Plateau pikas (Ochotona curzoniae) play a keystone role in the alpine meadow ecosystem on the Qinghai–Tibetan Plateau (QTP). For decades, QTP grasslands have become degraded to some degree, largely as a result of heavy livestock grazing. Concomitantly, the abundance of plateau pikas has increased dramatically as grassland degradation has altered the vegetation community structure and dominant species, shifting the plant nutrient contents toward higher protein conditions that favor them. Considerable research supports the hypothesis that the quantity and quality of food limit herbivore populations. Here, we examined the relationship between the availability of essential amino acids in the diets of plateau pikas and the degree of meadow degradation associated with livestock grazing intensity through a field survey, as well as the fitness of individuals by laboratory feeding experiments with 2 pelleted chows containing 0.26% and 0.45% methionine. Sulfur-containing methionine and cystine were the most limiting amino acids in the diets of pikas. During the pika breeding season, the concentrations of most essential acids, particularly methionine and cystine, were higher in heavily degraded meadows than in lightly and moderately degraded meadows. Individuals fed 0.45% methionine exhibited enhanced cell-mediated immunity, reduced intensity of coccidian infection, and increased concentrations of gonadotropin-releasing hormone, luteinizing hormone, estradiol, progesterone, and testosterone compared to those fed 0.26% methionine. These results showed that heavily degraded meadows provided relatively high-quality food that improved individual fitness, suggesting that the high-methionine food in the heavily degraded meadows may be a key factor in the generation and maintenance of high-density populations of plateau pikas.
