A spider monkey feedings on fruits of spondia mombin.
Credit: Nicholas Chapoy
Anecdotes abound of wildlife behaving "drunk" after eating fermented fruits, but despite this, nonhuman consumption of ethanol has been assumed to be rare and accidental. Ecologists challenge this assumption in a review published October 30 in Trends in Ecology & Evolution. They argue that since ethanol is naturally present in nearly every ecosystem, it is likely consumed on a regular basis by most fruit- and nectar-eating animals.
"We're moving away from this anthropocentric view that ethanol is just something that humans use," says behavioral ecologist and senior author Kimberley Hockings of the University of Exeter. "It's much more abundant in the natural world than we previously thought, and most animals that eat sugary fruits are going to be exposed to some level of ethanol."
Ethanol first became abundant around 100 million years ago, when flowering plants began producing sugary nectar and fruits that yeast could ferment. Now, it's present naturally in nearly every ecosystem, though concentrations are higher, and production occurs year-round in lower-latitude and humid tropical environments compared to temperate regions.
Most of the time, naturally fermented fruits only reach 1–2% alcohol by volume (ABV), but concentrations as high as 10.2% ABV have been found in over-ripe palm fruit in Panama.
Animals already harbored genes that could degrade ethanol before yeasts began producing it, but there is evidence that evolution fine-tuned this ability for mammals and birds that consume fruit and nectar. In particular, primates and treeshrews have adapted to efficiently metabolize ethanol.
A capuchin eating fruits.
Credit: Julia Casorso
"From an ecological perspective, it is not advantageous to be inebriated as you're climbing around in the trees or surrounded by predators at night—that's a recipe for not having your genes passed on," says molecular ecologist and senior author Matthew Carrigan of the College of Central Florida.
"It's the opposite of humans who want to get intoxicated but don't really want the calories—from the non-human perspective, the animals want the calories but not the inebriation."
It's unclear whether animals intentionally consume ethanol for ethanol's sake, and more research is needed to understand its impact on animal physiology and evolution. However, the researchers say that ethanol consumption could carry several benefits for wild animals.
First and foremost, it's a source of calories, and the odorous compounds produced during fermentation could guide animals to food sources, though the researchers say it's unlikely that animals can detect ethanol itself.
Ethanol could also have medicinal benefits: fruit flies intentionally lay their eggs in substances containing ethanol, which protects their eggs from parasites, and fruit fly larvae increase their ethanol intake when they become parasitized by wasps.
"On the cognitive side, ideas have been put forward that ethanol can trigger the endorphin and dopamine system, which leads to feelings of relaxation that could have benefits in terms of sociality," says behavioral ecologist and first author Anna Bowland of the University of Exeter. "To test that, we'd really need to know if ethanol is producing a physiological response in the wild."
There are a lot of unanswered questions regarding the significance of ethanol consumption to wild animals. In their future research, the team plans to investigate the behavioral and social implications of ethanol consumption in primates and to more deeply examine the enzymes involved in alcohol metabolism.
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