Cats are the ultimate groomers of the animal kingdom
Did you know that domestic cats can spend up to 24% of their waking time grooming? It is well known that cats are intensive groomers, from our feline friends at home to large cats like tigers or lions. Cats groom to remove loose fur and odors, redistribute protective oils, regulate body temperature, and even to bond with their fellow cat. Ungroomed fur can lead to tangles and matting, causing painful tugging of the skin or even infection. To aid in grooming, cats have small rigid spines on their tongue called filiform papillae. In this study, we investigate the role of the filiform papillae in grooming using micro computed tomography (CT) scanning, high-speed videography, and a lab-built grooming machine.
The history of cat tongues
Cats have existed for nearly 11 million years, with the first domestic cat Felis catus appearing around 10,000 years ago in Southwest Asia. The first study on cat papillae was conducted in 1982 by J. Boshel et al., where scanning electron micrographs were taken of the tongue surface (as shown to the left). In their study, they concluded that these papillae were conically shaped. In later studies by S.-i. Iwasaki and K. Ojima, this observation remained undisputed.
The 4 phases of cat grooming
Using high-speed videography, we filmed short hair domestic cats Felis catus grooming their own fur. We enticed the cats to groom by wetting their fur with water. In the video to the right, we put a small portion of baking flour on cat fur – qualitatively, you can see just how efficient the tongue is at grabbing and removing particulate. We found that a cat’s groom consisted of four phases: extension of the tongue, lateral expansion and stiffening of the tongue tissue, a sweep of the tongue through the fur, and finally retraction of the tongue in a U-shaped curl. During expansion, the papillae rotate until they are perpendicular to the tongue. This allows the papillae to stand erect to increase their contact area with fur. During grooming, the domestic cat’s tongue traveled a distance of 63 ± 20 mm, at a speed of 220 ± 9 mm/s, and at a frequency of 1.4 ± 0.6 licks per second.
We collect 6 different cat tongues, and find they all have rigid papillae
The image on the left shows the tongues of six species of cats which we collected post-mortem. These cats include the domestic cat Felis catus, bobcat Lynx rufus, cougar Puma concolor, snow leopard Panthera uncia, tiger Panthera tigris, and lion Panthera leo. A previously reported phylogenetic tree shows that these cat species are distantly related. All 6 tongues were found to have sharp, rigid papillae on the tissue surface. These tongues were generously donated from Zoo Atlanta, Carter Taxidermy, and UT College of Veterinary Medicine in conjunction with Tiger Haven.
CT scans reveal two regions of papillae on the tongue
We insert a domestic cat tongue, collected post-mortem, into a micro computed tomography (CT) scanner. A CT scanner produces many cross-sectional images (or “slices”) of an object using an X-ray, and combines them to create a 3D image, as shown in the two images on the right. We find that the front half of the tongue (from tongue tip to midsection) contains large filiform papillae composed of rigid keratin. The back half of the tongue (from midsection to throat) contains smaller, more dense filiform papillae that are soft. From the high-speed videos, we find that only the front half of the tongue is used during grooming. In our study, we refer to the “front half” papillae as our grooming papillae.
Unique cavity shape accidentally discovered in rigid filiform papillae
The discovery of the U-shaped cavity present in a grooming papilla was actually accidental. In our quest to understand why cats have rigid papillae on their tongues, we explored the possibility that these papillae are used to help rip meat off of bones. In the video to the left, we show a cat tongue being pulled through a store-bought piece of pork. The meat is easily ripped to shreds by the papillae. As we washed the tongue off in the sink, we noticed that the papillae were dyed red. Using tweezers, we pulled a single papilla off and stuck it under a microscope, shown below (the yellow is UV dye left over from a previous test):
A chunk of meat had been lodged into the tips of these papillae. This was contradictory to previous literature, which stated these filiform papillae were conical, not hollow. Using a macro lens on a Canon 1D, and some food coloring for visualization, we found that these grooming papillae had U-shaped cavities, as shown below.
We generated a 3D image of a domestic cat papilla using the micro CT scanner, and found two cavities – the top cavity at the tip, and a bottom cavity used for securing the papilla to the tongue tissue.
Size and shape of grooming papillae is constant across species
We took a grooming papilla from the center of each cat tongue, and generated a 3D image using the CT scanner. Despite the six species of cats spanning over 30-fold in body weight, their grooming papillae have a constant height of 2.3 ± 0.2 mm, as shown below.
The hollow cavity wicks up fluid like a straw
We find that a single papilla wicks up fluid into its cavity using surface tension, like a straw. A cat papillae can hold 0.014 μL of fluid, for a total of 4.1 μL across 290 papillae, or a tenth of an eyedropper drop. While this is not a large volume, papillae penetration into fur allows saliva to reach areas that the tongue surface cannot.
When a single cat hair is dragged through the wetted papilla cavity, it pulls on the fluid, causing the hair to become coated, as shown below.
Measuring fur properties
The cat fur coat has two layers, the topcoat and the undercoat. The topcoat consists of thick guard hairs, which are used to protect the undercoat from the environment. Although hidden from sight, the undercoat primarily consists of thin down hairs, which can outnumber guard hairs 24 to 1, and are used for thermoregulation.
We measured the dimensions of down fur for 6 cat species (cheetah Acinonyx jubatus, caracal Caracal caracal, Caucasian wildcat Felis sylvestris caucasica, leopard Panthera pardus, snow leopard Panthera uncia, and tiger Panthera tigris) at the Museum of Comparative Zoology at Harvard University. We further measured fur samples of 3 other species (bobcat Lynx rufus, cougar Puma concolor, and Eurasian lynx Lynx lynx) from furs procured from Promise Land Tannery. We measure the diameter and length of down hairs, and gather fur density values from literature.
Papillae tip can penetrate through fur to reach skin, for most cats
A cats fur is composed mostly of air, with the fraction of air vs total volume reaching 0.97 for the domestic cat, 0.98 for a tiger, and 0.99 for a snow leopard. Like a down jacket, this large air fraction provides the cat excellent insulation from the elements. As the cat tongue presses down on fur, air evacuates and hairs compress together, as shown in image B on the left. Just how far can hair compress down? We can answer this question by considering the geometry of the hairs. If hairs are compressed to an ideal hexagonal lattice, there will be a minimum height to which the fur can compress to, hfur. If this height is greater than the height of a papilla hpapillae (when fully erect), then the papillae will never be able to reach the skin to distribute saliva. Using our measured fur values, we find that most cats have no problem with their papillae reaching skin – we term these cats “groomable”, as shown in image C on the left. There is one exception, we find Persian cats are “ungroomable”, where their papillae are just not long enough to reach through their long fur to the skin layer. Long-haired domestic breeds, such as Persian domestic cats, are notorious for their matted fur if not cared for properly. According to VCA animal hospitals, owners of Persian cats should comb their cat daily and give it baths monthly to redistribute the hair’s natural oils.
Wetted hair clumps together, preventing fluid from spreading
When fluid is introduced to dry fur, surface tension causes hairs to clump together, as shown in the image on the left. If the cat tongue were completely smooth, as shown in the schematic on the left, this clumping effect would cause fluid to wet only the top layer of fur, preventing fluid from seeping to hair layers close to the skin. In our study, we show that theoretically saliva on a smooth tongue would only penetrate to a 0.54 mm depth in fur. Although the papillae only hold up to 5% of the tongue’s saliva volume, the height of the papillae can penetrate past this clumped hair layer and spread saliva to the dry hair underneath.