Kidney-specific lipid metabolism underlies stable chemical individuality in domestic cats and other felids

Domestic cats uniquely accumulate lipid droplets (LDs) in renal proximal tubular epithelial cells, but their chemical composition and biological functions remain unknown. Here, we identify a distinctive renal specialization in felids: lipid droplets enriched in branched-chain fatty acids (BFAs). These BFAs accumulate as triglyceride components and are excreted in urine as free fatty acids with stable and individually distinctive profiles. Behavioral assays show that cats discriminate urine samples based on BFA composition, even against complex volatile backgrounds, suggesting BFAs as robust, metabolically encoded olfactory signatures for individual recognition. Comparative analyses of non-domesticated felids reveal that BFA production is broadly conserved but exhibits both species-specific and individual variation. These findings demonstrate how a core metabolic pathway has been evolutionarily co-opted into a chemical signature system. This discovery highlights an unusual integration of renal physiology with social communication and expand our understanding of lipid metabolism, organ-level adaptation, and the evolution of chemical individuality in mammals.