Alcohol consumption by humans is a well-documented aspect of history and culture, but it appears humans might not be the only species regularly exposed to ethanol. A recent review in the journal Trends in Ecology & Evolution explores how ethanol, naturally present in ecosystems worldwide, could be a routine part of the diets of many animals.
The authors argue that the consumption of ethanol by animals might be more common and ecologically significant than previously believed, challenging the prevailing assumption that such behavior is rare or accidental.
“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, commonly known as alcohol, is a chemical compound formed when yeast ferments sugars. Yeast, particularly species like Saccharomyces cerevisiae, evolved to produce ethanol as a defense mechanism against bacterial competitors. This process has been occurring for about 100 million years, coinciding with the emergence of sugar-rich fruits, nectars, and saps from flowering plants. These sugary substances serve as substrates for yeast fermentation, making ethanol a naturally occurring element in virtually every ecosystem.
The concentration of ethanol in the wild varies widely depending on factors like climate and the specific plant species involved. For example, ethanol levels in fruits and nectars can range from trace amounts to over 10% in some cases. In tropical environments, where warm and humid conditions favor fermentation, ethanol concentrations are generally higher.
Ethanol is also present in floral nectars, which are often consumed by insects and birds, and in overripe or fallen fruits, which may attract a variety of animals.
In their review article, the researchers highlight several reasons why animals might ingest ethanol, often as an incidental or intentional byproduct of their regular feeding habits:
1. Accidental or incidental ingestion
For many animals, ethanol consumption is likely unintentional. Ethanol can be present in the fruits and nectars they naturally consume, especially if these foods are fermenting. Anecdotal accounts abound of animals appearing intoxicated after eating such foods—for example, birds crashing into objects after consuming fermented berries or moose getting stuck in trees after eating fermented apples. However, the researchers caution that these events, while dramatic, are not the norm and do not reflect most animals’ typical interactions with ethanol.
2. Nutritional benefits
Ethanol itself is a calorie-rich molecule, providing energy to animals. While fermentation reduces the total caloric content of sugary foods, the resulting ethanol still offers a significant energy source. Animals might also use ethanol as a chemical cue to locate high-sugar food sources, particularly in environments where food is scarce or competition is high.
“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.”
3. Medicinal properties
Low levels of ethanol can have antimicrobial properties, potentially helping animals fend off infections or parasites. For instance, some insects, like vinegar flies, are known to lay their eggs in ethanol-rich environments, which helps protect their offspring from parasitic attacks. This form of “self-medication” could extend to other species, though more research is needed to confirm such behavior in vertebrates.
4. Behavioral and social effects
Ethanol can influence animal behavior in ways that might have evolutionary benefits. For example, ethanol’s ability to reduce anxiety and increase sociability could enhance group cohesion or mating opportunities in some species. However, the researchers note that such effects come with risks, as ethanol also impairs coordination and decision-making, potentially increasing an animal’s vulnerability to predators or environmental hazards.
“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.”
The researchers highlighted significant gaps in our understanding of how animals interact with ethanol in nature. Future studies could investigate the abundance of ethanol in ecosystems and its relationship with environmental factors like climate and plant species. They could also explore why some animals are attracted to fermented foods while others avoid them, as well as the health impacts and genetic adaptations related to ethanol consumption. Additionally, research into the evolutionary role of ethanol and the effects of human activities on its availability could offer valuable insights into its ecological and evolutionary significance.
The article, “The evolutionary ecology of ethanol,” was authored by Anna C. Bowland, Amanda D. Melin, David J. Hosken, Kimberley J. Hockings, and Matthew A. Carrigan.