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.