INTRODUCTION
Dens used by cavernicolous animals have a vital impact on their survival. Dens provide animals with a place to rest, hide, and hibernate and help them avoid bad weather and predators (Ross et al. 2010; Tsunoda et al. 2018). The ecological characteristics of a den can accurately reflect the life, behavior, and social networks of cavernicolous animals. For example, the yellow-bellied marmot (Marmota flaviventris ) in North America uses dens that resist harsh environments and predators during hibernation and provide a protective shelter in summer (Svendsen 1976). Pallas’s cat (Otocolobus manul ), which inhabits the Mongolian plateau, cannot dig burrows by itself, so it often uses Marmota sibiricadens to give birth, raise offspring, and avoid natural enemies; the availability of M. sibirica dens thus has a very important impact on the stability of the O. manul population (Ross et al. 2010). The puma (Puma concolor ) living in the American continent uses dens to defend against predators (e.g., adult Ursus arctos ) and to maintain microclimate stability, therefore increasing the survival rate of its cubs (Bleich et al. 1996; Benson et al. 2008). American black bears (Ursus americanus ) use various types of dens (excavated, trees, rock cavities) for hibernation, and their dens provide safe places for female black bears to give birth and rear their cubs; in addition, some individuals use tree dens that can resist flooding (White et al. 2001). Therefore, dens offer an excellent means of studying the ecological adaptation of cavernicolous animals.
Except for plateau rabbits (Lepus oiostolus ), Marmota himalayana (hereafter referred to as the marmot) is the only large rodent in the alpine meadows of the Qinghai-Tibet Plateau (QTP). Marmots are herbivores and forage mainly on the leaves and stems of Cyperaceae, Gramineae, and the flowers of legume species. Marmots are of great significance to the stability of the alpine meadow ecosystem. First, the marmot is an important part of the food chain in the grassland ecosystem. It is important prey for large raptors, foxes, and wolves. In addition, marmots feed on grasslands that cannot be reached by livestock, which is equivalent to grazing in these areas and is good for the health of the grasslands (Oesterheld and McNaughton 1991). The burrowing and digging behavior of marmots is conducive to the circulation of organic matter in the grasslands. However, marmots do also cause obvious harm, as they compete with livestock for fine pasture, dig grass roots, and destroy turf all year round. In the pasture, marmot holes are the main hidden danger that causes livestock leg fractures (Chen 1982). Each of their excavated mounds covers a large area of grassland (~2 m2), causing ground collapse, soil erosion, and desertification. Marmots are also one of the main spreaders of the plague in this region (Xu et al. 2020), which seriously threatens the health of grassland herders and livestock.
Habitat selection is a reflection of the environmental, ecological, and physiological requirements of a species (Kohji and Kenichi 1998). When cavernicolous animals excavate dens, they typically show a strong selectivity with respect to the surrounding environment. For example, the Japanese red fox (Vulpes vulpes ) prefers to dig dens closer to a water source (Kohji and Kenichi 1998). On the north coast of Alaska, the arctic fox (Alopex lagopus ) prefers to choose dens on the southern slope in an area with thin permafrost (Garrott et al. 1983), which helps to keep the den warm.
In this study, marmot dens that were present in different terrains (sunny slopes, shady slopes, and flat areas) were located, and the physical characteristics of these dens were measured under harsh environmental conditions (low oxygen, low temperature, and high precipitation) (Zhang et al. 2019). Here we determined (1) the general physical characteristics and ecological functions of marmot dens and (2) whether there are differences in den traits under different terrains. Our findings highlight the adaptation mechanism of M. himalayanawith respect to its environment, which is of great significance for further understanding the ecological characteristics of the marmot or other cavernicolous animals all over the world.