When it comes to imbibing, humans don't have a monopoly on the hard stuff. Chimpanzees, it turns out, go ape over alcohol.
That's according to a nearly two-decade-long study that recorded chimpanzees in the West African nation of Guinea using leaves to drink.
Their tipple of choice? The fermented sap of raffia palm trees.
The 17-year study, detailed in this week's issue of Royal Society Open Science, marks the first time researchers have recorded voluntary alcohol consumption in nonhuman primates.
Residents in the town of Bossou, Guinea, tap raffia palm trees with plastic jugs to catch the trees' sap, which quickly ferments into a cider-like wine.
The town's nonhuman primate residents quickly caught on. Making use of their well-recorded tool-making skills, chimps fashioned drinking tools from handfuls of crushed and chewed leaves that they used to dip into the palm wine and suck out the fermented sap.
The researchers saw 51 instances between 1995 and 2012 in which 13 chimps used leaf sponges to partake in the jungle ethanol.
Some chimps consumed so much alcohol – as much as the equivalent of a bottle of wine – that they showed “visible signs of inebriation," such as falling asleep immediately or swinging around the jungle trees restlessly, said the leader of the research team, Kimberley Hockings from Oxford Brookes University and the Centre for Research in Anthropology in Portugal.
The sap averaged about 3.1 to 6.9 percent alcohol, according to the study, similar to most types of beer.
Why do chimps drink? Most animals show no interest in alcohol, a neurotoxin. A handful of species seem to have evolved a tolerance: For instance, fruit flies eat alcohol-producing yeast to poison parasites. And there's a Malaysian tree shrew that chronically consumes high-proof palm nectar, with no apparent ill effects.
Primates, it seems, are different. Monkeys in the Caribbean have been known to steal swigs of tourists' cocktails. And humans, of course, have been voluntarily consuming alcohol since as early as 9,000 years ago, far longer than any other addictive substance, as far as we know.
The findings, which mark the first recorded instance of voluntary, habitual consumption of alcohol by primates, may support the so-called Drunken Monkey Hypothesis, which proposes that roughly 10 million years ago, apes developed a genetic mutation that makes it easier to break down alcohol. That evolutionary adaptation gave ancient primates a huge advantage: the ability to consume and digest the additional calories found in fermented fruits and sap.
Alcohol also acts as an appetite stimulant, which could further help a primate consume more calories.
As reports have indicated, the olfactory senses of humans and primates "are particularly well tuned to detect high sugar and alcohol concentrations," the better to get the most calorie bang for our buck.
As Richard Byrne, an evolutionary biologist from the University of St Andrews in Scotland, told the BBC, this adaptation, exhibited for the first time in wild chimps, "opened access to good energy sources – all that simple sugar – that were accidentally 'protected' by noxious alcohol," which prevented other animals from consuming them.
If this is true, then those tipsy chimps, while surprising, are no accident.