Why can’t I taste with my feet?

Fish taste with their fins. Butterflies taste with their legs. Octopuses taste with everything. Cats can’t taste sugar. So, why do humans taste only with their tongues, when there are taste receptors all over the human body?

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Giselle Clarkson

A gurnard walks along the sandy bottom of the Hauraki Gulf, tasting the sea floor with fins it has modified into legs. An octopus hiding in a nearby reef tastes the water with its entire body, even its eyelids. A Bryde’s whale passing overhead takes a gulp of zooplankton and small fish, but the entire mouthful will probably just taste salty—whales have lost the ability to detect sweet, sour, bitter and umami.

Above the water, a butterfly tastes flowers with its feet, a female parasitic wasp detects a potential host with taste receptors in her ovipositor, and a tūī sings with pleasure after lapping up the sweet nectar in a harakeke flower. Your pet cat, however—like many carnivores—gets no joy from sugar at all. In cats, the genes responsible for making the sweet-taste receptors are riddled with mutations, and no longer function.

Technological advances over the past decade are revealing all kinds of surprising things about how animals and humans taste, the bizarre body parts involved, and what this most pleasurable of senses can teach us about evolution itself.

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What is taste? At its most basic level, it’s one of the ways we detect chemicals in the environment. When I put something in my mouth, different kinds of taste receptors on my tongue and in my throat respond to chemical compounds in the object. The receptors then send a signal to the taste cortex in my brain and create the perception of saltiness, sweetness, sourness, bitterness or umami.

The taste receptors are just messengers. The real action happens in the grey matter, says Rachel Herz, a neuroscientist at Brown University and Boston College in the United States, who specialises in smell and taste. “If I were to tickle different spots in your brain, I would be able to get you to perceive salty, bitter and so forth without you actually having anything in your mouth.”

Although humans have at least 25 different kinds of bitter receptor, the brain converts them all to a single sensation, says Rob Dunn, an ecologist at North Carolina State University and the co-author of a new book about the evolution of flavour. “The bitter of coffee and the bitter of hops are the same bitter. They have a different flavour, because their aromas are different.” Flavour involves more than the tongue—as anyone with a blocked nose can testify.

“Taste is confined to those things appreciated by the taste receptors on the tongue, and flavour is the full experience you get when something is in your mouth. It’s aroma, it’s mouthfeel, it’s spiciness—all those things together.”

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Taste probably evolved to keep us safe and well fed, leading us to the nutrients we need for survival and away from what could be dangerous.

“Bitter says, ‘Poison! Don’t eat this,’” says Emma Beckett, a food and nutrition scientist at University of Newcastle in New South Wales. “Sweet and umami say, ‘There’s energy in this.’” Salty and sour, however, are more complex: a little can be delicious, but a lot is disgusting. In the case of salt, that makes evolutionary sense, says Beckett, “because some salt is essential for survival, and too much salt will kill you”. An aversion to sour tastes would keep us from unripe fruit and spoiled food. Why do we like a little bit of sour, then? Perhaps because some fermented foods are good for us, says Beckett.

Every person’s sense of taste is unique. Scientists have found wide variation in how we perceive bitterness, for example. Some of Beckett’s research involves comparing the 25 per cent of people who are hypersensitive to bitterness (“supertasters”) with the 25 to 30 per cent who are insensitive to it (“non-tasters”) to see if there are any links between sense of taste and health problems such as obesity, irritable bowel syndrome and other gastrointestinal diseases.

Studies have found that supertasters tend to eat fewer vegetables, because vegetables taste unpleasantly bitter to them, and that these people compensate by eating more salty, sweet and fatty foods.

Yet being a non-taster of bitterness seems dangerous—you might accidentally eat something poisonous. Non-tasters are also more likely to smoke, as they aren’t put off by the bitterness of nicotine. One theory holds that insensitivity to bitterness may have evolved to protect us from malaria. Some bitter-tasting plants produce the anti-malarial compounds quinine and cyanide. At strong doses, cyanide is a fatal poison, but evidence shows that humans regularly consuming small amounts of the stuff can be off-putting to the malaria parasite. In places where malaria is prevalent, people who enjoy eating these plants may be more likely to survive. “Of course, it’s very hard to prove,” says Beckett.

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We think of taste as something that happens in our mouth, but recent research has shown that—just like the octopus and the butterfly—humans actually have taste receptors all over our bodies. There are some in the nose, the gut, the airways, the pancreas, the liver… even in the testicles. “Taste receptors are everywhere,” says Beckett, “and they’re doing lots of different jobs.”

Like the taste receptors in our mouths, the receptors in other parts of our bodies detect chemicals—the only difference is they’re not hooked up to the taste cortex in our brains, and so we’re not conscious of the signals they are sending.

This last point was lost on the dozens of teenage boys who filmed themselves dipping their testicles in various substances, claiming they could taste them, and posted the videos on TikTok. Beckett has had to field more questions about this than any other topic. “People hear, ‘Taste receptors are everywhere’, and they’re like, ‘Sweet, let’s dip our balls in soy sauce’.”

For the record: no, males can’t taste with their testicles. The taste receptors aren’t on the surface—they’re inside the structures responsible for making sperm, and like all the other taste receptors, their job is to detect the chemical environment. Scientists think these receptors may be signalling the presence of toxins, or whether the body has enough energy to switch on sperm production.

It’s not surprising that we have the same receptors in lots of different types of cells around our bodies, doing very similar jobs; we evolved from single-celled organisms, after all. And if all of those receptors were bombarding our conscious minds with signals, life would become overwhelming. The taste receptors in our mouths—the ones we’re actually aware of—are really the odd ones out, compared to the ones all over our bodies. “But they’re crucial for ensuring we actually pick up the food we need and put it in our mouth,” says Beckett.

Without taste, it’s hard to get excited about eating, she says: people who lose their sense of taste need to work really hard to get enough calories and nutrients in.

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Ultimately, taste is about pleasure, says Rob Dunn, though that part is often ignored by science.

“As grubby as a possum or a racoon in a trash can seems, it’s still making choices when it’s in there—and it’s doing that on the basis of taste and flavour, on the basis of the pleasures associated with those things.”

Scientific studies tend to focus on how animals—or even people—use taste to locate the calories they need. “But what we seem to have neglected is the only way they know if they have found good stuff is if their senses reward them for it. There are groups of people who get more than 60 per cent of their calories from honey during part of the year. If you read the anthropology literature about this, it’s like, ‘They’ve found a good way to optimise their caloric intake.’ No—it’s that honey tastes good.”

Flavour is part of an evolutionary dance between plant and herbivore, predator and prey. Fruit, for example, evolved to tempt animals to spread plants’ seeds around. Some of them even try to trick us.

Pentadiplandra brazzeana is a central African fruit nicknamed “oubli”—French for “forgetfulness”—because the bean-sized berries allegedly taste so sweet they can make children forget their mothers. But although oubli is abundant in the forests where fruit-loving western lowland gorillas live, the apes don’t touch it. That’s because oubli doesn’t actually contain sugar. The sweet taste comes from a protein called brazzein—and unlike sugar, brazzein provides barely any useful energy.

Lowland gorillas have a genetic mutation that means they can’t taste the fruit’s fake sweetness, so they don’t fall for the plant’s tricks. Is this an example of evolution at work? Gorillas that stuffed their faces with sweet but unnourishing fruit would have been less successful breeders than those that avoided the artificial sweetness of oubli in favour of real sugar, says Dunn. Natural selection would have favoured those with the mutation.

The pleasure of eating delicious food has likely driven human evolution, too. “The reason you would use fire is because when you cook stuff, it tastes better,” says Dunn. “Chimpanzees use tools to get honey, ants, or termites. What do those foods all have in common? The foods that the chimps eat with tools are tastier than the foods they get otherwise.

“There are all these consequences that follow—you get more calories, you feel fuller, you’re more likely to come back and do it again. That then allows evolution of bigger brains. But when it starts, ultimately, you have something in your hand and you’re choosing to eat it or not eat it.”

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Like cats, birds have lost the sweet receptors on their tongues. And, yet, tūī delight in sweet flax flowers. Australia is full of raucous nectar-loving songbirds, and in the Americas hummingbirds will flock to a feeder full of sugar water. Could it really be true that birds can’t taste sweet?

“Birds are dinosaurs, of course, and their ancestors were very carnivorous,” says Maude Baldwin, a lead researcher at the Max Planck Institute for Ornithology in Germany. One theory is that those meat-eating ancestors were so focused on the salty, umami taste of blood that sweetness didn’t matter to them any more—and over time, the unused gene mutated, eroded and eventually disappeared.

So, why are birds flocking to flowers rather than flesh? Baldwin set out to solve the mystery. By sequencing hummingbird taste receptors and recreating the proteins in the lab, her team found that the tiny jewel-like birds had modified their umami taste receptor—via a set of complex changes over millions of years—so that it was also able to detect sweetness.

“It seems they may not be able to distinguish between the two,” says Baldwin. In other words, umami and sweet are the same sort of yummy to a hummingbird.

Next, Baldwin’s team looked at songbirds—a huge group of around 4000 species that include Australian honeyeaters and lyrebirds, northern hemisphere crows and sparrows, and New Zealand korimako (bellbirds) and tūī. Songbirds had also modified the umami receptor in order to taste sweetness, but they had changed a different part of it. That leaves Baldwin wondering whether hummingbirds and songbirds might process sugar differently, just as they sense it differently. “It may have repercussions that we don’t initially appreciate,” she says. “In all aspects of biology, we see these cases where there are different solutions to similar problems, and understanding how those solutions evolve tells us a lot about how evolution works.”

This kind of convergent evolution is surprisingly common, says Baldwin: two groups of birds have independently trained themselves to like nectar rather than flesh.

Baldwin is also curious about kea, which enjoy nectar and fruit alongside other delights such as window rubber, lead and trampers’ tents. Her team is pretty sure some parrots can taste sweet. Australian lorikeets are notorious nectar-lovers, but kea are from a much older lineage of parrots. Baldwin is currently looking into kea and other parrots’ genomes to find out whether they have a sweet tooth (or rather, beak). Could the ancestor of all parrots taste sugar? Did the ability evolve multiple times? “New Zealand birds may play a very interesting role in this story,” says Baldwin.

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Finding out how taste works in birds could also have implications for human health. Hummingbirds live on sugar, and yet are capable of extreme athletic performance: they can migrate thousands of kilometres without stopping, flap their wings more than 50 times a second and reach speeds of 55 kilometres per hour. How do they cope with high-sugar diets without getting sick?

Humans share the planet with millions of other species, each of which has evolved different ways of meeting the unique challenges of its environment. Like all other animals, our sensory systems and our physiology are products of our evolutionary history, says Baldwin. “If we understand different physiologies, we’ll also understand ourselves better.”

It may also help us to have more empathy for others. “The thing I love about this research,” says Beckett, “is that it’s a way of explaining to people that we are all different on the inside.” Genetics doesn’t just determine the colour of our hair or eyes; it also affects how our taste receptors function.

It’s thought the taste receptors in the gut could play a role in signalling satiety—how full you feel—and that people with less sensitive receptors might receive a weaker signal in their brain.

“You and I could both eat the same meal and one of us would be full and one of us would still be hungry,” says Beckett. “Sometimes those subtle differences add up to have big consequences.” Unpredictable consequences, too: a random tweak of genetic code leads to an adaptation that helps a species survive sweeping changes to their environment. “So, from an evolutionary point of view, it would be nice if we could all be a little bit less judgey about what we’ve inherited.”

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