Secret 'messages' in cat pee let felines identify each other — and scientists just cracked the code

Cats can identify each other based on the scent of their pee, and researchers have just decoded the secret "messages" that make that possible.

A group of chemicals called branched-chain fatty acids (BFAs) help give cats their unique scent marks, a new study found. Unusual fat droplets in the cats' kidneys likely help them maintain that chemical fingerprint in each mark. The researchers reported their findings Aug.ust 19 in the journal Current Biology.

While most animals use a combination of visual, audible and scent-related cues to recognize each other, some also rely on specific chemical signatures. A number of insects, for example, use certain hydrocarbons to determine whether other insects are part of the same colony, and mice excrete urinary proteins that provide identifying information about the individual that left them. But not much is known about how other mammals leave unique scent marks that remain stable over time.

"The way I think of the mouse proteins is kind of equivalent to a human face," said Michael Sheehan, a neurobiologist at Cornell University who was not involved in the study. While a person might recognize someone they know well from less obvious signals such as their hairstyle or the way they walk, people typically recognize others by their faces. "My hunch is probably these branched-chain fatty acids from the cat urine [are] the same deal," Sheehan told Live Science.

In the new study, the researchers investigated domestic cats' flehmen response ‪—‬ an odd, open-mouthed expression many cat owners will recognize. The flehmen response is an innate behavior that draws certain chemicals into a cat's vomeronasal organ when it is investigating interesting odors. The vomeronasal organ helps cats detect pheromones and certain other chemicals in their environment.

The researchers found that cats showed the flehmen response less often, on average, the more they encountered the same urine sample from another cat, but the response increased when urine from a new cat was introduced.

"I found it surprising that an innate behavior could be so strongly influenced by familiarity and learning, rather than being a simple fixed response to a single pheromone," study co-author Masao Miyazaki, a biomolecular scientist at Iwate University in Japan, told Live Science in an email.

The team then used a technique known as high-performance liquid chromatography to split a sample of cat urine into its components, and then showed the cats each part of this sample. Cats consistently showed the flehmen response when they smelled a portion that contained a handful of branched-chain fatty acids.

Each cat had a unique mix of BCFAs in its urine, and these BFAs remained stable for at least a day after the cat left a scent mark, the scientists found. This suggested BFAs could leave information about the cat that made the mark, even after the rest of the urine had evaporated.

The team found BCFAs in lipid (fat) droplets in the renal cortex, the outer part of the kidney. Researchers have known for more than a century that these droplets exist in cat kidneys, but their purpose was unclear. The droplets could act as reservoirs that keep a cat's individual BCFA profile constant even after dietary or health changes, the scientists proposed in the study.

Large cats, including lions and tigers, also have BFAs and lipid droplets in their renal cortices. The specific BCFAs and the number of droplets differed among species, however, which suggests that these features may have diversified as cats evolved.

"We now want to understand how these lipid droplets are formed, maintained, mobilized and linked to secretion into urine," Miyazaki said. "We are also very interested in whether disruption of these processes contributes to chronic kidney disease in cats."

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