If you’ve ever lived with a cat, you’ve no doubt slipped into a peaceful trance by watching it meticulously clean, comb, and rearrange its fur using its sandpaper-like tongue. And if said cat is fond of you, there’s also a pretty good chance that you’ve been on the receiving end of such grooming services.
Has this experience ever led you to wonder about the unexplored dynamics of the feline tongue? No? Well, perhaps you need to be a biomechanical engineer focused on soft tissue adhesion in the animal kingdom, like Georgia Tech's Alexis Noel, for such curiosity to strike.
After examining the tongues from six species of cat, Noel and her colleague, professor David Hu, have discovered how long-overlooked structural properties enable the rough appendage to fulfill the important task of coat maintenance.
“The cat tongue is most recognized for its hundreds of sharp, backward-facing keratin spines called filiform papillae,” the duo wrote in their paper, published in PNAS. “A 1982 study concluded that a cat papilla has the shape of a solid cone, an observation that remained undisputed for two decades. In our study, we show that the papilla is in fact scoop-shaped, enabling it to use surface tension forces to wick saliva.”
According to Noel and Hu’s first wave of analysis – which involved micro-CT scanning of tongue tissue from a deceased domestic cat, bobcat, puma, snow leopard, tiger, and lion – many if not all types of felines have scooped tongue papillae, and the total length of these spines corresponds to the length of the downy hairs that make up each species’ undercoat. This layer of fuzz evolved to keep cats toasty by holding body-heat warmed air close to the skin. Yet because cats can only sweat from their paw pads, cats are believed to manage excess heat through evaporative cooling of licked-wet fur.
Through a highly unique series of experiments involving food coloring, domestic cat and tiger tongues, fur samples, and a machine designed to mimic licking, the duo then showed that papillae readily wick saliva produced in the mouth, via capillary action, then efficiently redistribute it over the undercoat hairs during grooming.
“The saliva deposited contains enzymes that can dissolve blood and other contaminants. Moreover, as the saliva evaporates, it directly cools the skin,” they wrote. “While the saliva in the papillae cavities only accounts for 5% of the total fluid on the top of the tongue, saliva deposition near the cat’s skin can provide up to 25% of the needed cooling for a cat’s thermoregulation.”
Without the hollow papillae, the authors add, saliva on the tongue’s surface would only wet the top layer of fur, leaving hairs underneath untouched. The flexible base of the papillae, which lay flat against the cat’s tongue until activated by the flick of a special muscle, allows them to penetrate thick and tangled fur and prevents them from being ripped out when the cat tries to remove hairs stuck to its tongue.
Using their CT structural findings as a template, Noel and Hu 3D-printed a scaled-up version of the papillae in silicone and formed them into a brush. The cutely named (TIGR) tongue-inspired grooming brush combed with less force and was easier to clean than a standard human hairbrush during tests on synthetic nylon fur.
Moving beyond cosmetic applications, Noel believes that insights from the cat tongue "could help us apply fluids, or clean carpets, or apply medicine," she told the Associated Press.