As if sharing 99 percent of our DNA was not proof enough of our close relationship with chimpanzees, here's another thing we have in common: like us, chimps enjoy alcohol.

A new study published in the journal Royal Society Open Science surveyed field data collected between 1995 and 2012 that documented chimps outside the town of Bossou, Guinea, drinking fermented palm sap, known by locals as palm wine. Some of the chimps even drank to the point of intoxication.

Locals gather the sugary sap by tapping the trees and collecting the fermented ooze in plastic containers. But the clever chimps have figured out a way to get at the alcoholic sap: by using palm leaves to sop up the liquid. The chimps snatch leaves from trees, fold or crumple them in their mouths, and then insert the wad into the container, which enables them to scoop or soak up the palm sap. They then plunge the soaked wad into their mouths, compress it between their tongue and palate, and swallow the sweet elixir.

"The consumption of ethanol by modern-day humans is nearly universal, being found in every society with fermentable raw materials," says Kimberley Hockings, an anthropologist at Oxford Brookes University and lead author of the study. "However, aside from enforced ingestion in captive experiments or anecdotal observations in wild apes, the habitual and voluntary consumption of ethanol has been documented until now only in humans."

The research bolsters what has come to be known as the "drunken monkey hypothesis," which proposes that a genetic mutation arose around 10 million years ago that gave the common ancestor of modern humans and apes the ability to digest fermented fruit. This ability would have conferred an advantage, for it enabled our ancestors to broaden their diet by including over-ripe fruit that had fallen to the forest floor.

The palm wine contains around 3 percent alcohol. Researchers estimated the wild chimps consumed around a quart per "drinking session" and even observed one chimp guzzling 3 liters of the wine.

"Some of the chimpanzees at Bossou consumed significant quantities of ethanol and displayed behavioral signs of inebriation," the researchers wrote. Some of the chimps were even seen kicking back and resting following their drinking sessions.

The study has further implications for chimp behavior, for it demonstrates the primates' ability to produce and manipulate tools, and may even hint at chimp culture.

"They applied their knowledge of how to make and use leafy tools to exploit a new liquid resource, palm wine," Hockings says. "Although our data set do not allow us to test for how this behavior spread through the community, the behavior is consistent with some degree of social transmission."