Raw data of results in S6 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.氮平衡（nitrogen balance）是草食动物摄入低氮日粮时面临的主要挑战。肠道菌群介导的尿素氮循环（urea nitrogen recycling）可在氮缺乏时期协助维持宿主的蛋白质稳态（protein homeostasis），但其与野生非冬眠小型哺乳动物的关联仍尚不明确。本研究结合同位素示踪（isotope tracing）、宏基因组学（metagenomics）、靶向短链脂肪酸分析（targeted short-chain fatty acid analysis）与粪便菌群移植（fecal microbiota transplantation）技术，探究了冬季蛋白质限制对青藏高原高原鼠兔（*Ochotona curzoniae*）尿素氮循环的影响。冬季蛋白质限制条件下，宿主肝脏的尿素循环代谢（hepatic urea-cycle metabolism）通路被下调，同时伴随具备解脲能力（ureolytic capacity）的有益菌（如别样杆菌属*Alistipes*）丰度、肠道脲酶活性、尿素转运蛋白（urea transporters）表达量以及乙酸盐（acetate）生成量的提升，最终使鼠兔蛋白质库中的氮再整合水平得到增强。至关重要的是，在低蛋白日粮中添加牦牛粪便菌群可通过提升别样杆菌属的丰度增强解脲能力，揭示了同域物种间的跨物种水平微生物转移可增强宿主蛋白质稳态的关键机制。本研究结果阐明了肠道菌群在尿素氮循环中发挥的功能作用，可维持冬季活跃的草食性小型哺乳动物的蛋白质平衡，有助于我们理解物种共存以及哺乳动物对高海拔环境的适应性机制。本研究证实，菌群驱动的尿素氮循环是冬季活跃草食动物维持蛋白质稳态的关键适应性策略。本研究为哺乳动物适应高海拔环境的机制以及物种间共存动态提供了全新的见解。