Tree Shrews Love Hot Peppers Because They Don’t Feel the Burn

A genetic mutation prevents Chinese tree shrews from feeling the heat of capsaicin, making them the only other mammal besides humans that enjoys hot foods

Tree shrew
Two tacos, extra ghost pepper sauce, please! Wikimedia Commons

In recent years, the lines that traditionally separate humans from other animals have gotten very blurry; crows and chimps use tools, some insects know how to farm, and species across the spectrum communicate in ways that are much more complex than we previously thought. One of the only traits left to distinguish us from other mammals is a tolerance—and in many cases deep, passionate love—for spicy foods. Richie Hertzberg at National Geographic reports that even that is under assault. A new study reveals that a species of Chinese tree shrew also seeks out hot peppers, and it’s probably got a higher tolerance than you.

According to a press release, chili peppers do not grow naturally in the range of Chinese tree shrews, Tupaia belangeri chinensis, but they do feed heavily on Piper boehmeriaefolium, another species of plant that produces copious amounts of capsaicinoids, the compounds that give peppers their kick. Cara Giaimo at Atlas Obscura reports that researchers at the Kunming Institute of Zoology stumbled upon the shrew's tolerance while trying to figure out what the animals like to eat. The shrews, not true shrews at all but relatives of primates, are more genetically similar to humans than other lab animals like mice. So the institute houses about 2,000 of the 10-inch-long mammals for research. As they presented foods to the shrews, they were stunned to find the animals preferred hot peppers, something a self-respecting rabbit or macaque would never eat. (Some other animals, like birds, don’t have capsaicinoid receptors, so they can munch peppers all they want.)

To understand the phenomenon, Chinese researchers collected five wild tree shrews and six wild mice to serve as controls. They fed the animals corn pellets spiked with capsaicin. Predictably, the shrews loved the spicy noms while the mice turned away. The researchers also collected bunches of Piper boehmeriaefolium from a local botanical garden. After synthesizing the capsaicin produced by the plant, they injected it into the animals. They then watched how often the animals licked the site, since licking is a response to pain, finding that mice licked the spot more often, a sign that they were irritated by the capsaicin. The shrews hardly licked the spots at all. After that they euthanized the animals to analyze their brains.

Unlike human pepper-heads, who enjoy the tingling on their lips, the slowly building heat and a rush of endorphins that comes from eating hot food, the shrews simply don’t feel the burn much, if at all. That’s because, the study in the journal PLoS Biology reveals, they have a mutation of the TRPV1 ion channel, also known as the capsaicin receptor. In other mammals, including the control mice, the receptor activates in the presence of capsaicin, causing pain and burning sensations.

While the amount of TRPV1 ion channels in the shrews and mice were the same, a single amino acid was missing in shrew receptors, making it difficult for the capsaicin to bind to pain receptors, giving the shrews their chili-chomping superpower.

It’s not difficult to see why this mutation would be beneficial—if other mammals avoid hot plants, the shrew could have an entire super-spicy ecological niche to itself. That’s what the researchers think too. “We propose that this mutation is an evolutionary adaptation that enabled the tree shrew to acquire tolerance for capsaicinoids, thus widening the range of its diet for better survival,” lead author Yalan Han says in the press release.

In fact, Nathaniel Dominiy, a Dartmouth evolutionary biologist not involved in the study, tells Kat Eschner at Popular Science that the shrew and the peppery plant may have coevolved, with the shrews getting the plant to themselves while the plant gets its seeds dispersed by the shrew. Dominiy says the shrews are “living fossils” that haven’t changed much in millions of years, which is plenty of time to develop an “incredibly exquisite relationships with...plants.”

Eating chilis seems to make a lot of sense for tree shrews. So why do people seek out ghost peppers and other tongue-numbing, esphogus-tearing, lung collapsing peppers? Gastro-psychologists think it’s because of something called "benign masochism," in which we seek out pain and negative stimuli, like the burning feeling of peppers, when we know there is no actual damage being done. In other words, as a species, we have psychological issues.

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