Wild chimpanzees forage for fermented fruits, consuming the equivalent of two glasses of wine daily, adjusted for their body size relative to humans.

It has long been recognized that many primates, including chimpanzees, consume substantial alcohol from diets rich in ripe fruits and other vegetation. Some suggest this could explain the human affinity for alcoholic drinks.

Alec Malo at the University of California, Berkeley, and his team sought to quantify the ethanol intake of chimpanzees by collecting fruits from two locations frequented by wild populations and measuring their alcohol content.

The research focused on two habitual populations: Eastern chimpanzees (Pan troglodytes schweinfurthi) in Ngogo, Uganda, and Western chimpanzees (Pan troglodytes verus) in the Tai Chimpanzee Project, Ivory Coast.

The researchers first observed the fruits being consumed by the chimps via camera traps and only collected those they confirmed were targeted. For fruits in the canopy, they gathered only samples that had recently fallen or were seen in the fall.

Eastern chimpanzees preferred ripe fruits from fig trees (Ficus mukuso), while Western chimpanzees often favored Guinea plums (Parinari excelsa) and fruits from the bitterburk tree (Sacoglottis gabonensis).

The study determined an overall alcohol concentration of roughly 0.3-0.4% in the fruit. Wild chimpanzees consume about 10% of their body weight in fruit pulp daily, Malo explains. They calculated that the chimpanzees ingest around 14 grams of pure ethanol per day at both sites.

A 125 ml glass of wine at 12.5% contains about 12 grams of ethanol. “This also accounts for the fact that chimpanzees average 40 kilograms versus 70 kg for humans,” says Malo. “Thus, we can assert that chimpanzees consume the equivalent of two glasses of wine each day.”

While chimpanzees and humans diverged millions of years ago, both likely encountered fermented fruits similarly, Malo notes.

“These findings indicate that our ancestors were similarly exposed to dietary alcohol,” he states. “The drunken monkey hypothesis suggests this exposure may have evolved an association between alcohol consumption and the rewards from fruit sugars, explaining humanity’s attraction to alcohol today.”

To gauge how much alcohol chimpanzees have in their systems, Malo collected urine samples beneath them, using an umbrella for protection. He’s currently analyzing this data.

Miguel Rulente, from the University of Girona in Spain and not part of the study, states that this research offers the first quantitative estimate of daily ethanol consumption in wild chimpanzees. “It supports the notion that alcohol exposure has deep evolutionary roots within primate diets and possibly influenced human evolution,” he asserts.

However, he cautions that this study has limitations in its implications for humans. “Unlike humans, ethanol consumption in apes is incidental and not intentional, making the leap from natural exposure to our species’ addiction challenging,” he adds.

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