Some of the chimps observed by researchers displayed typical symptoms of drunkenness after partaking in drinking sessions. Martin Harvey / Alamy

Wild chimpanzees that enjoy the odd booze-up on palm wine have helped shed light on a theory about evolution, scientists said Wednesday.

During a 17-year study, chimps in the West African country of Guinea were observed on numerous occasions imbibing a fermented milky sap from raffia palms, tapped by local people to make into an alcoholic drink.

Incidents of lone drinkers and communal sessions were seen, according to a paper published in the British journal Royal Society Open Science.

Researchers suggested the findings give insight into the social habits of chimpanzees in the wild. They also back the "drunken monkey" theory, which holds that apes and humans share a genetic ability to break down alcohol that was handed down from a common ancestor.

Under observation, the apes scrunched up leaves in their mouths, molding them into spongy pads that they then dipped into the sap-gathering container, which villagers attach to the tree near its crown.

Tests showed that the beverage's alcoholic content varied from 3.1 percent to 6.9 percent — the equivalent of strong beer.

Some of the chimps displayed typical symptoms of drunkenness.

"[They] consumed significant quantities of ethanol [alcohol] and displayed behavioral signs of inebriation," according to the study, which was headed by Kimberley Hockings of Oxford Brookes University.

"Researchers rarely collected detailed behavioral data before versus after exposure to ethanol, but some drinkers rested directly after imbibing fermented sap," the paper added.

The chimps are part of a closely observed colony at Bossou in southern Guinea. In 2008, one of the animals made headlines when he was found to use a stick to "fish" for ants — an important discovery in the use of tools by our primate cousins.

Cases of animals ingesting alcohol are not exceptional. They include Swedish moose that get drunk on fermented apples and monkeys on the Caribbean island of St. Kitts that sneak gulps from vacationers' cocktails.

But the Bossou chimps, observed over 17 years, are the first to provide serious data about how much alcohol can be knocked back in the wild and when.

Sometimes just a single chimp would go to the top of the palm, the researchers found. At other times, there would be drinking sessions in which several chums would gather in the crown of the tree.

"Individuals either co-drank, with drinkers alternating dips of their leaf-sponges into the fermented palm sap, or one individual monopolized the container, whereas others waited their turn," according to the study.

Over the 17 years, the researchers recorded 51 drinking events, 20 of which were drinking sessions. They identified 13 adult and young chimps.

The animals made their sponges from leaves that villages had placed over the top of the containers to prevent dust and insects from contaminating the sap.

According to the "drunken monkey" theory, apes and humans gain an evolutionary edge by being able to tolerate overripe foods. By metabolizing alcohol, according to this idea, our forerunners could eat fermented fruit found on the forest floor, gaining a precious additional source of calories and nutrients.

Agence France-Presse