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Scientists are serving up the best evidence yet that chimpanzees don't mind the taste of alcohol — and in fact will go out of their way for a drink. The findings support the view that the common ancestor of modern apes and humans became adapted to the consumption of alcohol millions of years ago.
That view, known as the "drunken monkey hypothesis," proposes that a genetic mutation made it easier for human ancestors to break down alcohol, and thus made it easier to digest fermenting fruit when necessary. One team of scientists even identified the potential target genes.
The study detailed in this week's issue of Royal Society Open Science doesn't prove the hypothesis — but it does show scientifically that the presence of alcohol, or ethanol, doesn't deter wild chimps from taking a drink. That's a first for non-human primates in the wild, said lead author Kimberley Hockings, an anthropologist at Oxford Brookes University.
"The consumption of ethanol by modern-day humans is nearly universal, being found in every society with fermentable raw materials," Hockings told NBC News in an email. "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."
Scientists have found that tree shrews in Malaysia habitually sip palm nectar with an alcoholic punch. They've shown that bats occasionally consume alcohol in ripened fruit. They've tested monkeys in the Caribbean that sneak sips from tourists' cocktails. But the newly published research sets a new standard for the study of non-human alcohol use.
For 20 years, Hockings and her colleagues have been monitoring a community of chimpanzees near the town of Bossou in the West African nation of Guinea. The area's human residents tap into Raffia palm trees and hook up plastic containers to catch the flow of the sap, which quickly ferments into a tasty wine.
It turns out that the chimps like the stuff as well: The researchers documented 51 instances between 1995 and 2012 in which chimps climbed up into the palm trees and stole some tastes of the wine. The chimps turned palm leaves into sponges of a sort, dipped them into the containers and sucked the sweet liquid down.
'Signs of inebriation'
The researchers went out of their way to calculate the alcohol content of the drink, and came up with an average of 3.1 percent. That's about a quarter of the strength of your typical Riesling. They also estimated how much the chimps were drinking — on average, about a quart (1 liter) per session for each adult chimp. During one of the drinking sessions, a chimp drank 3 liters of wine.
"Some of the chimpanzees at Bossou consumed significant quantities of ethanol and displayed behavioral signs of inebriation," the researchers wrote. Although the scientists didn't collect detailed data about the behavioral effects, they noted that some of the chimps rested right after a drinking bout.
Does all this mean that chimps are natural-born drinkers? Hockings acknowledged that Bossou's chimps took advantage of an alcoholic drink that humans had collected, but she pointed out that "they learned and continue to ingest palm wine on their own accord."
"They applied their knowledge of how to make and use leafy tools to exploit a new liquid resource, palm wine," she wrote. "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."
For Hockings, the bottom line is that it's worth conducting more research into the drinking habits of non-human primates. Further studies could show whether the "drunken monkey hypothesis" stands up to scrutiny — or should be thrown into the gutter.
"In the absence of data we can only speculate, but maybe fermented fruits were consumed more by the last common ancestor of living African apes and humans during certain times of year, when wild fruits were scarce and resources limited, hence providing an adaptive advantage through caloric gain," Hockings said. "We need more research in this area."
Update for 9:15 p.m. June 10: Berkeley primatologist Katharine Milton has been skeptical about previous reports of alcohol use by non-human primates in the wild, and she sent along this email responding to the latest research:
"The chimps at Bossou are highly habituated to humans, and routinely eat many cultivated foods the villagers produce including oranges, bananas, papayas and mangoes. Over a period of 17 years, some of these habituated chimps have occasionally been seen raiding unprotected containers of a sweet-tasting fermented palm sap beverage. Such observations do not strike me as surprising. Chimpanzees are very strongly attracted to substances with a pleasant sweet taste, especially when such items are produced and provided for them by humans! There are no data to show how these chimpanzees were first introduced to the palm beverage — perhaps a villager somehow encouraged this, or perhaps the beverage is stored in containers identical to the ones that villagers use to store palatable foods chimps also like to eat. No data are presented to show ethanol is serving as the attractant, or that the chimps are even aware of it. There are no data suggesting that wild apes or monkeys are attracted to overripe (rotting) fruits — the stage at which fruits would contain notable ethanol. ... There is a vast literature available on this broad and complex topic for those interested to explore."
This research paper from 2004 provides an introduction to the topic from Milton's perspective.
In addition to Hockings, the authors of "Tools to Tipple: Ethanol Ingestion by Wild Chimpanzees Using Leaf-Sponges" include Nicola Bryson-Morrison, Susana Carvalho, Michiko Fujisawa, Tatyana Humle, William C. McGrew, Miho Nakamura, Gaku Ohashi, Yumi Yamanashi, Gen Yamakoshi and Tetsuro Matsuzawa.