What Does a Study of Slow Lorises Actually Say About Cat Allergies?

Earlier this month, a team of researchers published a paper proposing an unusual theory about the origin of cat allergies. Felines, they wrote, evolved to manufacture and exude a protein called Fel d 1—the bothersome culprit that tickles and tortures the airways of millions of people worldwide—as a defense mechanism to deter their enemies.

However, the researchers behind the paper, published in the journal Toxins, didn’t actually study cats or humans to generate their hypothesis, instead focusing on a venomous primate called a slow loris. After finding remarkable similarities between the proteins found in slow loris venom and cat dander, the team suggested that both species might deploy the molecules as a form of protection—with unfortunate consequences for those that cross their paths.

For now, their hypothesis is just that: a hypothesis—one that has yet to be rigorously tested, as Wayne Thomas, an allergy expert at the University of Western Australia who wasn’t involved in the study, tells Natalie Parletta at the Guardian. Until that happens, some skepticism is warranted.

But even if the team’s theory doesn’t pan out, there’s some interesting science to explore around the slow loris, a fascinating but woefully understudied creature, study author Bryan Fry, a biologist at the University of Queensland, says in a statement.

When threatened, these wide-eyed critters will hiss and clasp their paws atop their heads, undulate like snakes, and suck up a mouthful of venom from glands in their armpits. Brimming with noxious liquid, a well-targeted nip from a riled-up loris can be enough to trigger anaphylactic shock—and in at least one case, death—in humans, not unlike a very severe allergic reaction. (Normally, slow lorises mostly reserve their venom for fights with other slow lorises.)

To home in on what gives these peculiar primates their baneful bite, Fry and his colleagues collected the underarm secretions from a handful of wild slow lorises housed at a research station in Indonesia and isolated the proteins within. When the researchers ran the amino acid sequences of the molecules through a database, they discovered the closest match for the toxic loris protein was Fel d 1, a molecule found in cat skin glands and saliva that often gets wicked into fur.

Small, light and sticky, Fel d 1 is a particularly potent allergen, capable of spreading through air and glomming onto most surfaces, where it can wait for up to months at a time for a hapless human to pass by and breathe it in, reports Lindsey Konkel at Live Science. Researchers still don’t know its original function, though some have proposed that the well-traveled protein may play roles in hormone transport. Nevertheless, its side gig of irritating human airways has been a lucrative one.

“The fact that so many people are allergic to cats mightn’t be a coincidence,” Fry says in the statement. “Our theory is that since this protein is being used as a defensive weapon in slow lorises, it makes sense that cats may be using [an] allergen as a defensive weapon, too.”

Speaking with the Guardian, Thomas advises caution, pointing out that plenty of nature’s proteins resemble well-known allergens like Fel d 1, which has never been shown to have a toxic function. Sometimes these similarities can truly be chalked up to coincidence.

On the whole, the evolution of allergies remains a topic of fervent discussion among scientists. Though some of these reactions may have come about as a way to protect humans from natural toxins, not all the molecules that raise the body’s alarm bells are necessarily nefarious by nature. Many researchers support the idea that these overzealous and inappropriate reactions are an unfortunate byproduct of excessive hygiene in the Western world, which has purged urbanites of beneficial microbes necessary to calibrate the immune system to ignore harmless substances—including, perhaps, pet dander.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.