Berkley finds chimps consume same as two glasses of wine daily

By Stephen Beech

Boozy chimps down the equivalent of two glasses of wine every day in their native habitat, according to new research.

A survey of the ethanol content of fruits eaten by the primates suggests they are "chronically exposed" to alcohol, say American scientists.

The first-ever measurements of the ethanol content of fruits available to chimpanzees in their native African habitat showed that they could "easily" consume the equivalent of more than two standard alcoholic drinks each day.

Researchers from the University of California, Berkeley, say it's not clear whether they actively seek out fruit with high ethanol levels - which are usually 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.

Study first author Aleksey Maro, of UC Berkeley's department of integrative biology, said: "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.

"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 glass of wine contains around 14 grams of ethanol if it's a standard five-ounce (150ml) glass of 12% ABV wine.

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 10lbs (4.5 kilos) of fruit per day, on average, and that fruit makes up about three-quarters of their diet.

Researchers have also recorded for each site the approximate proportion of each fruit species in the chimp diet.

That information allowed the Berkeley biologists to calculate an average rate of dietary ethanol consumption.

Study senior author Professor Robert Dudley, of UC Berkeley, said: "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.

"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."

Maro said fruit consumption occurs throughout the day, but the chimps show no obvious signs of intoxication.

Weighted by the proportion of time chimps eat each type of fruit, the average alcohol content of fruit was 0.32% by weight at Ngogo and 0.31% at Taï.

The most frequently consumed fruits at each site - a fig, Ficus musuco, at Ngogo, and the plum-like fruit of the evergreen Parinari excelsa at Taï - were the highest in alcohol content, according to the findings published in the journal Science Advances.

Troops of male chimpanzees often gather in the canopy of F. musuco trees to consume fruit before going on boundary patrols of their community, Maro noted. And the fruit of P. excelsa is also very popular among elephants, which are known to be attracted to alcohol.

To get a buzz on, the researchers say 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 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.

Maro said: "Chimpanzees consume a similar amount of alcohol to what we might if we ate fermented food daily.

"Human attraction to alcohol probably arose from this dietary heritage of our common ancestor with chimpanzees."

Dudley first began to suspect more than 20 years ago that the human appetite for alcohol was inherited from our primate ancestors, and wrote a 2014 book about his theory: "The Drunken Monkey: Why We Drink and Abuse Alcohol."

The "drunken monkey" hypothesis was met with scepticism from many scientists who told him that chimps and other primates don't eat fermented fruit or nectar.

But over the years, Dudley's theory has gained an increasing number of adherents.

More primatologists now report seeing monkeys and apes eating fermented fruit, a practice that was recorded earlier this year among chimps in Guinea-Bissau.

And it's not only mammals that get a daily dose of alcohol from their diet.

In a paper published earlier this year, Dudley and his Berkeley colleagues reported that the feathers from 10 of 17 bird species tested contained secondary metabolites of alcohol, indicating that their diet included substantial amounts of ethanol.

Dudley said: "The consumption of ethanol is not limited to primates.

"It's more characteristic of all fruit-eating animals and, in some cases, nectar-feeding animals."

He says that one theory about why animals seek out ethanol is that its odor helps animals find food with a higher sugar content, providing greater energy returns over time.

He added that alcohol also may increase the pleasure of eating, similar to sipping wine with dinner.

Dudley said: "It just points to the need for additional federal funding for research into alcohol attraction and abuse by modern humans.

"It likely has a deep evolutionary background."

He added: "I think the strength of Aleksey's approach is that it used multiple methods.

"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."