Taste is a sort of chemical touch. Taste cells are specialized skin cells. If you have hands for picking up foods and putting them into your mouth, it makes sense for taste cells to be on your tongue. But if, like flies, you don’t, it may be more expedient to have them on your feet. “They land on something and go, ‘Oooo, sugar!’’’ Rawson does her best impersonation of a housefly. “And the proboscis automatically comes out to suck the fluids.” Rawson has a colleague who studies crayfish and lobsters, which taste with their antennae. “I was always jealous of people who study lobsters. They examine the antennae, and then they have a lobster dinner.”
The study animal of choice for taste researchers is the catfish,[12] simply because it has so many receptors. They are all over its skin. “Catfish are basically swimming tongues,” says Rawson. It is a useful adaptation for a limbless creature that locates food by brushing up against it; many catfish species feed by scavenging debris on the bottom of rivers.
I try to imagine what life would be like if humans tasted things by rubbing them on their skin. Hey, try this salted caramel gelato, it’s amazing. Rawson points out that a catfish may not consciously perceive anything when it tastes its food. The catfish neurological system may simply direct the muscles to eat. It seems odd to think of tasting without any perceptive experience, but you may be doing it right now. Humans have taste receptor cells in the gut, the voice box, the upper esophagus, but only the tongue’s receptors report to the brain. “Which is something to be thankful for,” says Danielle Reed, Rawson’s former colleague at Monell. Otherwise you’d be tasting things like bile and pancreatic enzymes. (Intestinal taste receptors are thought to trigger hormonal responses to molecules, such as salt and sugar, and defensive reactions—vomiting, diarrhea—to dangerous bitter items.)
We consider tasting to be a hedonic pursuit, but in much of the animal kingdom, as well as in our own prehistory, the role of taste was more functional than sensual. Taste, like smell, is a doorman for the digestive tract, a chemical scan for possibly dangerous (bitter, sour) elements and desirable (salty, sweet) nutrients. Not long ago, a whale biologist named Phillip Clapham sent me a photograph that illustrates the consequences of life without a doorman. Like most creatures that swallow their food whole, sperm whales have a limited-to-nonexistent sense of taste. The photo is a black-and-white still life of twenty-five objects recovered from sperm whale stomachs. It’s like Jonah set up housekeeping: a pitcher, a cup, a tube of toothpaste, a strainer, a wastebasket, a shoe, a decorative figurine.
Enough stalling. Time to try the palatant. I raise the cup to my nose. It has no smell. I roll some over my tongue. All five kinds of taste receptors stand idle. It tastes like water spiked with strange. Not bad, just other. Not food.
“It may be that that otherness is something specific to the cat,” says Rawson. Perhaps some element of the taste of meat that humans cannot perceive. The feline passion for pyrophosphates might explain the animal’s reputation as a picky eater. “We make [pet food] choices based on what we like,” says Reed, “and then when they don’t like it, we call them finicky.”
There is no way to know or imagine what the taste of pyrophosphate is like for cats. It’s like a cat trying to imagine the taste of sugar. Cats, unlike dogs and other omnivores, can’t taste sweetness. There’s no need, since the cat’s diet in the wild contains almost nothing in the way of carbohydrates (which include simple sugars). Either cats never had the gene for detecting sweet, or they lost it somewhere down the evolutionary road.
Rodents, on the other hand, are slaves to sweetness. They have been known to die of malnutrition rather than step away from a sugar-water drip. In an obesity study from the 1970s, rats fed an all-you-can-eat “supermarket” diet that included marshmallows, milk chocolate, and chocolate-chip cookies gained 269 percent more weight than rats fed standard laboratory fare. There are strains of mice that will, over the course of a day, consume their own bodyweight in diet soda, and you do not want the job of changing their bedding.
Does that mean rodents feel pleasure in tasting sweet things the same way we do? Or is it simply a sequence of programmed responses, receptors sending signals and signals driving muscles? Video footage Danielle Reed sent me suggests that rodents do consciously perceive and savor the taste of something sweet. One clip shows a white mouse that has just been drinking sugar solution. She is shown in ultra-slow motion, filmed from below through a clear plastic floor, licking the fur around the sides of her mouth. (The caption uses the scientific term for lip-licking: “lateral tongue protrusion.”) Another clip shows a mouse that has just tasted denatonium benzoate, a bitter compound that parents used to paint on their children’s fingertips to discourage nail-biting. The mouse is doing everything it can to rid itself of traces of the chemical. It shakes its head and rubs its face with its hairy white forelegs. It pulls a “gape”: mouth opened wide, tongue stuck out to eject the offending food. (Humans do this too. The scientific term here is “the disgust face.”)
“If it’s exceedingly nasty,” Reed told me, “they will actually drag their tongue on the bedding to try to get it off.” Clearly taste matters to them.
Conversely, do animals with no taste buds derive no pleasure from eating? Is it just a daily chore? Has anyone observed—in, say, a python eating a rat—those same parts of the brain that light up when humans are experiencing taste delight? Reed doesn’t know. “But no doubt somewhere in the world there’s a scientist trying to get a live python into an fMRI machine.”
Rawson points out that although snakes can’t taste, they have a primitive sense of smell. They’ll extend their tongue to gather volatile molecules and then pull it back in and plug it into the vomeronasal organ at the roof of the mouth to get a reading. Snakes are keenly attuned to the aroma of favored prey—so much so that if you slip a rat’s face and hide, Hannibal Lecter–style, over the snout of a non-favored prey item, a python will try to swallow it. (University of Alabama snake digestion expert Stephen Secor did this some years back to reenact a scene for National Geographic television. “Worked like a charm,” he told me. “I can get a python to eat a beer bottle if I put a rat head on it.”)
For part of their development, human fetuses have a vomeronasal organ, though no one knows whether it’s functional. You can no more ask a fetus about these things than a python. Rawson surmises that the organ is a holdover from “when we were crawling out of the primordial soup,[13] and we needed to sense the chemicals in the environment and know which ones to go toward or away from.”
Rawson has an idea of what it is like to eat without perceiving tastes, because she has talked to cancer patients whose taste receptors have been destroyed by radiation treatments. The situation is well beyond unpleasant. “Your body is saying, ‘It’s not food, it’s cardboard,’ and it won’t let you swallow. No matter how much you tell your brain that you need to eat to survive, you’ll gag. These people can actually die of starvation.” Rawson knows a researcher who has been experimenting with using potent flavors—which, as we know from the last chapter, are mainly smells—to make up for absent tastes. Taste and smell are intertwined in ways we don’t consciously appreciate. Food technologists sometimes exploit the synergy between the two. By adding strawberry or vanilla—aromas we associate with sweetness—it’s possible to fool people into thinking a food is sweeter than it really is. Though sneaky, this is not necessarily bad, because it means the product can contain less added sugar.
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This explains the perplexing odor of swamp water on certain floors of the Monell Chemical Senses Center during the 1980s. The basement was a big catfish pond.