Ian Sample Science editor
Tue 29 October 2024
Male fruit flies turn to alcohol when they are rejected as a mate, while females of a closely-related species become less picky about their mates after imbibing. Photograph: Surapol Usanakul/Alamy
Humans may have turned drinking into something of an art form but when it comes to animals putting alcohol away, Homo sapiens are not such an outlier, researchers say.
A review of published evidence shows that alcohol occurs naturally in nearly every ecosystem on Earth, making it likely that most animals that feast on sugary fruits and nectar regularly imbibe the intoxicating substance.
Although many creatures have evolved to tolerate a tipple and gain little more than calories from their consumption, some species have learned to protect themselves with alcohol. Others, however, seem less able to handle its effects.
“We’re moving away from this anthropocentric view that alcohol is used by just humans and that actually ethanol is quite abundant in the natural world,” said Anna Bowland, a researcher in the team at the University of Exeter.
After trawling research papers on animals and alcohol, the scientists arrived at a “diverse coterie” of species that have embraced and adapted to ethanol in their diets, normally arising through fermented fruits, sap and nectar.
Ethanol became plentiful on Earth about 100m years ago when flowering plants began to produce sugary fruits and nectar that yeast could ferment. The alcohol content is typically low, at around 1% to 2% alcohol by volume (ABV), but in over-ripe palm fruit the concentration can reach 10% ABV.
In one study, wild chimpanzees in south eastern Guinea were caught on camera bingeing on the alcoholic sap of raffia palms. Meanwhile, spider monkeys on Barro Colorado Island, Panama, are partial to ethanol-laden yellow mombin fruit, revealed to contain between 1% and 2.5% alcohol. “Evidence is growing that humans are not drinking alone,” the authors write in Trends in Ecology and Evolution.
Whether consumption leads to drunkenness is another question. Tales of inebriated animals are abound, from elephants and baboons drunk on marula fruit in Botswana, to a moose found with its head stuck in a tree in Sweden after chomping on fermented apples. But in none of these cases, at least, was alcohol measured in the animals or the fruit.
Many animals seem to have impressive alcohol tolerance. Despite the “prodigious ethanol consumption” of pen-tailed treeshrews, the scientists found no evidence that the animals became intoxicated, but concede it was “unclear how an inebriated treeshrew would behave”.
Animals that regularly eat fermented foods tend to metabolise alcohol rapidly, sparing them its worst effects. But some creatures encounter ethanol less often and can suffer the consequences. Tests on Cedar waxwings, which died from crashing into fences and other structures, found they had been flying under the influence after gorging themselves on the over-ripe berries of the Brazilian pepper tree. “Inebriation is not beneficial in an environment where you are fighting to survive,” Bowland said.
Perhaps the most striking effects of alcohol are seen in insects. Male fruit flies turn to alcohol when they are rejected as a mate, while females of a closely related species become less picky about their mates and have sex with more males after imbibing. Fruit flies lay their eggs in ethanol-rich foods, protecting them from parasites.
Earlier this month, researchers led by Eran Levin at Tel Aviv University found that oriental hornets may be the only animals capable of consuming an unlimited amount of alcohol without suffering ill effects. “They can ingest up to 80% ethanol solution without any negative effects on their mortality or behaviour,” said Dr Sophia Bouchebti, an author on the study.
“Aside from the fruit fly, aye aye and slow loris, it is unclear if animals prefer ethanol-containing food,” said Matthew Carrigan at the College of Central Florida, who worked on the review with Bowland. “One of our next steps is the test whether animals in the wild prefer ethanol-containing food or eat it only when ethanol levels are too low to detect or unfermented fruit is hard to find.”
Alcohol consumption among non-human animals may not be as rare as previously thought, say ecologists
Cell Press
image:
A spider monkey feedings on fruits of spondia mombin
view moreCredit: 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 publishing October 30 in the Cell Press journal 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 (@KJHockings) 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.
“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 implication of ethanol consumption in primates and to more deeply examine the enzymes involved in alcohol metabolism.
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This research was supported by the Primate Society of Great Britain, the Wenner-Gren Foundation, the Canada Research Chairs program, and the Natural Sciences and Engineering Research Council of Canada.
Trends in Ecology & Evolution, Bowland et al., “The evolutionary ecology of ethanol” https://cell.com/trends/ecology-evolution/fulltext/S0169-5347(24)00240-4
Trends in Ecology & Evolution (@Trends_Ecol_Evo), published by Cell Press, is a monthly review journal that contains polished, concise, and readable reviews and opinion pieces in all areas of ecology and evolutionary science. It aims to keep scientists informed of new developments and ideas across the full range of ecology and evolutionary biology—from the pure to the applied, and from molecular to global. Visit http://www.cell.com/trends/ecology-evolution. To receive Cell Press media alerts, please contact press@cell.com.
A capuchin eating fruits
Credit
Julia Casorso
Journal
Trends in Ecology & Evolution
Method of Research
Literature review
Subject of Research
Not applicable
Article Title
The evolutionary ecology of ethanol
Article Publication Date
30-Oct-2024
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