Media playback is unsupported on your device Media caption The wild chimps use sponges made of crushed leaves to drink palm wine

They have shown an understanding of language and a sense of fairness, and now humans' closest primate cousins have even been found to share a taste for alcohol.

Scientists studying chimpanzees in Guinea have seen evidence of long-term and recurrent ingestion of ethanol by apes.

The 17-year study recorded chimps using leaves to drink fermented palm sap.

Some drank enough alcohol to produce "visible signs of inebriation".

The study - published in the journal Royal Society Open Science - revealed their tipple of choice is naturally fermented palm wine, produced by raffia palm trees.

Opportunistic drinking

In the Bossou area of Guinea, where this research took place, some local people harvest "palm wine" from the trees - tapping them at the crown, and gathering the sap in plastic containers, which they collect in the mornings and evenings.

Researchers working in the area had already witnessed chimpanzees climbing the trees - often in groups - and drinking the naturally fermented palm sap.

Image copyright G Ohashi Image caption The chimpanzees use leaf sponges in their palm wine "drinking sessions"

The chimps used drinking tools called leaf sponges - handfuls of leaves that they chew and crush into absorbent sponges, dip into the liquid and suck out the contents.

To work out the extent of the animals' indulging, the scientists measured the alcohol content of the wine in the containers and filmed the chimps' "drinking sessions".

The research team, led by Dr Kimberley Hockings from Oxford Brookes University and the Centre for Research in Anthropology in Portugal, worked out that the sap was about 3% alcohol by volume.

"Some individuals were estimated to have consumed about 85ml of alcohol," she said, "the equivalent to 8.5 UK units [approximately equal to a bottle of wine]".

"[They] displayed behavioural signs of inebriation, including falling asleep shortly after drinking.

"On another occasion after drinking palm wine, one adult male chimpanzee seemed particularly restless.

"While other chimpanzees were making and settling into their night nests, he spent an additional hour moving from tree to tree in an agitated manner. Again pure speculation, but it's certainly something we would like to collect further data on in the future," the researcher told BBC News.

Alcohol can be toxic, and although there have been unconfirmed anecdotes of non-human primates consuming it in the wild, this is the first time that researchers have recorded and measured voluntary alcohol consumption in any wild ape.

In addition, chimpanzees' apparent taste for a tipple adds to an evolutionary story about humans' common predilection for alcohol. Another recent study by Matthew Carrigan, from Santa Fe College in the US, showed that humans and African apes shared a genetic mutation that enabled them to effectively metabolise ethanol.

Prof Richard Byrne, an evolutionary biologist from the University of St Andrews, commented that the evolutionary origin of that gene could be that it "opened access to good energy sources - all that simple sugar - that were accidentally 'protected' by noxious alcohol".

"And presumably, whatever its evolutionary origin, it is that adaptation which makes me able to enjoy a good malt," he added.

Dr Catherine Hobaiter, from St Andrews University, said: "It would be fascinating to investigate the [behaviour] in more detail: do chimps compete over access to the alcohol? Or do those who drank enough to show 'behavioural signs of inebriation' have a bit of a slow day in the shade the next morning?"

Dr Hobaiter added: "Even after 60 years of studying [chimpanzees], they are constantly surprising us."

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