The price of survival: comparative adaptation to high altitudes between yaks and cattle.

The yak (Bos grunniens) serves as an exceptional model for studying high-altitude adaptation mechanisms due to its evolutionary success in the hypoxic environment of the Qinghai-Tibet Plateau. Previous research has largely focused on genetic and physiological traits of yaks; however, the interactions between rumen microbiota and host physiology under hypoxic conditions are poorly understood. As the largest digestive organ in ruminants, the rumen and its microbiota play a central role in digestion and host nutrition. In this study, a comparative analysis of digestive metabolism and rumen microbiota was carried out in yaks and cattle (Bos taurus) under two distinct atmospheric oxygen scenarios: baseline (2,200 m) and hypoxic (3,800 m). Our findings reveal that yaks have developed unique microbial strategies to cope with energy deficits in hypoxic stress. These strategies include a shift in rumen microbiota toward amino acid degradation, providing more available energy substrates for host utilization, and enhanced long-chain fatty acid biosynthesis, enabling more efficient energy storage and utilization. This improves energy acquisition in yaks despite their reduced nutritional intake. However, this metabolic adaptation comes at a physiological cost - reduced microbial crude protein (MCP) synthesis, leading to elevated ruminal NH3-N levels, and increased fatty acid metabolism and urea cycle activity contributing to hepatic stress. Our results showed that under high-altitude conditions, yak MCP synthesis decreased by 47.3%; and ruminal NH3-N and serum ALT (a hepatic stress marker) increased by 147.2 and 19.7%, respectively. This study presents evidence of potential metabolic trade-offs in high-altitude adaptation, indicating that yaks may optimize microbially mediated energy production at the cost of liver health. These insights deepen our understanding of host-microbiome coevolution mechanisms in extreme environments and highlight biological costs associated with adaptation to high altitudes.