There are many stories about wildlife appearing ‘drunk’ after eating fermented fruits. However, the idea that animals intentionally consume ethanol has been considered uncommon and often accidental. Recently, ecologists have started to question this belief, stating in a review published on October 30 in the journal Trends in Ecology & Evolution from Cell Press that ethanol is found naturally in almost every ecosystem, suggesting that many animals that eat fruits and nectar likely consume it regularly.
“We’re shifting away from the human-centered perspective that considers ethanol solely a human product,” explains behavioral ecologist and senior author Kimberley Hockings from the University of Exeter. “Ethanol is far more prevalent in nature than we previously realized, and most animals that snack on sugary fruits will be exposed to some ethanol.”
Ethanol became widely available around 100 million years ago when flowering plants started to produce sugary nectar and fruits conducive to yeast fermentation. Today, ethanol is naturally present in nearly every ecosystem, with higher concentrations and year-round production occurring in tropical and humid areas compared to temperate climates. Typically, naturally fermented fruits contain only 1% to 2% alcohol by volume (ABV), but some overripe palm fruits in Panama have been found to have alcohol levels as high as 10.2% ABV.
Many animals already had genes capable of breaking down ethanol long before yeast began to generate it, but evidence shows that evolution has refined this capacity in mammals and birds that consume fruits and nectar. Primates and treeshrews, in particular, have evolved to metabolize ethanol effectively.
“From an ecological standpoint, being inebriated while climbing trees or being around nighttime predators isn’t beneficial — that’s not a good recipe for survival,” says molecular ecologist and senior author Matthew Carrigan from the College of Central Florida. “Unlike humans, who seek intoxication without wanting excess calories, animals want the calories but not the inebriation.”
It’s still unknown whether animals deliberately seek out ethanol for its own sake, indicating a need for further research to understand the impact on animal physiology and evolution. However, the researchers suggest that consuming ethanol might offer several advantages for wild animals. Primarily, it serves as a source of calories, while the aromatic compounds released during fermentation could guide animals towards food sources, though it’s improbable that they can actually sense ethanol. Additionally, ethanol might possess medicinal properties: fruit flies actively lay their eggs in ethanol-laden environments to protect them from parasites, and their larvae reportedly increase ethanol consumption when infested by wasps.
“Cognitively, it’s been suggested that ethanol might activate the endorphin and dopamine systems, promoting relaxation benefits that could enhance social interactions,” said behavioral ecologist and first author Anna Bowland of the University of Exeter. “To evaluate this, we need to establish whether ethanol triggers a physiological response in wild animals.”
Many questions remain about the role of ethanol consumption in wild animal behavior. The research team plans to delve deeper into the social and behavioral consequences of ethanol intake among primates and further explore the enzymes involved in alcohol metabolism.
This study received support from the Primate Society of Great Britain, the Wenner-Gren Foundation, the Canada Research Chairs program, and the Natural Sciences and Engineering Research Council of Canada.
