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

## TL;DR

Tibetan yaks adjust their diet seasonally, being selective in summer and more general in winter, driven by resource availability and energy needs.

## Contribution

This study reveals how seasonal resource heterogeneity shapes yak dietary strategies, supporting optimal foraging theory in high-altitude environments.

## Key findings

- Yaks show stronger dietary selection in summer when resources are abundant, and broader diets in winter to compensate for food scarcity.
- Forbs are consumed most in both seasons, while sedges and grasses are eaten more in winter, indicating preference for high-protein plants.
- Dietary diversity is higher in winter, contrasting with lower plant diversity, suggesting a compensatory foraging strategy.

## Abstract

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 trnL 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.

Using DNA metabarcoding, we show that Tibetan yaks are highly selective foragers in the resource‐rich summer but switch to a diverse, generalist diet to survive the harsh winter. This flexible foraging strategy is driven by seasonal plant availability, providing strong support for optimal foraging theory and highlighting the critical need to protect winter forage resources for high‐altitude conservation.