Chimpanzees have a fondness for alcohol. Among our closest primate relatives, they frequently consume fermented fruits in their natural habitats. So much so that the “drunken monkey hypothesis” suggests chimpanzees, and perhaps numerous other animals, actively ingest and even seek out alcohol in their diets. As descendants of fruit-eating, inebriated apes, humans likely inherited the same inclination.
Now, there is further evidence of their intoxicating habits—found in their urine. After UC Berkeley graduate student Alexei Makuro and integrative biologist Robert Dudley determined that the alcoholic fruits consumed by chimps were equivalent to two standard drinks (14 grams), they needed to verify the total amount of alcohol the apes had ingested. Since a breathalyzer isn’t practical for wild chimpanzees, they turned to urine analysis.
Keeping Clear of the Spray Zone
In August 2025, Makuro collaborated with Sharifa Namaganda, a Ugandan graduate student from the University of Michigan who already possessed experience collecting urine samples at Ngogo, a habitat within Kibale National Park. Under her guidance, Makuro fashioned apparatuses from forked branches, covering the ends with plastic bags. These bags, supported by the branch framework, created small plastic cups for collecting urine samples. They found that longer handles helped maintain distance from the spray zone.
Makuro then patiently waited beneath trees where chimpanzees were feeding, watching for movement signs, as they often urinate before vacating a feeding spot. The makeshift collector proved effective, also gathering urine from puddles on the forest floor. When chimps are lower to the ground, they sometimes perch on small logs, defecating on one side and urinating on the other.
After 11 days of urine collection, Makuro possessed enough samples to fill a crucial gap in the drunken monkey hypothesis. Most urine samples contained ethyl glucuronide, a metabolic byproduct of alcohol. This byproduct confirms they absorb substantial quantities of ethanol through their diet, most likely from fermenting fruits. The team’s findings are detailed in a study published today in the journal Biology Letters.
“We have found widespread physiological evidence of alcohol consumption by chimpanzees,” Makuro stated. “If there were any lingering doubts about the drunken monkey hypothesis—that there is enough alcohol in the environment for animals to experience it similarly to humans—those have now been dispelled.”
chimpanzee sitting in a fruit tree
A Western chimpanzee perched in a fruit-laden tree in Ngogo, Uganda. UC Berkeley graduate student Alexei Makuro tracked these chimps during the summer of 2025 as they feasted on the sweet fruit of the African star apple, Gambeya albida. Makuro then collected their urine to check for metabolic byproducts from ethanol ingestion. Photo: Alexei Makuro/UC Berkeley
Counting the Drinks
Of the 20 urine samples taken from 19 distinct chimpanzees, 17 tested positive using commercial strips sensitive to 300 nanograms per milliliter (ng/mL) or more of ethanol. Of the 11 samples tested with strips sensitive to 500 ng/mL and higher, 10 were positive (only 4 out of 20 fell below the 500 ng/mL threshold). For a human, 500 ng/mL represents an expected level following one or two standard drinks over the preceding 24 hours. Similar levels are anticipated from a chimpanzee that has been eating slightly fermented fruits throughout the morning.
“The levels are high, and this is a conservative estimate considering the duration of exposure throughout the day,” Dudley commented. “In nanograms per milliliter, they significantly exceed some clinically and forensically significant human thresholds.”
Makuro only collected samples from chimpanzees that he and the Ngogo staff could recognize. Both males and females registered positive for ethanol products in their urine, though negative results were disproportionately common in females and younger individuals. Dudley speculated that males might accumulate more of the alcoholic fruit intake.
The Human Connection
The team suggests that future research should assess the impact of dietary ethanol on chimpanzee physiology and behavior over time, including how consuming fermented fruits affects aggression or the timing of female fertility.
“Food and alcohol, it turns out, are evolutionarily linked, particularly in the life of chimpanzees,” Makuro remarked.
This connection might also extend to humans, raising questions about whether we evolved a predisposition toward alcohol consumption and how much this influenced the domestication of yeasts to create our own alcoholic beverages.
“The final piece of the puzzle for the drunken monkey hypothesis has yet to be demonstrated: whether chimpanzees selectively choose fruits with high ethanol content,” Makuro concluded. “This has not been shown for any taxon in the wild. So, the future direction will be to conclusively prove the universal hypothesis of alcohol attraction.”
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