Many odor molecules evaporate, degrade, or change over time. That creates a fundamental question in animal communication: if scent marks are constantly changing, how can animals determine who left them?

Researchers from Japan, Germany, and Spain, led by scientists at Iwate University, may have found part of the answer in domestic cats. Their experiments identified 13 branched-chain fatty acids, or BFAs, that may act as a persistent chemical signature in cat urine.

The specific mixture and proportions of these compounds varied from cat to cat, but remained relatively consistent within the same animal. Behavioral tests also showed that cats could detect differences between BFA profiles when researchers controlled for other lipids in the urine.

The findings suggest that BFA profiles may function as durable chemical “calling cards,” helping cats recognize the identity of individual animals. The study was published in Current Biology.

Cats can remember the smell of individual urine

Before investigating the chemicals involved, the researchers first needed to establish whether cats could distinguish between the urine of different individuals.

When cats encountered the same urine sample repeatedly, they gradually spent less time investigating it. When urine from another cat was introduced, however, their interest increased and they spent more time sniffing.

Remarkably, the cats continued to show a reduced response to previously encountered urine odors even after several months. This pattern suggests that cats may retain long-term memories of specific urine smells.

The researchers also studied the Flehmen response, a characteristic open-mouthed expression that cats often display while investigating certain odors. The cats showed this reaction more frequently when they smelled unfamiliar urine than when they smelled their own.

When the same urine sample was presented several times, Flehmen responses declined. When urine from another cat was introduced, the response increased again.

Professor Masao Miyazaki of Iwate University, who led the research project, said, “After confirming that cats can distinguish between individual urine odors, we used the Flehmen reaction as a clue to identify molecules in urine that may contribute to the recognition of individual odors.”

Thirteen fatty acids create a distinctive cat urine profile

Using the cats’ behavior as a guide, the scientists focused on the lipid fraction of urine that contained unusual BFAs.

They ultimately identified 13 of these compounds. After reviewing existing scientific literature, the researchers found no previous reports of the same BFA compounds occurring in mammalian excreta or secretions.

What stood out was not simply the presence of the compounds, but the patterns they formed. Each cat had a BFA profile made up of different fatty acid combinations and relative abundances.

These profiles varied widely between animals but remained relatively stable when samples were taken from the same cat on different days.

Genetics may also play a role. Closely related cats generally had more similar BFA patterns, but each animal still maintained a distinguishable profile, even within the same family.

The compounds also appeared to be relatively durable. Many volatile chemicals responsible for urine odor begin to change rapidly after urine is deposited. BFA compounds are semi-volatile and evaporate more slowly.

In urine-soaked samples stored at 25°C, the characteristic BFA profiles associated with individual cats remained relatively stable for at least 24 hours.

Behavioral tests show that cats detect BFA differences

The researchers then tested whether cats could actually distinguish between different BFA patterns.

They controlled for other lipid components in the urine samples and changed only the donor-derived fraction containing BFAs. Cats that had become accustomed to the original sample began sniffing again when the BFA fraction was switched.

This behavioral change provided evidence that cats can recognize differences between individual BFA compositions.

The result strengthens the idea that these fatty acids are not merely unusual chemical byproducts. Instead, they may carry meaningful information about an animal’s identity.

Cat kidney lipid droplets may help preserve scent identity

The investigation also produced unexpected clues about the kidneys.

Researchers detected BFAs in the kidneys but not in the other tissues they tested. BFA-containing lipids were also found among the neutral lipids stored in droplets in the renal cortex.

These kidney lipid droplets have puzzled scientists for more than a century. Cats are known to have large numbers of them, but their biological purpose remains unknown.

The new findings raise the possibility that the droplets act as a reservoir for BFA-containing lipids.

Such stores may help keep a cat’s chemical signature relatively stable when diet or physiological condition changes temporarily. By buffering short-term fluctuations, the kidneys may help maintain a more consistent individual profile in the urine.

“Although the presence of lipid droplets in cat kidneys has been known for more than a century, it remained a mystery why cats have so many lipid droplets,” Professor Miyazaki said. “Our findings suggest that one of the functions of BFA may be to support a stable chemical signature in urine. How BFA stored in renal lipids is ultimately released into the urine is an important question for future research.”

Similar BFA chemistry appears across the cat family

The researchers also examined whether the same traits occur in other feline species.

BFA-related compounds in urine and kidney lipid droplets have been detected in several members of the cat family, including lions, tigers, leopards, jaguars, lynx, and Iriomote lynx.

However, the exact BFA profiles varied between species. The researchers also observed differences in the amount and distribution of lipid droplets within the kidneys.

Differences were found between the Iriomote wildcat and the Tsushima wildcat, a geographically isolated wildcat that lives in Japan.

Together, these observations suggest that BFA-related chemistry and kidney physiology are widespread across the cat family, while also changing and potentially diversifying during feline evolution.

It remains to be proven whether lions, tigers, and other wild cats actually use these compounds to recognize specific individuals.

How changing scent marks may preserve animal identity

The discovery may also help address a broader problem in animal communication.

Scent marks begin changing chemically as soon as they are deposited, yet they still need to convey useful information. Understanding how animals leave stable identity signals despite these chemical changes has long been a fundamental scientific challenge.

Mice provide one known solution. In mice, a major urinary protein helps store identity information in urine. Scientists have not established similar protein-based identification systems in many other mammals.

Cats may use a different strategy.

Instead of relying primarily on proteins, cats may produce unique combinations of semi-volatile, lipid-derived molecules. Because these compounds evaporate more slowly and may be supported by lipid stores in the kidneys, they could help preserve an individual’s chemical identity over time.

Potential applications of cat odor research

For now, this is basic research and is not expected to lead immediately to new products or technologies. Still, the findings suggest several possible future applications.

A better understanding of BFA chemistry could eventually contribute to new ways of managing urine odor in cats.

The kidney findings may also help researchers investigate why lipid accumulation is a normal physiological feature in some situations but is associated with disease in others.

The research could also have implications for wildlife conservation. If scientists can show that BFA profiles reliably identify the same animal across multiple urine samples, environmental urine could be monitored as a non-invasive way to track rare wild cats without capturing or directly observing them.

What began as an investigation into the chemistry of cat odor recognition could help explain both the century-old mystery of lipid droplets in cat kidneys and the broader question of how animals leave recognizable identities in their environments.