Scientists Find a Chemical 'Calling Card' in Cat Urine

Scent marks are meant to identify the animal that left them, but odor molecules evaporate, break down and change after release. A team from Japan, Germany and Spain, led by Iwate University, reports a possible answer in domestic cats in the journal Current Biology.

Cats can tell individual urine samples apart: they sniffed a repeated sample less each time, but grew interested again when a different cat's urine appeared, and still responded less to already-encountered urine months later. Cats also performed the flehmen response, the open-mouthed expression used to investigate scents, more often with unfamiliar urine than their own. Using that response as a clue, the team traced the effect to 13 branched-chain fatty acids (BFAs) in urine's lipid fraction, not previously reported in mammalian excretions or secretions.

Each cat carried its own combination and relative amounts of the 13 compounds, stable for that individual across dates though profiles differed between cats. The BFAs evaporated more slowly than other urine odor molecules; stored at 25°C, individual patterns stayed stable for at least 24 hours. When researchers altered only the BFA fraction, cats accustomed to the original sample sniffed more at the altered one, confirming they could detect the change.

BFAs turned up in kidney tissue but not in the other tissues examined, stored among neutral lipids inside droplets in the renal cortex. Cats have been known to carry abundant kidney lipid droplets for more than a century without a known purpose. "Our findings suggest that one of their functions may be to support a stable chemical signature in urine," said Professor Masao Miyazaki of Iwate University, who led the study.

The researchers also detected BFA-related compounds in urine and kidney lipid droplets of lions, tigers, leopards, jaguars, lynxes and the Iriomote cat, with profiles varying by species and between the Iriomote cat and Japan's Tsushima leopard cat. Whether wild cats use BFAs to recognize individuals has not yet been determined.