Fallback foods (FBF), classified into staple and filler types, are low-quality food resources chosen by animals due to a shortage of preferred food during a specific period. The selection of lichens as FBF for Yunnan snub-nosed monkeys (Rhinopithecus bieti) represents a unique ecological adaptation and evolutionary development in the animal kingdom. This study investigates the yearly dietary selection of five R. bieti groups to address the issues and elucidate the nutritional value and ecological selection of lichens for this monkey species, which resides at the highest altitude among nonhuman primates. The results indicate that the consumed lichens serve as the staple FBF. Two main lichen species taken by the monkeys are Bryoria spp. and Usnea longissimi, with Bryoria spp. being the primary choice (67.25 ± 12.20% compared to 15.79 ± 11.66% from U. longissimi). Bryoria spp. provides a higher level of digestible fiber (NDF) and a lower level of tannin, fat, ADF, and energy compared to U. longissimi, which offers higher availability. Lichens are the dominant food and nutritional resource for the monkey species during the dry season, while they serve as a primary food source rather than a nutritional resource during the wet season. Therefore, they compensate for nutrients from other food types, such as fruits, seeds, and leaves. Compared to other Asian colobine counterparts, this species consumes the highest amount of lichens but the lowest proportions of leaves, flowers, and seeds. This dietary pattern demonstrates a specific type of ecological selection and evolutionary development during the Quaternary. The biomass of lichens in the monkeys’ habitat has significantly decreased due to environmental degradation. This study also provides evidence and information to develop or amend conservation strategies and guidelines for the dietary management of captive Yunnan snub-nosed monkeys.

Understanding how ecological and environmental changes, anthropogenic activities, and climate have driven and will direct animals’ development and predicting their prospective distribution profiles in the Quaternary are essential to making a tangible conservation strategy. Macaques (Macaca) distributed in mainland East Asia provide an ideal research model for such an effort. We reconstruct macaques’ geographic distribution profiles during the Quaternary, from the last inter-glaciation (LIG, 120,000 - 140,000 years BP), the Last Glacial Maximum (LGM, 22,000 years BP), and the present (1970-2000) – based on which we deduce their perspective distribution in the 2050s. The results show their suitable habitats during LIG and LGM were mainly in Southwest, Central, and Coastal China. A noticeable distribution reduction started in LIG and persisted until the current (1970-2000). Their distribution centroid would shift northward to mountainous regions, mainly in Southwest China, where more migration corridors would be reserved for their future development. Also, the results indicate that China’s Protected Area currently does not cover more than 87% of macaques’ habitats, a dismal situation for their conservation. Finally, this study proclaims that the conservation priority of the macaques in the years to come should focus on Southwest China – their future refuge region in Quaternary.