Scientists have discovered a unique chemical signature in cat urine that could help cats recognize each other even after a scent mark has been left behind for some time. A study led by researchers from Iwate University in Japan, along with collaborators from Germany and Spain, identified 13 unusual branched-chain fatty acids (BFAs) in cat urine. These compounds create a stable chemical profile that differs between individual cats, suggesting they may function like a durable "calling card" for scent-based identification. In the study, researchers found that cats can tell the difference between urine samples from different individuals. Cats showed less interest in urine samples they had encountered before, but their curiosity increased when exposed to samples from unfamiliar cats. This behavior remained consistent even after months, suggesting that cats might remember specific urine scents over a long period. The flehmen response, a behavior where cats open their mouths to better detect odors, was also more common when they encountered unfamiliar urine samples. The study found that the BFA profiles in cat urine remained stable even after being stored at 25°C for at least 24 hours. This is in contrast to many other volatile chemicals in urine, which change quickly after being deposited. Researchers also discovered that BFAs are present in the renal cortex of cats, stored in lipid droplets that have puzzled scientists for over 100 years. These droplets may act as a reservoir for BFAs, helping to maintain a consistent chemical signature in urine, even as a cat's diet or health changes. The study also found similar BFA-related compounds in the urine and kidney lipid droplets of several other big cat species, including lions, tigers, leopards, jaguars, lynxes, and the Iriomote cat. However, the specific BFA profiles and the distribution of lipid droplets varied across species. This suggests that the use of BFAs for scent marking and the related kidney structures may be common among cats, having evolved and diversified over time. The findings raise questions about how wild cats use these compounds to recognize each other and highlight a broader challenge in animal communication: how scent marks can provide stable information even as the chemicals in them change over time. While mice use proteins to preserve information in their urine, cats may rely on these semi-volatile lipid-based molecules, supported by a reservoir in the kidneys. The study's results could have future applications in managing cat urine odor, understanding lipid-related health issues, and using urine analysis for non-invasive wildlife monitoring. The research was published in the journal Current Biology.