L181 The microscopic structure of a cat’s tongue helps keep its fur clean

T.S. eliot's mystery cat, Macavity, besides being a criminal mastermind able to evade the combined ranks of British law enforcement, had a coat that was “dusty from neglect”. Criminality is one thing, but this truly strains the imagination. Real cats are champion groomers.

Of the ten hours a day that a domestic cat deigns to remain awake, it spends a quarter licking dirt, fleas, blood and loose hairs from its fur. Cats’ tongues, specialised for this task, are covered in hundreds of backward-facing keratin spines. But exactly how these cone-shaped protuberances 突起, called filiform papillae, work to give the animals such mastery over their cleanliness has remained unknown until now.

To crack the mystery Alexis Noel and David Hu, a pair of engineers at the Georgia Institute of Technology, in Atlanta, examined the grooming mechanisms of six feline species—from domestic pets and bobcats to snow leopards and lions. Studying the activity of tongues inside the mouths of living creatures proved tricky, so instead Dr Noel and Dr Hu built an automated grooming machine fitted out with tongues and furs from animals whose lives had ended at places such as the Tiger Haven in Tennessee, a sort of retirement home for rescued big cats. They attached the tongues to a mechanical arm and made them “lick” the furs. High-resolution cameras and scanners took pictures.

The two researchers found that the filiform 丝状 papillae were shaped not, as had previously been thought, like solid cones. Rather, they resembled tiny scoops. Each had a small groove—named a cavo papilla by the team—at its tip.

This structure permits surface tension to wick saliva from a cat’s mouth and release it into the farthest recesses of the animal’s fur. During each lick, about half of the saliva on the tongue is so transferred. Saliva serves as a multi-purpose cleaning agent and the cavo papillae also assist the absorption, for the return journey, of any dirt or blood that needs removing. The cat’s tongue therefore “acts like a loofah and a sponge at the same time”, says Dr Hu.

The pair’s findings, just published in the Proceedings of the National Academy of Sciences, could inspire new ways to clean complex hairy surfaces. The authors themselves demonstrated one such application, which they call the tongue-inspired grooming brush. To make this they employed 3D printing to create structures, shaped like cat papillae, attached to a silicone base. The TIGR brush pulled on cat hairs and fur with less force than existing brushes, and was easier to clean. Such a brush could also be used to spread medicines deep into a cat’s fur or onto its skin, without the usual distressing practice of having to shave the animal first.