Raw data of results in S3 Fig.

Nitrogen balance is a major challenge for herbivores when consuming a low-nitrogen diet. Gut microbiota-mediated urea nitrogen recycling facilitates protein homeostasis during times of nitrogen deficiency, yet its relevance to wild nonhibernating small mammals remains unclear. Here, through a combination of isotope tracing, metagenomics, targeted short-chain fatty acid analysis, and fecal microbiota transplantation, we investigated the effects of protein restriction in winter on urea nitrogen recycling in plateau pikas (Ochotona curzoniae) of the Qinghai–Tibetan Plateau. Hepatic urea-cycle metabolism was downregulated during winter protein restriction, accompanied by increases in beneficial bacteria with ureolytic capacity (such as the genus Alistipes), gut urease activity, and urea transporters, and acetate production, with a consequent increase in nitrogen reincorporation into the pika’s protein pool. Critically, supplementing a low-protein diet with yak fecal microbiota enhanced the ureolytic capacity by increasing Alistipes abundance, revealing a critical mechanism whereby interspecies horizontal microbial transfer between sympatric species enhances host protein homeostasis. Our results reveal a functional role for the gut microbiota in urea nitrogen recycling to maintain protein balance in winter-active herbivorous small mammals and contribute to our understanding of species coexistence and mammalian adaptation to high-altitude environments. Our findings establish that microbiota-driven urea nitrogen recycling is a key adaptive strategy for protein homeostasis in winter-active herbivores. This work provides new insights into the mechanisms of mammalian adaptation to high-altitude environments and the dynamics of interspecies coexistence.