The “drunken monkey hypothesis” might sound like a creative drinking game straight out of a college party, but it’s actually a scientific theory that could explain our predilection for alcohol. Primatologists have plenty of anecdotal evidence that suggests that apes—and by extension our ancestors—imbibe from time to time, but nothing too solid. Now we have data to back it up.
Researchers have recorded several chimpanzees in the wild drinking fermented palm sap, or palm wine, from raffia palm trees, according to a study published in the journal Royal Society Open Science. Palm wine has an average of 3.1% alcohol by volume but is sometimes as high as 6.9%—more than most beers. And the chimpanzees aren’t just having a sip.
Researchers estimated that some of the chimpanzees were consuming as much as 85ml of alcohol, or about a bottle of wine. Some became visibly inebriated, even falling asleep soon after drinking. One even became restless, agitatedly moving from tree to tree while the other chimps were preparing to sleep, Dr. Kimberley Hockings, who led the research team, told BBC News.
The joys of palm wine aren’t just known to chimpanzees. The local people in the Bossou area of Guinea, where the study occurred, also drink the palm wine, harvesting it by tapping the trees at the crown and collecting the sap in plastic containers. The chimpanzees, though, use a different method. Here’s Victoria Gill, reporting for BBC News:
Researchers working in the area had already witnessed chimpanzees climbing the trees – often in groups – and drinking the naturally fermented palm sap.
The chimps used drinking tools called leaf sponges – handfuls of leaves that they chew and crush into absorbent sponges, dip into the liquid and suck out the contents.
To work out the extend of the animals’ indulging, the scientists measured the alcohol content of the wine in the containers and filmed the chimps’ “drinking sessions.”
It plays right into the drunken monkey hypothesis, which was first posited by Robert Dudley back in 2004. Dudley suggested that human’s primate ancestors may have followed the scent of fermenting fruit to find food. Once they got there, the naturally occurring ethanol may have spurred their appetites, and so those who ate the fruit may have increased their caloric consumption. So, at one time, it was beneficial to be attracted to ethanol.
Chimps, which share a common ancestor with humans, could have the same inclination toward ethanol that humans do. In fact, a recent study showed that humans and African apes shared the genetic mutation that enabled them to metabolize alcohol.
Still, it remains rare for chimpanzees to voluntarily consume alcohol. Just 51 drinking events by individual primates were observed from 1995 to 2012, and one adult male accounted for 14 of those events.