Den characteristics and functions
The shape of the den entrance is typically determined by the morphological structure of the inhabitant. Marmots have a flattened body shape. This body shape is conducive to remaining close to the ground to avoid being discovered by predators when the marmots are active outside the den. Field measurements were used to determine that the long axis of the oval-shape entrance to these dens was significantly longer than the short axis (Fig. 3). These dimensions are larger than the den entrance diameter (27 and 19 cm for the long and short axis, respectively) of the steppe marmot (Marmota bobak Müll.) that inhabits the Ukraine (Nikol’Skii and Savchenko 2002). The shape and size of the den entrance facilitate the rapid entry of the marmot while preventing predators from entering (Rodrick and Mathews 1999). There was no significant difference in the area of the den entrance among different terrains, which suggests that the size of the den entrance helps to maintain a stable temperature inside the den (Nikol’Skii and Savchenko 2002), which is an adaptive strategy used by marmots to protect themselves and to reduce the possible impact of the external environment (Jia et al. 1991). We also found that dens in flat areas had a longer long axis relative to the long axis of dens on slopes. This may be due to the wider field of vision that marmots have access to on slopes; thus, a larger den entrance makes it easier for a marmot hiding inside the den to observe potential predators.
We found that the average first tunnel length was 248.64 cm (Fig. 4), which is longer than that (127.3 cm) of dens in other parts of the QTP (Zhang et al. 2019); this difference is most likely due to the different geographical and climatic conditions across different regions. The first tunnel is deep and long, which can increase the infiltration distance of rainwater after entering the tunnel, thereby minimizing the accumulation of water deeper within the den. This is especially important in rainy areas (i.e., the QTP). In addition, the shortest length of the first tunnel in this study was 100 cm, which is greater than the average body length of M. himalayana (55 cm) and thus ensures that they can enter the den quickly to avoid predators (Zhang et al. 2019).
Tunnel volume reflects the internal structural traits of the marmot den. We found that on average the den volume was 0.29 m3, slightly larger than that (0.23 m3) in other parts of the QTP (Wang 1992), but there was no significant difference in the den volume among the different terrains analyzed here (Fig. 5). The underground den system has a complex internal structure and multiple nests, which are places for marmots to breed, hibernate, and store food (hay). The truncated cone-shape mound near the den entrance represents an observatory for the resident marmot(s). They often stand on the mound to observe the environment around the den entrance (sometimes for >1 h), and they become active around the entrance only after confirming that there is no potential danger (Yang and Xie 1983). These mounds do, however, form a large number of naked spots on the grassland and represent one of the main reasons for grassland degradation in this region (Chen 1982).
Whereas the aspect of a den may confer certain advantages, the direction of the den opening may be related to site-specific conditions such as vegetation, drainage, or climate (Rodrick and Mathews 1999). Among the 131 dens surveyed in this study, there were no dens oriented toward the north, and only a few (9.09%) dens located on shady slopes displayed a northeast aspect (Fig. 6). Danilov (1961) suggested that dens oriented southward have a more favorable microclimate because of the protection from the prevailing northeasterly winds. For example, den orientation for the red fox on Assateague Island, Maryland, is often associated with local prevailing wind directions (Krim et al. 1990). Chesemore (1969) found that most entrances of arctic fox dens had a southerly, easterly, or westerly orientation, possibly indicating a preference for a warmer exposure. We did not, however, assess any microclimatic variables of the dens in this study.
The angle between the tunnel and the ground has an important influence on the structural stability of the den. We found that on average the angle of the den entrance was 36 ± 4.82° (Fig. 7), which is smaller than that of a previous study (45°) (Zhang et al. 2019). A smaller angle of the den entrance results in structural instability, and makes it easy for the den to collapse, especially in pastures, where this hidden danger leads to leg fractures among large livestock. In contrast, a larger angle makes it difficult to dig the burrow and the excavated soil can easily backfill the tunnel; in addition, rainwater easily collects in the den. Thus, the moderate angle noted here is the result of a trade-off between structural stability and better drainage (Wang and Wang 2006).
Marmots are timid by nature. When they are active outside their dens, they walk along a fixed route, trampling the grass and forming a path. The number of paths reflects the activity intensity of a marmot near its den entrance. We found that these paths are arranged radially around the entrance, with an average of 2.68 ± 0.82 paths near each entrance (Fig. 8). Marmots are one of the main spreaders of bubonic plague in this region (Xu et al. 2020), and thus one method to control the spread of this disease is the use of annular mousetraps to trap and kill marmots. If mousetraps are laid out on the path near the den entrance, the capture rate can be increased (Zhou et al. 2019).