# The Seasonal Dietary Shift and Niche Resilience of Yaks on the Qinghai–Tibetan Plateau

## TL;DR

Yaks on the Qinghai–Tibetan Plateau shift their diet from diverse summer forbs to specialized autumn grasses, using a strategy to maximize energy in harsh environments.

## Contribution

This study reveals yaks contract their dietary niche in autumn, challenging traditional foraging theory and offering new insights into high-altitude herbivore adaptation.

## Key findings

- Yaks shifted from diverse forbs in summer to grass-dominated diets in autumn.
- Dietary richness and niche width decreased, but phylogenetic diversity remained stable.
- Autumn foraging became more integrated and synchronized, reflecting energy-maximization strategies.

## Abstract

Ecological theory posits that seasonal food availability drives dramatic temporal variation in dietary niche breadth, yet alternative paradigms offer contrasting predictions about its effect. Traditional Optimal Foraging Theory predicts that herbivores should broaden their dietary niche when resources become scarce. However, it remains unclear how high-altitude specialists like the yak adjust their fine-scale foraging strategies to navigate the extreme transition from resource-rich summers to resource-limited autumns. By sequencing plant DNA from fecal samples, we analyzed the seasonal diet dynamics of grazing yaks on the Qinghai–Tibetan Plateau. We found a dramatic dietary shift, as instead of expanding their niche under autumn scarcity, yaks significantly contracted a dietary niche, switching from diverse high-quality forbs to specialized abundant graminoids. These results suggest that yaks function as adaptive mixed feeders, employing a selective energy-maximization strategy to persist in one of the world’s harshest environments.

Understanding how herbivores adjust their foraging strategies to cope with seasonal resource fluctuations has been central to the nutritional ecology. Optimal Foraging Theory (OFT) predicts that generalists should broaden their dietary niche when high-quality resources are scarce, but empirical evidence in extreme environments remains poorly understood. We used trnL-P6 metabarcoding of fecal samples (n = 10/season) and a local reference library of 120 plant species to quantify diet composition and niche metrics of free-ranging yaks (Bos grunniens) on the Qinghai–Tibetan Plateau in June (summer) and October (autumn) 2024. Yaks shifted from a diverse, forb-dominated diet (e.g., Polygonaceae, Rosaceae) in summer to a specialized diet dominated by grasses in autumn. Although dietary richness and total niche width (TNW) decreased in autumn, phylogenetic diversity remained stable, indicating a strategic shift to distinct evolutionary lineages to ensure functional redundancy. Furthermore, food network analyses demonstrated a transformation from a flexible, modular foraging pattern in summer to a highly integrated, synchronized network in autumn. These findings suggest that under the distinct quality–quantity trade-off of high-altitude ecosystems, yaks adopt an energy-maximization strategy by minimizing search costs, aligning with the opportunity cost constraints of OFT, rather than randomly expanding their niche. This insight into selective foraging dynamics is critical for developing sustainable grazing practices that accommodate the natural adaptive behaviors of alpine herbivores.