On an average, a cat spends nearly 2.5 hours (one-fourth of its waking hours) licking its fur every day. Ever wondered why would an animal spend so much time on an activity such as this? (Photo: WikiCommons, Creative Commons CC BY)

What was that funny, cute and awwwww-inspiring video of a cat that you last saw on the internet? Was it on one of the thousand Facebook pages, a WhatsApp forward, Instagram or on YouTube? Was it a cute kitty amazed by a laser beam on a wall; one trying to catch a fish floating on the screen of a smartphone or was it that beauty basking luxuriously in the sun, licking its fur to spotless brilliance?

Whatever you were watching, or even if you aren't a 'cat person', you surely must have observed that cats spend much of their time (actually nearly 25 per cent of their waking hours) licking themselves. It is estimated that on an average, a cat spends nearly 2.5 hours licking its fur every day.

Ever wondered why would an animal spend nearly one-fourth of its waking time on an activity such as this? And what on earth has the grooming and cleaning industry got to do with this?

Cats like to remain clean, that's true. But there is more to their infatuation for licking. Researchers say this also helps them remove excess heat and regulate body temperature. And now, armed with studies on cats (actually their multifarious tongues and the dynamics of their licks), bio-engineers are planning to devise devices that may make grooming a hell lot easier for humans and animals alike.

So, how does this happen? How are cats able to keep their fur spotless clean but dogs can't? What can we learn from their tongues?

To answer this, Alexis C Noel and David L Hu, bio-engineers at the Georgia Institute of Technology, Atlanta (US), conducted a study and analysed tongue pattern of six different cat species - domestic cat, bobcat, cougar, snow leopard, tiger and lion (see photo below).

The study was published in the Proceedings of the National Academy of Sciences, one of US's most reputed science journals, on December 4, 2018.

Tongues of six cat species--domestic cat, bobcat, cougar, snow leopard, tiger and loin--were used by researchers from the Georgia Institute of Technology for this study. (Photo: Alexis Noel/GIT)

The team found that cats have thousands of hollow rigid spine-like structures on their tongue which help them wick saliva (moisture) deep into the fur and clean it. These spines are called filiform papillae (see photo below). Besides helping the tongue deliver saliva into the fur, papillae also act as a brush that helps in untangling and removing broken hairs from the skin.

A microscopic image of papillae in cat tongue. (Photo: Alexis Noel/Georgia Institute of Technology)

Researchers say findings of the study will help the grooming and cleaning industry come up with more efficient products that rely upon the natural cleansing ability of a cat's tongue. A direct use can be in the development of better-quality brushes.

What did the scientists do

To study the cleansing ability of a cat's tongue and the role of papillae, the team collected post-mortem tongues of six cat species - domestic cat, bobcat, cougar, snow leopard, tiger and lion.

It then created 3D images of each tongue and observed the structure of papillae in them. Surprisingly, the size and structure of papillae in all six tongues was more or less the same (despite the size variation between a domestic cat, a tiger and a lion).

To study the dynamics of cat grooming, the team used sophisticated cameras capable of high-speed videography and recorded the tongue movement of a domestic cat when it was licking her fur. When the footage was played in slow-motion, the team saw the papillae in action.

It was observed that there are four phases in which a cat grooms itself -- 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 (see photo below).

The four phases that occur when a cat licks her fur. (Photo: Alexis Noel/Georgia Institute of Technology)

"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," Dr Alexis Noel writes about her study.

Explaining it further, she says the 3D printed images (see below) show that a cat's tongue has two regions of papillae. In the first region (from the tip to the middle part) the papillae are long. In the second half of the tongue (from middle to the throat) the papillae are shorter, softer but denser. "From the high-speed videos, we find that only the front half of the tongue is used during grooming," she writes.

3D images of a cat tongue that was used to study papillae. (Photo: Alexis Noel/Georgia Institute of Technology)

In the slow-motion video (see below), it was found that when the cat's tongue contacts the fur, the rear-facing papillae become perpendicular to the surface so as to allow maximum transfer of saliva into the fur.

See: How spines in a cat's tongue (papillae) flare out when it licks its fur. (Video courtesy: PNAS) pic.twitter.com/AvPY3GHylP Mukesh Rawat (@mukeshrawat705) December 18, 2018



Once the saliva has been transferred, the forward motion of the tongue (while licking) has a combing effect that untangles locked hair and cleans broken ones. So, when a cat licks her skin, the tongue bends outward and most papillae are as perpendicular to the skin as possible (see photo below).

The papillae on the tongue of a cat become perpendicular while making contact with the fur so that maximum saliva can be released. (Photo: Alexis Noel/Georgia Institute of Technology)

"Slow-motion footage of several housecats grooming revealed the felines flared their tongues outward as if taking a big lick of an ice cream cone so the papillae stood perpendicular as they move through the fur," Science said in a report on the study.

Besides cleaning the skin, the papillae transfer moisture (in the form of saliva) to the fur and the skin. This helps cats remove excess heat and regulate body temperature during summer and also distribute essential body oil across the fur (now you know why it's so shiny!). The papillae in also helps cats, tigers, lions and others in ripping off meat.

For better observation and results of the papillae in action, the researchers measured the force and speed used by a domestic cat when it licks her skin.

Once the force and speed were measured, an artificial brush (see below) designed using tongue of a dead cat was used for the experiment. It was brushed over a cat skin with the same force and speed to the see the amount of moisture that is deposited on the fur.

A brush prepared using tongue of a dead cat to study how papillae function. (Photo: Alexis Noel/Georgia Institute of Technology)

"These experiments revealed that U-shaped hollows at the tips of papillae wick saliva from the mouth, each wicking action capturing up to 4.1 µL of saliva," the study said, adding that this amount is nearly the same as one-tenth of the drop of a normal eyedropper.

"Each lick of the tongue deposits nearly 50 per cent of the fluid on the tongue onto fur and can deliver a substantive fraction of the cooling effect required for regulating body temperature," it said.

As far as the quality and ease of grooming is concerned, it is directly dependent on whether the papillae can penetrate the fur enough to reach the skin. So, if a cat (or any other member of the cat species) has outgrown the fur to such an extent that the thickness of the fur is greater than the length of the papillae, it won't be able to clean and groom itself on its own.

An example of this is the Persian domestic cats that are known for their thick fur and matted hair. Ask anyone who has a Persian cat at home how arduous a task it is to groom them and prevent their fur from matting.

TIGR brush: How grooming & cleaning industry can benefit

Explaining the significance of the study's findings, the researchers say that when it comes to the multibillion-dollar grooming and cleaning industry, the focus has so far mostly been on the development of better cleaning fluids and less on developing more effective brushes.

The study says that structure and functions of the papillae can help in "development of soft robotics and biologically inspired technologies" that can help in sorting and cleaning furry surfaces, among others.

In fact, the researchers have themselves developed an artificial brush inspired from the papillae on the cat tongue. The cat-inspired grooming (TIGR) brush incorporates the 3D-printed cat papillae into a silicone substrate.

"The unique shape of the cat's papillae may inspire ways to clean complex hairy surfaces. We demonstrate one such application with the tongue-inspired grooming (TIGR) brush (see video below), which incorporates 3D-printed cat papillae into a silicone substrate. The TIGR brush experiences lower grooming forces than a normal hairbrush and is easier to clean," the researchers said in the study.

Watch: A bio-inspired 3D printed TIGER brush that is designed after studying how a cat cleans her fur. (Video courtesy: PNAS) pic.twitter.com/oRuZ7QcqEi Mukesh Rawat (@mukeshrawat705) December 18, 2018



This aside, the learning outcomes can also help in treatment of cat-skin-related diseases.

Researchers hope that they can develop artificial brush that will act like papillae and help vets deposit medicines directly into the skin of cats or other furry animals.

It seems we should all brace up for times when bio-inspired brushes will flood our homes, for humans and animals alike.

Till then, watch that cute kitty, sitting on a cosy chair, licking herself under a bright sun on a lovely Sunday. Share her photos and videos for mutual awwwwww.

(You can also listen to the brains behind this research in this podcast by the National Academy of Sciences, USA)

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