The deep-rooted affinity some monkeys show for ripe fruit could explain why our own species has such a fondness for alcohol.
When studying the diets of black-handed spider monkeys (Ateles geoffroyi) in Panama, researchers found the palm fruits these animals regularly eat contain small doses of ethanol.
Urine samples from two of the spider monkeys also revealed ethanol-specific metabolites, which suggests the alcohol isn't just passing through their bodies, but is being digested and utilized in some way.
"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," says primatologist Christina Campbell from 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."
The drunken monkey hypothesis was first put forward by the UC Berkeley biologist Robert Dudley in 2000. It posits that the strong attraction monkeys show to the smell and taste of ethanol is an evolutionary advantage that allows them to hunt down ripe, energizing fruits and gobble them up before other animals can get to them.
The same hankering for ethanol still exists in our own species but has since been divorced from the nutritional benefits of the whole fruit. Instead, humans have learned to distill spirits, and "the once advantageous appetite for alcohol" seen in our primate ancestors has now become a danger to our wellbeing.
It's an interesting idea, but until now, the evidence for this hypothesis has been limited and mostly anecdotal. Wild chimpanzees (Pan troglodytes), for example, have been caught eating fermented sap from palm trees, and this sap has later been found to contain ethanol concentrations of nearly 7 percent. Yet it's unclear whether the ethanol itself is drawing the chimps to the fruit, or whether they are actually getting drunk.
The research done in Panama is the first to directly measure the ingestion of alcohol-heavy fruits eaten by primates.
Captive spider monkeys have previously shown sensitivity to the odors of ripe fruits that contain ethanol, but this is the first study to show preferential consumption of those fruits in the wild.
Not only do naturally foraging spider monkeys appear to eat a lot of fruit containing ethanol, they also seem to be metabolizing the fermented sugars.
"The monkeys were likely eating the fruit with ethanol for the calories," says Campbell.
"They would get more calories from fermented fruit than they would from unfermented fruit. The higher calories mean more energy."
A similar propensity might also exist in humans. In fact, the fruits eaten by spider monkeys are the same ones used by indigenous human populations in Central and South America to make chicha, which is a fermented alcoholic beverage.
The popularity of this drink could be a byproduct of our craving for ripe fruit. When yeast feeds on sugar, it produces alcohol, probably as a way to fight off other competitors. The volatile compound then wafts through the air, drawing animals like ourselves to the juicy snack.
The more fermented fruit we eat, the more energy we receive, and, possibly, the drunker we get.
In the case of spider monkeys, however, Dudley suspects there is little inebriation. The partially consumed fruits researchers tested only contained a percent or two of ethanol.
"They're probably not getting drunk, because their guts are filling before they reach inebriating levels," explains Dudley.
"But it is providing some physiological benefit. Maybe, also, there's 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. You can't rule that out."
If there is some evolutionary advantage to alcohol, passed on for tens of millions of years from a shared ancestor between ourselves and modern primates, then you'd expect it to show up in an animal's DNA.
And it does. Genes encoding for ethanol metabolism are widespread among mammals that eat fruit and nectar. In fact, humans, chimpanzees, bonobos, and gorillas all share a mutation in a gene that improves an ethanol enzyme by 40-fold.
What benefits that gene ultimately gives animals still needs to be researched, but accessing extra calories likely provided them with an evolutionary edge in an environment where finding calories takes a lot of hard work.
"Given that positive selection on those genes encoding for ethanol catabolism has been substantial among fruit- and nectar-consuming mammalian species more generally, the natural consumption of fermented carbohydrates is likely to be more widespread than is currently realized," the authors conclude.
The study was published in Royal Society Open Science.