Mechanism of Dietary Variation of Grazing Yaks on Tibetan Plateau: The Role of Seasonal Heterogeneity of Resources

Diet composition is a crucial yet understudied, dimension of animal ecology, with seasonal dietary shifts being a key factor in the population dynamics of large herbivores. However, characterizing these variations and their drivers in free-ranging animals has been challenging due to their high mobility and the diverse plant species in their diet. According to optimal foraging theory, animals select their diet to maximize energy intake, a decision process that involves evaluating the abundance and quality of potential food sources. We determined the seasonal dietary shifts and food network in high-altitude grazing yaks using DNA metabarcoding targeting the <i>trn</i>L region of fecal samples. Seasonal shifts in yak diet composition were structured by resource heterogeneity and influenced by plant community diversity and aboveground biomass. Dietary diversity and richness were greater in winter than summer, while plant community diversity and species richness exhibited opposite trends. This pattern indicated that yaks exhibited the strongest dietary selection during the summer with high resource abundance. Less selection in winter led to more diet dissimilarities, possibly reflecting a compensatory strategy to mitigate energetic deficits by broadening dietary niche breadth and maximizing resource availability under food limitation conditions. The proportion of forbs consumed by yaks was highest in both summer and winter, while the intake of sedges and grasses increased significantly in winter, suggesting that yaks selected high-protein forbs over grasses or sedges. Our results support the predictions from optimal foraging theory, demonstrating that the energetic basis of dietary selection governs niche width in seasonal environments. Consistent with predictions from optimal foraging theory, our study shows that the energetic drivers of diet selection determine niche breadth in seasonal environments.