Wildlife behaving like drunk humans after eating fermented fruits is generally considered anecdotal rare, and is not well documented in academic literature. However, one group of ecologists is challenging the assumption that animals don’t eat ethanol-laden fruits and nectars that often. They argue that since ethanol-laden fruits are present in nearly every ecosystem on the planet, these fruits are likely consumed on a more regular basis by animals that eat nectar and fruits. The argument is detailed in a paper published October 30 in the Cell Press journal Trends in Ecology & Evolution.
“We’re moving away from this anthropocentric view that ethanol is just something that humans use,” Kimberley Hockings, a study co-author and behavioral ecologist at the University of Exeter in the UK, said in a statement. “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.”
[Related: Different kinds of alcohol might make you feel different emotions.]
What is ethanol?
In fruit, ethanol is a naturally occurring substance–the result of fermentation of fruit sugars by yeast. It first became abundant about 100 million years ago, when flowering plants began producing the sugary nectar that yeast could ferment. Ethanol is now present in nearly every ecosystem, but concentrations of it are higher and occur year-round in lower-latitude and humid tropical endorsements.
Usually, naturally fermented fruits only reach 1 to 2 percent alcohol by volume (ABV). However, concentrations as high as 10.2 percent ABV have been found in over-ripe palm fruit in Panama. By comparison, most alcoholic drinks that humans consume range from 3 percent ABV (a light beer) to 50 percent ABV (distilled spirits).
While animals already have the genes needed to degrade ethanol before yeasts began producing it, there is some evidence that evolution fine-tuned the ability for mammals and birds that eat fruit and nectar. Primates and small mammals called treeshrews have adapted to efficiently metabolize ethanol particularly well. Spider monkeys feed on fermented fruits of yellow mombin, which have fruit ethanol levels between one and 2.5 percent. Wild chimpanzees also have been observed repeatedly ingesting fermented palm. Elephants, baboons, and other wildlife also apparently get “drunk” on marula fruit in Botswana. However, ethanol was not measured in the fruit nor was ethanol confirmed in the animals in these cases.
“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,” study co-author and College of Central Florida molecular ecologist Matthew Carrigan said in a statement. “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.”
Potential benefits
In the new paper, the team analyzed the available scientific literature to understand ethanol prevalence and role in the natural world and what the evolutionary benefits might be for animals.
They found that it is still unclear whether animals intentionally consume ethanol for the sake of it and more research is needed to understand how it impacts animal physiology and evolution.
However, they found that ethanol consumption might carry several benefits for wild animals. It is a source of calories and the smells that are produced during fermentation could guide animals towards food sources. The team does not believe that animals can detect the ethanol in the fruit itself by smell alone.
[Related: Why fruit bats can eat tons of sugar without getting diabetes.]
Ethanol also may have some medicinal benefits. For example, fruit flies intentionally lay their eggs in substances that have ethanol. This protects the eggs from parasites. The fruit fly larvae also 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,” study co-author and University of Exeter behavioral ecologist Anna Bowland said in a statement. “To test that, we’d really need to know if ethanol is producing a physiological response in the wild.”
According to the team, there are still several unanswered questions about the significance of ethanol consumption to wild animals. In future research, they plan to investigate the behavioral and social implication of ethanol consumption by primates and take a closer look at the enzymes involved in alcohol metabolism.
