Cats may carry a built-in identity card in their urine, according to new research that finally explains a feline kidney mystery that has puzzled scientists for more than a century.
An international team led by researchers at Iwate University in Japan, working with collaborators in Germany and Spain, identified 13 branched-chain fatty acids in domestic cat urine whose combined mixture and relative proportions differ from cat to cat, yet stay relatively stable within the same individual over time. The findings, published in Current Biology, suggest the compounds function as a durable scent signature rather than the fleeting odors typically associated with animal scent-marking.
Unlike most volatile compounds in urine, which break down or evaporate quickly once deposited on an object, the newly identified fatty acids persist far longer, according to the researchers. Behavioral trials showed cats could distinguish between different individuals' fatty-acid profiles even when other scent compounds were controlled for, suggesting the animals may actually rely on this signature to recognize one another.
"Cat urine serves, so to speak, as a 'scented business card.'"
Prof. Masao Miyazaki, Iwate University
The discovery also solves a longstanding anatomical puzzle. Scientists have known for more than 100 years that domestic cats' kidneys contain unusually abundant lipid droplets in the renal cortex, but the function of those structures was never established. The new research found the branched-chain fatty acids are manufactured and stored specifically within those kidney lipid droplets, tying a century-old structural oddity to a concrete biological purpose for the first time.
The team, led by researcher Shota Ichizawa, says the findings could have practical uses beyond satisfying curiosity about cat behavior. A stable, individually distinct chemical signature could eventually help shelters and veterinarians identify stray or lost cats, or assist researchers studying stress and territorial behavior in multi-cat households. For now, the study adds a new layer to how scientists understand feline communication, suggesting cats "read" each other's identities through a chemistry lab hidden in plain sight, right down in the litter box.
