Black-handed spider monkeys in Panama have a taste for ethanol-rich fruit, which could shed light on the evolutionary origins of humans’ predilection for booze. According to a new study in the journal Royal Society Open Science, the animals’ alcoholic tendencies may support the so-called “drunken monkey” hypothesis, which states that our love of liquor derives from the dietary habits of our primate ancestors.
First proposed by biologist Robert Dudley from the University of California, Berkely, the drunken monkey theory centers on the idea that fruit-eating animals are likely to benefit from consuming slightly over-ripe fruit due to its higher sugar content and calorific value. As these sugars ferment, however, they produce ethanol, which means that these animals may have evolved to guzzle alcoholic food.
While the consumption of ethanol-rich fruit has been documented throughout the animal kingdom, scientists had never previously determined whether non-human primates actually have the capacity to metabolize alcohol in order to harness its calories. To investigate, Dudley and his colleagues traveled to Panama’s Barro Colorado Island, where local spider monkeys spend a good deal of their time devouring the sugary fruit of the local jobo tree.
After examining the partially-eaten fruit dropped by foraging monkeys, the researchers found that the pulp contained an average of 1 to 2 percent ethanol, indicating that the animals do indeed have a preference for alcoholic food. "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," explained study author Dr Christina Campbell in a statement.
"The monkeys were likely eating the fruit with ethanol for the calories," she said. "They would get more calories from fermented fruit than they would from unfermented fruit. The higher calories mean more energy."
The researchers also collected urine samples from six foraging spider monkeys, and detected secondary metabolites of alcohol consumption in five of these. This indicates that the animals are indeed able to digest booze and access its calories.
"This is just one study, and more need to be done,” said Campbell. “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."
While Dudley insists that the quantity of alcohol consumed by the Panamanian monkeys isn’t enough to get them drunk, the authors state that “human ancestors may also have preferentially selected ethanol-laden fruit for consumption” due to its high-calorie content.
In other words, our love of booze may stem from the fact that we evolved from fruit-eating primates who sought out fermented sugars. “Contemporary patterns of alcohol consumption, in turn, may derive from these ancestral associations between ethanol and nutritional reward,” write the researchers.
Indeed, while our penchant for alcohol served us well when ripe fruit was our only poison, the present availability of inebriants has made drunken monkeys of us all and turned our evolutionary preferences into a major public health problem. Viewed from this perspective, the authors conclude that “excessive consumption of alcohol, as with diabetes and obesity, can then be viewed conceptually as a disease of nutritional excess.”