If you share a home with a cat, you already know two things: they do whatever they want, and they’re annoyingly good at it. What you probably didn’t know is that every time your cat pees outside the litter box or marks a corner of the yard, they’re leaving behind a chemically encoded message that other cats can read like a name tag. Scientists just figured out how.
A study published August 19 in Current Biology identified the compounds behind this system—a group of chemicals called branched-chain fatty acids, or BFAs, that give each cat a unique scent fingerprint in their urine. The signatures hold up even after the cat is long gone, and for an animal running its entire social life through scent, that’s the point. Cats spray to claim territory, advertise for mates, and flag their presence to rivals. A message that evaporates in an hour is useless. These don’t.
The more interesting question was how. Urine composition changes with diet, health, and age. How does a cat’s chemical ID card stay consistent through all of that? The answer turned out to be hiding in the kidneys—specifically in tiny fat droplets in the renal cortex that researchers have known about for over a century without understanding what they do. The new study found they function as reservoirs, maintaining each cat’s individual BFA profile regardless of what else is going on biologically. Some of the BFAs identified had never been reported in other mammalian excretions or secretions.
The system, essentially, is purpose-built for stability.
Scientists Say Cat Pee Works Like a Scent-Based ID Card
To confirm cats were picking up on those signatures, researchers measured the flehmen response — that wide-mouthed, briefly horrified expression that makes cats look like they’re personally offended. The behavior pulls odor compounds into the vomeronasal organ, a scent-processing structure that handles pheromones and other chemical signals. Cats showed the response less when repeatedly exposed to the same urine sample, but dialed it back up when a new cat’s sample appeared. They were reading individual IDs, not just detecting urine generically.
Study co-author Masao Miyazaki, a biomolecular scientist at Iwate University in Japan, told Live Science he didn’t expect an instinctive behavior to be so responsive to familiarity and learning. Cornell neurobiologist Michael Sheehan, who wasn’t involved in the research, compared the BFA system to a human face—the primary signal animals use to tell each other apart, even when subtler cues are available.
Lions and tigers use the same system, with their own kidney droplets and BFA profiles, though the specific compounds differ by species. The research team now wants to understand whether disruptions to the droplet system contribute to chronic kidney disease in cats—which would make this line of inquiry much more medically urgent than they thought.
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