Scientists first hypothesized an evolutionary advantage to humans’ taste for ethanol about 15 years ago, when a biologist at the University of California, Berkeley, proposed what has come to be called the “drunken monkey hypothesis.” Robert Dudley argued that our primate ancestors got an evolutionary benefit from being able to eat previously unpalatable fruit that had fallen to the ground and started to undergo fermentation. The hypothesis received a boost last year, when a team led by Matthew Carrigan—a biologist at Santa Fe College in Gainesville, Florida—found that the key enzyme that helps us metabolize ethanol underwent an important mutation about 10 million years ago. This genetic change, which occurred in the common ancestor of humans, chimps, and gorillas, made ethanol metabolism some 40 times faster than the process in other primates—such as monkeys—that do not have it. According to the hypothesis, the mutation allowed apes to consume fermented fruit without immediately getting drunk or, worse, succumbing to alcohol poisoning.
Nevertheless, researchers had turned up little evidence that primates in the wild regularly eat windfall fruit or are attracted to the ethanol that such fruit contains. Now, a team led by Kimberley Hockings, a primatologist at the Center for Research in Anthropology in Lisbon, concludes from a 17-year study of chimps in West Africa that primates can tolerate significant levels of ethanol and may actually crave it, as humans do.
The researchers used video cameras to observe a troop of 26 wild chimps living in Bossou, Guinea, between 1995 and 2012. The villagers living around Bossou routinely tap into the raffia palm tree and collect its sap, which ferments in plastic buckets before being drunk. The villagers collect the fermented palm wine, which has an alcohol content as high as 6.9%, in the early morning and late afternoon. While villagers were away, the chimps approached the buckets, fashioned drinking cups from folded leaves—a toolmaking skill widely observed among wild chimps—and proceeded to consume the beverage themselves. As the team reports online today in Royal Society Open Science, over 17 years it observed 20 “drinking sessions” involving 13 of the chimps, who lapped up the sap at an average rate of about nine leaf dips per minute. On the low end of the scale, that’s roughly equivalent to one liter of beer per session. The 13 chimps included males, females, and young chimps, although not babies. The other 13 animals were never observed drinking during the entire period.
Hockings says that although the team was able to collect quantitative data on chimp drinking habits for the first time, the researchers could offer only anecdotal evidence about whether the chimps actually got drunk from the palm wine. On one occasion, Hockings relates, a male chimp “seemed particularly restless” and spent an hour “moving from tree to tree in an agitated manner” while other chimps were settling down to sleep. But she says it would be “pure speculation” to say that he was actually inebriated.
Indeed, the team cautions that its observations should not be interpreted to mean that chimps actually crave ethanol, but only that they can tolerate it. The palm sap contains sugars like sucrose and glucose and minerals that chimps might want and need in their diet, the researchers point out. “An experimental trial to provide chimpanzees with access to fermenting and nonfermenting palm sap is needed to test whether ethanol is an attractant or not,” Hockings says, something that would not be easy with wild chimp populations.
Nevertheless, the study indirectly supports the “drunken monkey hypothesis” by showing that chimps, with whom humans share a common ancestor, are “not averse to ethanol” and “do not avoid food containing alcohol,” Carrigan says. Brenda Benefit, an anthropologist at New Mexico State University, Las Cruces, agrees, although she says the evidence would be even stronger if the chimps had extracted the sap from the trees themselves. “It still leaves the question of whether chimps or gorillas use foods high in ethanol, such as fallen fruit, without human intervention.”