Cats leave a 13-compound calling card in urine, and the kidney may be the warehouse

An international team led by Iwate University, with collaborators in Germany and Spain, says it has found 13 branched-chain fatty acids (BFAs) in domestic cat urine whose combination and relative abundance vary sharply between individuals while staying comparatively stable within the same cat across different collection dates ^[1][2]. The result, published in Current Biology, matters because scent-mark research has been stuck on a physical problem: marks persist after the sender leaves, but odor molecules evaporate, degrade and change after deposition ^[3][4].

The route to the compounds was behavioral rather than analytical. The team first confirmed that cats habituate to a urine sample, sniffing it less on repeated presentation, then sniff more when urine from a different donor appears ^[5]. That reduced response was still detectable after intervals of months, which the researchers read as long-term memory for individual urine odors ^[6]. They then used the flehmen response, the open-mouthed grimace, as a readout: cats showed it more often toward unfamiliar urine than their own, and it declined with repetition and rose again with a new donor ^[7]. "After confirming that cats can distinguish individual urine odors, we used the flehmen response as a clue to identify urinary molecules that may contribute to individual scent recognition," said Masao Miyazaki of Iwate University, who led the project ^[8].

Two properties make the BFAs interesting as an analyte. The researchers report finding no prior reports of these same compounds in mammalian excretions or secretions in their literature survey ^[9]. And they are semi-volatile, evaporating more slowly than typical urinary odorants: in urine-soaked samples held at 25 C, individual-specific profiles stayed comparatively stable for at least 24 hours ^[10]. Related cats had more similar profiles, but individual differences persisted within families ^[11]. The causal test was a swap: with other urinary lipid components held constant and only the donor-derived BFA fraction changed, habituated cats increased sniffing ^[12].

Then the anatomy. BFAs turned up in the kidney and not in the other tissues examined, and BFA-containing lipids were found among neutral lipids inside lipid droplets in the renal cortex ^[13]. Those droplets have been described in cat kidneys for more than a century without an established function ^[14]. "Our findings suggest that one of their functions may be to support a stable chemical signature in urine," Miyazaki said ^[15]. The proposed mechanism is a reservoir that buffers short-term swings from diet or physiological state ^[16], which is a hypothesis, not a demonstrated pathway: how the stored lipids reach urine is listed as future work ^[17].

The comparative data are the part with practical reach. BFA-related compounds also appeared in urine from lions, tigers, leopards, jaguars, lynxes and Iriomote cats, spanning seven felid groups including the domestic cat, which suggests the system is widespread in the family and diversified as felids evolved ^[18][19]. If a field-collected sample can repeatedly identify an individual, that is a monitoring method for elusive cats without sighting or capture ^[20].

Watch three things. Whether BFA profiles hold across an animal's lifetime, not just across months ^[17]. Whether the kidney work can distinguish harmless lipid storage from the accumulation that signals disease ^[21]. And the paper itself: neither press account supplied here reports how many cats were tested or on what instruments, which is where a claim about individual-level identification will be won or lost ^[22].