The fig at Kibale and the plum at Taï aren't super boozy; they only contain about 0.3 percent alcohol. But since a chimpanzee can ingest around 10 pounds of fruit a day, that amounts to an average daily dose of 14 to 15 grams of alcohol. A standard human drink is about 10 grams -- but since chimps weigh much less than humans do, 14 to 15 grams to them is the equivalent of around 2.5 human drinks per day.
"They're doing that over the course of breakfast, lunch, and dinner," says Maro, so to speak, because chimps eat more or less throughout the day. But when they find a tree heavy with their favorite fruits, they sometimes engage in "binge eating," stuffing their mouths with as many figs as they can fit, sucking out all the juice (and alcohol), then spitting the dry wad onto the forest floor. But unlike unhealthy binging behaviors in humans, this is considered normal and natural -- scientists say, it's just what chimpanzees do.
The study is not without shortcomings. Biologist Matthew Carrigan of the College of Central Florida, points out study authors did not directly consider how many fruits the chimpanzees really ate, just how much time they spent eating different types. Primatologist Karline Janmaat of the University of Amsterdam, the Netherlands, also points out it remains uncertain whether the fruits dropped or left behind by the chimpanzees may be higher or lower in alcohol than the ones they ate.
Maro acknowledges that it would be good to measure more directly how much alcohol chimpanzees have actually consumed, something that might be done with urine samples he has recently collected. That could cast more light on the possible role of alcohol in the aftermath of some large gatherings of chimpanzees observed in trees with abundant ripe figs in Kibale National Park.
Even if the study's estimates are accurate, Janmaat cautions that chimpanzees probably don't, and shouldn't, get drunk in the wild. "That's very dangerous when you need to climb high up in a tree to build a nest to sleep at night."
Still, if given unlimited access to alcohol, chimpanzees may not know when to quit. In the 1960s, an experiment that would not be permitted today showed that when chimpanzees could drink as much alcohol as they wanted, some didn't touch it, while others drank until visibly drunk.