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.

Seasonal fluctuations in energy pose major energetic challenges to temperate small mammals. Photoperiod is regarded as a highly reliable and anticipatory environmental cue, enabling animals to redistribute their energetic resources and prepare for upcoming seasonal stressors. However, the integrative mechanisms by which photoperiodic changes drive inguinal white adipose tissue (iWAT) browning to achieve this energetic redistribution remain poorly understood. Here, we investigated whether photoperiod alone can induce browning of iWAT and modulate thermogenic capacity in the Brandt's vole (Lasiopodomys brandtii). Combining field sampling across seasons and an 8-week controlled photoperiod experiment, we investigated how seasonal photoperiod regulates morphological and molecular remodeling of iWAT in Brandt's voles. In autumn, iWAT showed marked reductions in adipocyte size accompanied by increased expression of browning-related genes. Short-day exposure similarly decreased iWAT mass and adipocyte area while elevating UCP1 levels, indicating enhanced browning capacity in response to reduced day length. Transcriptomic analysis revealed that photoperiod-dependent molecular regulation was centered on the calcium signaling pathway, with Ca²+-mediated activation of CaMKII and CREB emerging as key drivers initiating the browning program. Our findings demonstrate that photoperiod alone is sufficient to induce functional browning of white adipose tissue, providing experimental evidence for a photoperiod-driven thermogenic program for the seasonality of physiological adaptation in temperate small mammals.

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 trn 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.