A gag in “Piker’s Peak,” a Looney Tunes cartoon from 1957, plays off the trope of the Saint Bernard rescue dog carrying brandy to warm up an avalanche victim. After digging Yosemite Sam out of the snow, the dog opens the barrel in its collar to reveal a cocktail shaker and martini glass, and ignores the frozen Sam as it mixes itself a drink.
There’s something charmingly funny about the image of an animal drinking alcohol; it seems so incongruously humanlike. Some documentaries that show wild animals getting drunk off boozy rotten fruit, like this one from Botswana, use music and narration to emphasize the unexpected comedy of the scene. Scientists once believed that such behavior was random and accidental, especially in species not closely related to humans and other great apes. But more recent studies paint a very different picture. A research review published in October 2024 in Trends in Ecology & Evolution found that wherever ethanol—the active ingredient in alcoholic beverages—occurs in nature, it is routinely consumed by a variety of species, from insects and birds to rodents and monkeys.
In nature, “ethanol ingestion is far more common than was previously thought,” says Anna Bowland, a Ph.D. student in bioscience at the University of Exeter, England, who worked on the review. The paper cites research mainly from tropical regions like Central America and Southeast Asia, where yeast and bacteria ferment the natural sugars in fruit and nectar into ethanol in the hot sun. But a similar effect has also been observed in completely different environments. In Finland, when wild berries thaw in the warm sun after being bruised by frost, “they ferment quite quickly,” says Bowland. “And then as the birds come and feed on them, they’re ingesting alcohol.”
Because animals that feed on fruit and nectar ingest more ethanol on average, many of them show evolutionary adaptations to tolerate it. The review cites a study from the rainforests of Malaysia, which found that arboreal mammals like treeshrews, lorises, and squirrels regularly feed on fermented palm nectar with an alcohol concentration as high as 3.8 percent, comparable to a light beer. But intriguingly, says Bowland, “they don’t—in our anthropogenic sense—seem to get drunk,” meaning that they don’t display behaviors associated with inebriation in humans, like drowsiness or reduced motor skills. This suggests that treeshrews are particularly good at metabolizing ethanol.
Bowland explains that from an evolutionary standpoint, “it’s not beneficial for [animals] to get drunk, because that can lead to predation and injury and reduce survival, so they might not pass on their genes.” Animals that feed on a boozy food source have a better chance of survival if they can hold their liquor, so the presence of ethanol creates evolutionary pressure for tolerance. Aside from treeshrews, fruit flies provide a well-known example. Not only do the insects tolerate relatively high concentrations of ethanol, they are attracted to its smell, which indicates prime egg-laying territory. Some studies have even shown behavior patterns around alcohol in fruit flies that seem to mirror those of humans. The insects develop higher tolerance to alcohol through repeated exposure, and are more likely to “drown their sorrows” in alcohol after repeated romantic rejection.
Another prominent example is the great apes. “Humans, chimpanzees, and gorillas—we all possess a mutation in one of our genes that greatly increases the rate at which we can metabolize and break down ethanol,” says Bowland, thanks to a digestive enzyme called ADH4. While other primates like Central American spider monkeys consume ethanol in fruit, and may, like fruit flies, be drawn to the smell, apes are particularly efficient at processing it. This has led some researchers to propose that the human fondness for ethanol goes back long before deliberate brewing and fermenting, to a dietary shift in our common ancestor with other apes.
Robert Dudley, a professor of integrated biology at the University of California, Berkeley, explored the evolutionary origins of human alcohol consumption in his book The Drunken Monkey Hypothesis. “Primates are ancestrally fruit-eaters, going back 45 million years,” says Dudley, based on evidence like the shape of their teeth. In modern apes, fruit still makes up 60 to 80 percent of the diet for chimpanzees and 90 percent for gibbons. According to Dudley, who was not affiliated with the 2024 study, genetic analysis of ADH4 in early apes shows that the enzyme became 20 times better at degrading ethanol about 12 million years ago. “This is right when the great apes are coming out of the trees and going bipedal, walking around, and, we think, now going after more ripe fruit crops that have already fallen down,” he says. Other foods becoming scarce, as well as the increased availability of fermenting fruit on the ground, may have increased pressure for prehistoric primates to adapt to an ethanol-rich source of nutrients.
“We’ve evolved with this molecule,” Dudley says of ethanol. His research suggests that the scent of ethanol “acts as a long-distance sensory cue” for primates, alerting them to the presence of edible fruit hidden among dense foliage. “Where there’s ethanol, there has to be sugar,” Dudley explains, and the scent of ethanol might even allow monkeys and apes to “assess individual fruits without wasting time biting into them.” Dudley also suggests that ethanol-rich fruit may stimulate the appetite of wild primates, encouraging them to take as much advantage of the available nutrition as possible. He compares this with the “aperitif effect” observed in humans, where consuming alcohol before a meal leads to increased food consumption, perhaps due to the stimulation of brain areas that regulate feeding behavior.
That wild animals regularly consume ethanol is clear, but figuring out exactly how much they consume and how often is “actually a very complex question,” says Dudley. “What you have to know is, A, what are they eating, and in what quantity over the year? And B, you have to know how much alcohol is in each food item. Now the problem is, if they consume a food item, you can’t measure its alcohol content, so you have to, by inference, sample from what you think they’re eating.” By correlating properties of fruit and other data collected in the wild, scientists can get a sense of how much ethanol animals are ingesting. Hair and urine samples can also show evidence of ethanol being metabolized. But these challenges of collecting data in the field mean that there is still “more work on the natural biology of alcohol exposure” to be done, says Dudley.
Bowland agrees that when it comes to the relationship between animals and ethanol “in the natural environment, there are still lots of unanswered questions.” One of these is whether ethanol consumption is purely incidental—just a consequence of eating sugar—or has other potential benefits to survival. Bowland references one hypothesis that suggests animals might ingest ethanol for a medicinal benefit. She describes experiments in which fruit fly larvae appear to deliberately drink more alcohol to kill off the eggs of parasitic wasps, which cannot tolerate as much ethanol as the larvae themselves. Dudley points to other fruit fly studies suggesting that “if you expose them to low-level alcohol, like 1 or 2 percent, they live 20 to 25 percent longer.” But exactly how much constitutes a beneficial versus a harmful amount of alcohol, in both animals and humans, is difficult to ascertain.
Bowland is hopeful that her review will encourage further research into the interactions between animals and ethanol. Dudley agrees. “I think that the nice thing about that paper [is it] just points out the ubiquity of ethanol,” he says of the 2024 review. Ethanol is just a part of nature, found wherever there is sugar and microbes to ferment it. And while there’s still much to be explored, one thing is clear: If there’s anything that truly separates human beings from animals, it’s not alcohol.
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