Urinary concentrations of a direct ethanol metabolite indicate substantial ingestion of fermenting fruit by chimpanzees

Frugivorous animals routinely ingest fruit sugars and the associated products of microbial fermentation. Yeast-derived ethanol within fruit is a recently described chronic feature (approx. 14 g d−1) of the chimpanzee diet, but physiological evidence of exposure has not yet been demonstrated. We assayed urine collected from 19 wild chimpanzees for the presence of ethyl glucuronide (EtG), a direct metabolite of ethanol. Urine samples were obtained from study individuals feeding almost exclusively in the canopy of mast-fruiting Gambeya albida (=Chrysophyllum albidum), freshly fallen fruits of which averaged a pulp-ethanol content of 0.09% (±0.01 s.e.; range 0.01–0.40%) across multiple ripe crops. Of 20 individual urine samples (nine from females and 11 from males, aged 10–46 years), 17 tested positive for EtG at an analytical threshold of 300 ng ml−1; out of a subset of 11 of these positive samples, 10 samples further tested positive at a threshold of 500 ng ml−1. These values are high relative to calibrations of the test method for modern humans and are consistent with substantial rates of ethanol consumption via frugivory in the wild.