The first-ever measurements of the ethanol content of fruits available to chimpanzees in their native African habitat show that the animals could easily consume the equivalent of more than two standard alcoholic drinks each day, according to researchers at the University of California, Berkeley.

It's not clear whether they actively seek out fruit with high ethanol levels, which are typically riper fruit with more sugars to ferment. But the availability of ethanol in many species of fruit that they normally eat suggests that alcohol is a regular part of their diet and likely was a part of the diets of our human ancestors.

"Across all sites, male and female chimpanzees are consuming about 14 grams of pure ethanol per day in their diet, which is the equivalent to one standard American drink," said UC Berkeley graduate student Aleksey Maro of the Department of Integrative Biology. "When you adjust for body mass, because chimps weigh about 40 kilos versus a typical human at 70 kilos, it goes up to nearly two drinks."

A "standard drink" in the U.S. contains 14 grams of ethanol, irrespective of the consumer's body size, although in much of Europe the standard is 10 grams.

The 21 species of fruit Maro sampled at two chimp study sites—Ngogo in Uganda and Taï in Ivory Coast—had an average alcohol content of 0.26% by weight. Primatologists who have studied chimps at these sites estimate that the animals consume about 10 pounds (4.5 kilograms) of fruit per day, on average, and that fruit makes up about three-quarters of their diet. The researchers have also recorded for each site the approximate proportion of each fruit species in the chimp diet. This information allowed the Berkeley biologists to calculate an average rate of dietary ethanol consumption.

"The chimps are eating 5 to 10% of their body weight a day in ripe fruit, so even low concentrations yield a high daily total—a substantial dosage of alcohol," said Robert Dudley, UC Berkeley professor of integrative biology. "If the chimps are randomly sampling ripe fruit as did Aleksey, then that's going to be their average consumption rate, independent of any preference for ethanol. But if they are preferring riper and/or more sugar-rich fruits, then this is a conservative lower limit for the likely rate of ethanol ingestion."

Fruit consumption occurs throughout the day and the chimps show no overt signs of intoxication, Maro said. In fact, to get a buzz on, a chimp would have to eat so much fruit its stomach would bloat. But chronic low-level exposure suggests that the common ancestors of humans and chimps—our closest living relative among the apes—were also exposed daily to alcohol from fermenting fruit, a nutrient that is missing from the diets of captive chimps and many humans today.

"Chimpanzees consume a similar amount of alcohol to what we might if we ate fermented food daily," Maro said. "Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees."