Both male and female chimpanzees were consuming approximately 14 grams of pure ethanol each day.

Aleksey Maro, a UC Berkeley graduate student, said: "Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees."

Their daily ethanol intake is equivalent to one standard drink in the US. "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," he said.

The chimpanzees were the biggest fans of figs and Guinea plum, which had the highest alcohol content among all the fruits available to them.

It is unclear whether the chimps were actively seeking out the fruit with a higher alcohol content or whether they were opting for riper fruits, which had more sugars to ferment.

Researchers said the availability of ethanol in many species of fruit they are consuming suggests that alcohol is a common part of their diet, and likely our ancestors' too.

University of California professor Robert Dudley said: "The chimps are eating five to 10 per cent of their body weight a day in ripe fruit, so even low concentrations yield a high daily total -- a substantial dosage of alcohol."

The 21 species of fruit that were studied had an average alcohol content of 0.26 per cent by weight. The chimpanzees consume about four and a half kilograms of fruit each day, which makes up around three-quarters of their diet.

Professor Dudley said: "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."

Despite their ethanol-rich diets, the chimps showed few signs of intoxication, the researchers said.

"In fact, to get a buzz on, a chimp would have to eat so much fruit its stomach would bloat," he added.

"One of the reasons this has been a tempting target but no one's gone after it is because it's so hard to do in a field site where there are wild primates eating known fruits. This dataset has not existed before, and it has been a contentious issue."

Chimps consume a pint of lager's worth of alcohol every day in fruit, scientists find