Alcohol consumption in the natural world is way more common than you thought

Ethanol is more than a human invention. New research reveals how animals across ecosystems encounter and adapt to ethanol.

30 Oct 2024, 22:28 by Mihai Andrei · ZME Science

Fruits and alcohol go hand in hand. Image credits: Andriyko Podilnyk.

Alcohol isn’t solely a human construct. Ethanol, a type of alcohol, naturally occurs across ecosystems. From jungles to deserts, wherever sugary foods ferment, ethanol is found. So, behavioral ecologist Kimberley Hockings from the University of Exeter challenges the traditionally human-centered view of ethanol, suggesting it has a broader ecological role.

“We’re moving away from this anthropocentric view that ethanol is just something that humans use. 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,” the researcher says.

Hockings and colleagues found ethanol to be a natural substance many animals encounter. They may even use it for nutritional, medicinal, and protective benefits.

Ethanol in nature

Ethanol is formed naturally through the fermentation process carried out by some yeasts which thrive in environments rich in sugars, such as ripe fruits, plant saps, and nectar. When yeasts metabolize these sugars anaerobically (without oxygen), they convert them into ethanol and carbon dioxide. This process likely evolved as a competitive advantage, allowing yeasts to inhibit bacterial growth by creating ethanol-rich environments that are inhospitable to many bacteria but tolerable for the yeast itself.

Ethanol became much more common with the advent of human agriculture, because we grow a lot of fruit that can ferment. But ethanol’s story traces back around 100 million years to the Cretaceous period, when the first flowering plants began producing sugary nectar and fruits. As yeasts began fermenting these sugars, they generated ethanol as a byproduct. Since then, ethanol has been a staple in natural ecosystems.

“Yeasts are widespread in the environment, and sugary foods such as fruits, saps, and nectars can ferment naturally; as such, it is possible that all species that incorporate these foods in their diet ingest some amount of ethanol,” explains Anna Bowland, co-author of this recent study, for ZME Science.

In the wild, ethanol concentrations in naturally fermenting foods typically hover around 1%-2% alcohol by volume (ABV). Still, even more potent examples exist. For instance, overripe palm fruit in Panama has been recorded at 10.2% ABV. Also, palm sap, which chimpanzees in Africa sometimes consume, can contain similar levels. This pervasive presence of alcohol begs the question: how do animals adapt to it?

Animals adapting to alcohol

It is unlikely that animals chase ‘intoxication’ or ‘inebriation’, Bowland tells ZME Science. If an animal is at the point of being ‘drunk’ it’s a recipe for disaster, leaving them susceptible to predation. So, animals have adapted in different ways to deal with ethanol.

For instance, the metabolic enzymes responsible for breaking down alcohol, like alcohol dehydrogenase (ADH), existed in various animals even before yeast’s evolutionary shift to ethanol production. But with the increasing availability of fermented fruits and nectars, evolution appears to have fine-tuned these enzymes in certain animals, especially those regularly exposed to fermented foods.

Primates, for instance, and specific mammals like treeshrews, have evolved to metabolize ethanol efficiently, avoiding the adverse effects of intoxication that humans might experience. Basically, they’ve found ways to digest alcohol without getting hammered. It’s the exact opposite way we humans treat alcohol: we (usually) want the inebriation but not the extra calories — animals want the calories but not the buzz.

Yet some animals use alcohol more creatively to their advantage.

How animals use alcohol

For some species, such as the vinegar fly (Drosophila melanogaster), ethanol-rich environments serve as prime locations for egg-laying. It offers their offspring protection from parasites that cannot tolerate the ethanol levels.

Also, ethanol is one of many olfactory (scent) indicators that guide animals to food sources. Yeast fermentation produces unique aromas, which may help animals locate edible resources. So, while animals may not be specifically attracted to ethanol’s scent, the bouquet of odors associated with fermentation could be an effective foraging signal in dense forests or during migration.

In certain cases, ethanol may even serve medicinal purposes. Some plant-derived alcoholic compounds show anti-malarial properties, which might influence an animal’s choice to consume fermented fruits in malarial regions. Notably, chimpanzees, which are prone to malaria, have been observed drinking palm wine. Malarial parasites are reportedly inhibited by ethanol concentrations attainable by alcohol consumption. This, though, is still speculative.

“For example, palm wine has locally been reported to hold anti-malarial properties. Chimpanzees are widely infected by malaria and have been found to ingest palm wine, although the reasons for ingestion remain unknown. Whether or not other animals use ethanol for medicinal purposes, particularly in natural contexts, still requires further research,” says Bowland.

Also, although getting drunk is definitely not good for animals, they may appreciate a bit of “social lubrication”.

Ethanol, even in low concentrations, may alter social and cognitive behaviors among some animals. This may extend and improve social interactions in some mammals, and some scientists speculate that ethanol’s impact on endorphins and dopamine could promote feelings of relaxation and social bonding.

Plenty more questions to answer

For years, we’ve had a predominantly human-centered perspective on alcohol. But as this study shows, animals have their own way of interacting with it. They have been doing so for millions of years — and we know surprisingly little about this interaction. Part of this is that it’s not an easy thing to study, says Bowland.

“To study this behaviour in the wild requires knowledge of the natural ethanol concentrations within foods ingested by wild animals and whether animals feed on them at times when ethanol is expected to be present. While we can estimate ethanol content in fruits, we can of course never know the exact ethanol concentration of each and every fruit ingested by an animal. Many wild animals are also unhabituated, so gaining an insight into natural feeding behaviour and selection can at times be challenging.”

While this study offers some answers, it raises even more compelling questions about the complex role ethanol might play in natural ecosystems. or example, do animals intentionally seek out fermented foods only under specific environmental conditions, such as food scarcity, or might certain species regularly incorporate them as part of a balanced diet? Additionally, does natural ethanol exposure influence animal behavior in ways that benefit social interactions or cognitive function, and if so, to what extent?

The field is ripe for additional studies, concludes Bowland.

“By reframing the ecological and evolutionary importance of ethanol, we hope that future research will begin to explore these and extend our knowledge of ethanol use across animals more broadly.”