So how could this new cell elude scientists and doctors for so long? In a way, it didn’t. Plikus and his graduate student scoured centuries of scientific papers for any lost trace of fatty cartilage. They found a clue in a German book from 1854 by Franz Leydig, a contemporary of Charles Darwin. “Anything and everything that he could stick under the microscope, he did,” Plikus says. Leydig’s book described fat-like cells in a sample of cartilage from rat ears. But 19th-century tools couldn’t expand beyond that observation, and, realizing that a more accurate census of skeletal tissue might be valuable for medicine, Plikus resolved to crack the case.

His team began their investigation by looking at the cartilage that’s sandwiched between thin layers of mouse ear skin. A green dye that preferentially stains fatty molecules revealed a network of squishy blobs. They isolated these lipid-filled cells and analyzed their contents. All of your cells contain the same library of genes, but those genes aren’t always activated. Which genes did these cells express? What proteins slush around inside? That data revealed that lipochondrocytes actually look very different, molecularly, from fat cells.

They next questioned how lipochondrocytes behave. Fat cells have an unmistakable function in the body: storing energy. When your body stores up energy, cellular stores of lipids swell; when your body burns fat, the cells shrink. Lipochondrocytes, it turned out, do no such thing. The researchers studied ears of mice put on high-fat versus calorie-restricted diets. Despite rapidly gaining or losing weight, the lipochondrocytes in the ears didn’t change.

“That immediately suggested they must have a completely different role that has nothing to do with metabolism,” Plikus says. “It has to be structural.”

Lipochondrocytes are like balloons filled with vegetable oil. They’re soft and amorphous but still resist compression. This contributes meaningfully to the structural properties of cartilage. Based on data from rodents, the tensile strength, resilience, and stiffness of cartilage increased 77 to 360 percent when comparing cartilage tissue with and without lipochondrocytes—suggesting that these cells make cartilage more pliable.

And the structural gifts appear to benefit all sorts of species. In the outer ear of Pallas’s long-tongued bat, for example, lipocartilage underlies a series ...