When researchers consider the classic five categories of taste—sweet, salty, sour, bitter, and umami—there’s little disagreement over which of them is the least understood. Creatures crave sweet for sugar and calories. A yen for umami, or savoriness, keeps many animals nourished with protein. Salt’s essential for bodies to stay in fluid balance, and for nerve cells to signal. And a sensitivity to bitterness can come in handy with the whole not-poisoning-yourself thing.
But sour? Sour’s a bizarro cue, a signal reliable neither for toxicity nor for nutrition. Really, it’s just a rough proxy for low pH, the presence of acid—the citric in lemons, the acetic in vinegar, and the like. “We don’t need sour to live,” Ann-Marie Torregrossa, a taste researcher at the University at Buffalo, told me. “It’s a weird sense to need.” It has been so scientifically neglected that Rob Dunn, an ecologist at North Carolina State University, considers it something of a “missing taste,” the gustatory litter’s forgotten runt. No one really knows for sure, Dunn told me, “what it’s all about.”
And yet we taste sour, strongly, and are not alone in doing so. When Dunn and his colleagues recently set out to investigate the sensation’s evolutionary roots, he told me, they couldn’t find a single backboned species that had definitively lost the ability to identify acidic foods, be they birds or mammals or amphibians or reptiles or fish. Admittedly, that may be a function of how few animals scientists have surveyed—just several dozen—but already, that makes sour a standout. Cats, otters, hyenas, and other carnivores have lost the ability to suss out sugar; giant pandas are immune to umami; dolphins, which swallow their prey whole, don’t seem to be able to savor sweetness or savoriness, and have booted bitter sensitivity too. But sour sensing appears to have staying power that its cousins do not—which means that it must be doing something important, perhaps something ancient.
What that something is remains a mystery, and it’s probably actually somethings, depending on the species. Part of the story, Dunn said, may begin with fish—the most ancient vertebrate group that’s had its sour-sensing superpowers assessed and confirmed. Fish have taste buds in their mouths, like we do, but also freckled all over their bodies (which you could think of as enormous scaled tongues). Some of these receptors can sense acid, which may have helped the animals navigate in and out of waters rich or poor in carbon dioxide, and kept their bodies’ fluids in chemical balance.
When the ancestors of today’s terrestrial creatures began their slow crawl ashore, sour sensing somehow stuck—and quickly splintered along species lines. Nowadays acidic foods are neither universally beloved among land animals nor universally reviled. Many apes, including us, seem to dig the taste, as do rats and pigs—at least up to a certain concentration, called a “bliss point,” past which the taste gets gross. “Just don’t give a tomato to a sheep,” Dunn warned me. “And certainly don’t give a lemon to a sheep.” (Dunn hasn’t tried to, but he and his colleagues did find a 1970 study that suggests that sheep think acidic stuff tastes baaaaad.)
It’s not totally clear why some species find sour so odious, but scientists have guesses. Maybe animals that have been documented as disliking the taste—horses, vampire bats, rabbits, and axolotls, to name a few—take it as a hint that their food is still unripe, or has gone rancid and is therefore unsafe. At an extreme, acid itself can gnaw away at tissues or erode tooth enamel; it can screw with a body’s chemistry or discombobulate the sometimes-fragile microbes that inhabit the gut. “A lot of the explanations are aimed at the negative,” Hannah Frank, a crop and soil-sciences researcher at North Carolina State who’s been working with Dunn to untangle sour’s evolutionary past, told me. But they also “haven’t been well substantiated,” she said. Proving the why of evolution is always something of a scientific nightmare. And it’s not like history is peppered with case studies of “sad sheep that died because they ate too many lemons,” Dunn told me.
Unlike sheep, though, we humans are, as a species, absolute sour stans. So are several species of apes and monkeys in our evolutionary vicinity—chimps, orangutans, gorillas, macaques, gibbons. Clearly, acid’s doing something right. For years, researchers have been floating a compelling reason: Sour can be a good indication that a food is rich in vitamin C, a nutrient that our ancestors lost the ability to manufacture about 60 to 70 million years back. A fresh appetite for sour might have helped spare us the ravages of scurvy.
Even in the simplest version of this tale, though, the relationship with acidity is messy. Sour fruit, though sometimes an excellent snack, can also be too raw. Here, a partnership with sweetness might be key, says Katie Amato, a biological anthropologist at Northwestern University who’s been collaborating with Dunn. Very tart, very sugary foods could even signal a bonus benefit: that a bonanza of beneficial microbes have colonized our cuisine and started to break its carbohydrates down. This process, called fermentation, adds the taste of tang; it can also keep dangerous microbes out, and pulverize gnarly plant fibers that our own bodies struggle to digest on their own. And humans (some of us, anyway) really, really dig it—think kimchi, kombucha, sauerkraut, or yogurt. If sour’s a marker for fermentation’s marvelous musk, then “it would be selecting for the right kind of overripe fruit,” Amato told me.
If those notions pan out, they open up far more questions than we have answers to. Paule Joseph, a nurse practitioner and taste and smell researcher at the National Institutes of Health, told me that scientists still don’t have a good explanation for variation for sour preference within species. Some of it might be inborn biology, drawn from genetics or age. (Some research has hinted that little kids might be more jazzed about sour foods than adults.) But Joseph says it’s also essential to consider how the foods in our environment shape our predilections. Even sort of “bad” tastes such as bitter and sour can become positive—black coffee, for example, has notes of both.
And the trends that pushed primates toward sourness won’t necessarily dictate tart tastes in other species. Pigs apparently think sour’s splendid, even though they can synthesize vitamin C just fine; Dunn ventures that their acid appetites might just be part and parcel of their propensity to “eat almost anything.” Then there are guinea pigs, which present the converse conundrum: They, like us, have lost their vitamin C–producing chops. And yet, a 1978 study showed that two guinea-pig species “rejected” citric acid in a taste test.
Taste-preference studies in nonhuman species, to be fair, aren’t very easy to do. A typical experiment involves offering an animal a choice between plain water and flavored water—infused with something sweet, salty, bitter, umami, sour—and seeing which liquid most captivates the creature. An avoidance of somewhat-acidic water might not say all that much; maybe it’s missing that crucial, sugary X factor. Or maybe acidic water just seems too unnatural. And though some animal species produce many of the same reactions we make when we encounter something grody-tasting—wincing, nose wrinkling, mouth gaping, even a bit of dramatic limb flailing—the further scientists get from studying humans, the tougher it is to suss out enjoyment, or lack thereof.
Hiro Matsunami, a chemosensory biologist at Duke University, pointed me to yet another complicating factor: Sour sensing’s apparent ubiquity among vertebrates may not necessarily be about taste. The same chemical receptors we use to zero in on acid in our mouths seem to perform other functions in the body that might be super essential. That evolutionary pressure alone could have made sour taste stick around too.
Since embarking on their science-of-sour shenanigans, both Frank and Dunn have been conducting some very informal investigations to expand sour’s evolutionary tree. Dunn’s been throwing lemons to crows; Frank has been feeding pickles and citrus to her dog, Maple June. Neither species seems that pleased with the offering, though Maple June still, with an agonized look on her face, wolfs raw lemons down. “She just pains her way through” as many other dogs do, Frank told me. Maybe she’s attracted to sour’s beguiling acerbicness—the appeal of a food that somehow bites back. Then again, Maple June’s a canine, and perhaps the story is simple, Frank said: “She’ll eat anything.”