the majority of the monkeys, prefer alcohol diluted in fruit juice, will only drink in the company of other monkeys, and not before lunch.
  • Regular drinkers: fifteen percent of the monkeys prefer their alcohol “neat” or diluted in water, not sweetened or diluted with fruit juice. Interestingly, steady drinkers do very well in social groups, and are good leaders. They run troops well, they keep order well, and they’re very dominant. This type of alcoholic monkey is a very functional animal.
  • Binge drinkers: five percent of the monkeys drink their alcohol fast, get in fights, and drink themselves into a coma. If this group has unrestricted access to alcohol, they will drink themselves to death within 2-3 months.
  • Teetotaler: fifteen percent of the monkeys prefer little or no alcohol.

Inebriated chimpanzees

In a 17-year-long study conducted by a group of researchers in Guinea, West Africa, wild chimpanzees, our closest primate cousins, were observed utilising crushed leaves as sponges in order to drink fermented palm sap produced by indigenous raffia palm trees.

Males and females were equally keen on the drink, but chimps varied in how much they imbibed. Of 26 chimps observed, 13 were apparently teetotal. Some of the chimps drank until they were ‘visibly inebriated’. At one event, the amount of alcohol ingested reached 85 millilitres, the equivalent of about three pints of Stella Artois.


So, our monkey cousins can, did, and still do, get drunk and – with increasing frequency and at earlier ages – so do we. While such a link can never be entirely proved, it provides an intriguing insight into our ancestral origins and an shrewd alternative to conventional explanations of why humans have such a taste for intoxication.

City Addictions, April 14, 2017


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