Dietary ethanol ingestion by free ranging spider monkeys (Ateles geoffroyi): an evaluation of the drunken monkey hypothesis

The Drunken Monkey Hypothesis posits that the proclivity of modern humans to consume alcohol may stem from a deep-rooted affinity present in all frugivorous nonhuman primates for naturally occurring ethanol within ripe fruit (Dudley 2000, 2002 2004; 2014; Stephens and Dudley 2004). Ethanol is a natural by-product of the metabolic activity of fermentative yeasts within fruit pulp that consume sugars made available as the fruit ripens (Spencer and Spencer 1997). Here I investigate key assumptions and predictions of the Drunken Monkey Hypothesis by examining the role that ethanol concentration in Spondias mombin fruits plays in the diet of free ranging spider monkeys (Ateles geoffroyi) on Barro Colorado Island, Panamá. Specifically, I aimed to answer the following questions: (1) Do ripe or over-ripe fruits consumed by spider monkeys have measurable ethanol content? (2) Do spider monkeys preferentially select and consume fruit with higher ethanol content? (3) Is higher ethanol content inversely correlated with sugar content? I predicted that I would find that: 1) ripe or over-ripe fruits consumed by spider monkeys have measurable ethanol content; 2) spider monkeys preferentially select and consume fruit with higher ethanol content; and 3) the fruits with higher ethanol content also have higher sugar content and thus greater caloric reward. This study demonstrates that Barro Colorado Island spider monkeys regularly consume fruits containing ethanol. Additionally, it showed a non-statistically significant trend towards spider monkeys preferentially selecting - and consuming more of - fruits that contain higher levels of ethanol. There was no observed relationship between ethanol content and sugar content. Though the data presented here are too sparse to demonstrate that nonhuman frugivorous primates exhibit a proclivity to consume ethanol, they nevertheless suggest that this is an important avenue for future research, both to further characterize the diet and foraging behavior of spider monkeys and to directly address the Drunken Monkey Hypothesis.