‘Drunk animals’ may explain humans’ love of alcohol

Tales of animals appearing “drunk” after eating fermented fruit have circulated for years. Still, many scientists believed that ethanol consumption among wild animals was rare and largely accidental. 

Challenging this assumption, ecologists now argue that animals frequently consume low levels of alcohol in their natural diets. 

Published in the journal Trends in Ecology & Evolution, a new review suggests that alcohol is ubiquitous across ecosystems and regularly consumed by many fruit- and nectar-eating species.

The role of ethanol in nature

Behavioral ecologist Kimberley Hockings of the University of Exeter believes that this finding broadens our understanding of ethanol’s role in nature. 

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

This perspective emphasizes that ethanol isn’t unique to human consumption but rather a natural compound that animals regularly encounter in their environments.

Ethanol dates back 100 million years

The presence of ethanol in the environment dates back approximately 100 million years, around the time flowering plants began to flourish. 

These plants produce sugars in nectar and fruits, creating an ideal environment for yeast fermentation, which naturally generates ethanol. 

Today, ethanol is present in ecosystems worldwide, with tropical and humid regions showing higher concentrations due to more consistent warmth and moisture. 

Alcohol in fermented fruits

In most cases, fermented fruits have alcohol levels of 1%-2% by volume (ABV), though over-ripe fruits, such as some palm fruits in Panama, have been found to reach up to 10.2% ABV.

Long before the appearance of ethanol-producing yeast, animals had already evolved genes to break down ethanol. Evidence suggests that evolution further adapted these genes for mammals and birds that consume fruit and nectar regularly, particularly in species like primates and treeshrews. 

This evolutionary adjustment likely enabled animals to tolerate the presence of ethanol in their food sources, allowing them to consume it without experiencing severe intoxication.

Animals need the calories in alcohol

“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,” said co-author Matthew Carrigan, a molecular ecologist at 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.”

Benefits of animal alcohol consumption

Though researchers are still investigating whether animals actively seek out ethanol, it’s becoming clear that its consumption may offer several benefits. 

First and foremost, ethanol provides an additional source of calories, which is vital for survival. Fermentation also releases odorous compounds that can lead animals to food, although it remains uncertain if animals are capable of detecting ethanol itself or are simply drawn to the overall scent of fermentation.

In addition to being a source of energy, ethanol may have protective effects against certain environmental threats. For example, fruit flies lay eggs in ethanol-containing substrates, which can deter parasites. 

Likewise, when parasitized by wasps, fruit fly larvae increase their ethanol consumption as a protective measure. Ethanol could have other biochemical or medicinal benefits in wild animals, an area researchers are keen to explore further.

Potential effects on animal behavior 

“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,” adds lead author Anna Bowland, a behavioral ecologist at the University of Exeter. 

“To test that, we’d really need to know if ethanol is producing a physiological response in the wild.” 

Such a physiological response could mean that ethanol’s presence in animals’ diets has broader social or psychological impacts, potentially influencing animal behavior in subtle ways.

Many questions remain 

Despite these intriguing insights, the extent and implications of ethanol consumption in wild animals remain largely unknown. 

The research team plans to examine the behavioral and social effects of ethanol intake in primates and investigate how alcohol-metabolizing enzymes function across species. 

They are particularly interested in whether animals consume ethanol intentionally or if it’s simply a byproduct of their search for food. 

This question opens the door to understanding how evolution might have shaped animals’ ability to tolerate or benefit from low ethanol levels in their diets.

“To really understand if animals consume ethanol intentionally, we need to observe how they interact with ethanol-containing fruits in their natural habitats,” Hockings said. 

These future studies could reveal whether ethanol has played an adaptive role in the diets of certain species, and if so, what evolutionary benefits it may have conferred.

Alcohol in the animal kingdom

This research challenges the assumption that ethanol consumption is unique to humans and raises new questions about its evolutionary significance for nonhuman species. 

By exploring the role of ethanol in natural ecosystems, scientists may uncover previously overlooked benefits that ethanol offers animals. 

Understanding how animals consume and process ethanol could reveal broader ecological patterns and even help us trace the evolutionary origins of how different species interact with this potent molecule.

“There is much more to learn about how ethanol affects wildlife and whether it plays a larger part in the ecological relationships that shape species interactions and evolution,” Carrigan concluded.

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