Urine tests confirm alcohol consumption in wild African chimpanzees

(Press-News.org) Aleksey Maro knows far more than he cares to know about the urination habits of chimpanzees. But if you want to measure the alcohol intake of chimps in a Ugandan rain forest, where a breathalyzer is impractical, collecting urine for analysis is your only choice.

Last year, Maro and adviser Robert Dudley, UC Berkeley professor of integrative biology, documented that the fruits chimps eat in the wild contain enough alcohol from fermentation to provide around 14 grams per day — the equivalent of two standard drinks. But the proof is in the urine.

To perfect his urine sampling techniques, Maro, a UC Berkeley graduate student, worked alongside Sharifah Namaganda, a Ugandan graduate student at the University of Michigan who has experience collecting urine samples for previous projects at Ngogo. Under her guidance, he gathered forked branches and covered the ends with plastic bags, creating shallow plastic bowls suitable for stealthy sampling. Longer handles proved best to stay clear of the spray zone.

Maro then hung out under trees with feeding chimps, looking for signs of movement — they tend to urinate before leaving their feeding spot. The improvised collector worked well, though he discovered that a more reliable and less icky method was gathering the urine from leaves under the trees. He thought about staking out trees where chimps sleep, since, like humans, they urinate upon waking. Perhaps next time.

He also got samples from puddles of urine on the forest floor. When the urge hits while chimps are out and about, they like to straddle small logs, defecating on one side and peeing on the other. Who knew?

Maro’s valiant efforts were successful. His 11-day collecting trip last August to Ngogo in Uganda’s Kibale National Park yielded enough urine samples for him and Dudley to fill in a crucial gap in their “drunken monkey” hypothesis — the idea that chimps and probably many animals naturally ingest alcohol in their diet and even seek it out. One consequence is that, as descendants of fruit-eating apes, humans likely evolved the same tendency.

Their new results, to be published next week in the journal Biology Letters, show that the urine of most chimps sampled contain a metabolic byproduct of alcohol, ethyl glucuronide, that proves they ingest significant quantities of ethanol in their diet — likely from those fermenting fruits.

“We find widespread physiological evidence of the consumption of alcohol by chimpanzees,” Maro said. “If there's any doubt about the drunken monkey hypothesis — that there's enough alcohol in the environment for animals to experience alcohol in a way analogous to humans — it's been cleared up.”

“It corroborates the inferred ingestion rates that Aleksey derived previously,” Dudley said. For the earlier paper, Maro had collected samples of the many types of fruit chimps are known to eat, measured the ethanol concentration within the fruit pulp and estimated how much alcohol an average chimp would consume given known feeding rates.

Of the 20 urine samples from 19 different chimps (the Western chimpanzee, Pan troglodytes), 17 tested positive on commercial strips sensitive to 300 nanograms per milliliter (ng/ml) or more ethanol. Eleven samples were tested with strips sensitive to 500 ng/ml or more; 10 were positive (making a total of 4 out of 20 below the 500 ng/ml cutoff). In humans, 500 ng/ml is a level expected after light drinking — 1 to 2 standard drinks — within the previous 24 hours. Similar levels would be expected in a chimpanzee that had spent the morning scarfing down slightly fermented fruit.

“The levels are high, and this is a conservative estimate given the time course of exposure through the day,” Dudley said. “In nanograms per milliliter, these are coming in way above some of the clinically relevant and forensically relevant human thresholds.”

Maro collected samples only from chimps that he could identify with the help of Ngogo staff who recognize each individual ape. That allowed him to document that males and females alike tested positive for ethanol byproducts in urine, and that the negative results came disproportionately from females in estrus and juveniles. One possibility, Dudley said, is that males hoard the more alcoholic fruits.

Maro also analyzed the alcohol content of the star apple the chimps were gorging on, thanks to a bumper crop available during a masting year. Based on undamaged fruits collected under the trees, the star apples contained less alcohol than the average of many varieties of fruit he had sampled at Ngogo in 2019. Those fruits averaged 0.32% by weight of ethanol. The star apples, which are about 20% sugar, contained only 0.09% ethanol by weight.

The chimps may have been eating riper, more fermented fruit in the trees than Maro was able to collect on the ground. Nevertheless, the relatively high levels of ethyl glucuronide in their urine suggest that the chimps were eating kilograms of the sweet treat. One estimate is that chimpanzees eat about 4.5 kg of fruit daily, or 10 pounds.

Dudley and Maro urge future studies to assess the effects of dietary ethanol on the physiology and behavior of chimps over time, including whether consumption of fermented fruit affects aggression or the timing of female fertility.

“Food and alcohol evolutionarily are, as it turns out, very much connected, especially in the lives of chimpanzees,” Maro said.

And possibly humans too, whose closest living relatives are chimpanzees and bonobos.

“It all comes back to the human side: Have we evolved predisposed to the consumption of alcohol, based on this ancestral lineage? And how did that predispose us to the domestication of alcohol via brewer’s yeast” and the subsequent abuse of alcohol, Maro said.

Alcohol consumption in other wild animals

The test strips — immunoassays similar to a pregnancy test — are the same as used to test humans engaged in activities, such as flying airplanes or operating dangerous equipment, that require abstinence from alcohol. The new study proves their value in field research, Dudley said, and the strips should make it easier to test all animals in the wild for alcohol consumption. He has convinced one UC Berkeley colleague to take the strips into the field in Madagascar to test the urine of fruit bats, who also ingest lots of fruit, much of it likely fermented.

“The hypothesis is, of course, that it has to be positive. The question is how much,” Dudley said. “With these strips you can do sampling through the year with their diet shifts and climate shifts. It's not just primates. It's anything else that's eating fruit.”

Maro added that “I have this slide (depicting) every animal besides chimpanzees that I camera-trapped eating those fruits, and it's everybody out there. So, it's a widespread phenomenon.”

Dudley emphasized that the new study doesn’t fill all the gaps in his drunken monkey hypothesis.

“The final link here with the drunken monkey hypothesis remains to be shown: that the chimps are selectively consuming fruits with higher ethanol content,” he said. “That hasn't really been demonstrated for any taxon in the wild. So that would be the next future direction on this — to definitively prove the universal hypothesis of attraction to alcohol.”

In addition to Maro, Dudley and Namaganda, Laura Byrne of San Francisco State University is also a coauthor of the paper. The research was supported in part by the German production company InOneMedia, which filmed Maro’s 2025 field work for the first segment of a two-part documentary about animal intoxication in the wild. The film, Tiere im Rausch (Wild on a High), was broadcast in Germany by ARD in December and will air in English in April or May.

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