Den location and field measurement
The entrance of a marmot den is oval in shape, with excavated
soil/gravel piled up near the entrance (Fig. 1), which results in a
truncated cone-shaped pile that is obviously different from the
surrounding grassland (Appendix Fig. A1). In addition, marmots often
traverse a fixed route around the den entrance, trampling on the grass
and forming a path that is easy identified (Appendix Fig. A2).
Tibetan foxes occasionally are
seen in the study area, and they may use dens that have been abandoned
by marmots. However, it is easy to distinguish between the den entrance
of a fox and a marmot (Appendix Fig. A1).
Marmot dens were investigated during the summer (July to September) of
2019 by searching the study area from unmanned aerial vehicles (UAVs)
flying 40−50 m above the ground at speeds of 30−50 km/h. Dens located
during aerial surveys were ground checked to verify their identity based
on the presence of a large amount of marmot scat (Appendix Fig. A3),
footprints, trails, and the presence of an adult or cubs (Garrott et al.
1983).
During the entire study period, we investigated 32 sites and 131 dens
(51 on sunny slopes (facing south or west), 45 on shady slopes (facing
north or east), and 35 in flat areas). We did not carefully distinguish
between temporary dens, summer lived dens, and hibernation dens. We
measured the following indicators of these dens and the surrounding
environment.
(1) Den density: We calculated the den density for each terrain based on
the area of the surveyed sites as scanned by UAVs (Qin et al. 2019) and
the number of dens recorded.
(2) Den entrance size: As the den entrance ofM.
himalayana is oval in shape, it thus has two parameters, the long axis
(a ) and the short axis (b ). The entrance area (S) was
calculated using the following formula:
\(\mathrm{S}\mathrm{\ }\mathrm{=}\mathrm{\ }\mathrm{\pi}\mathrm{\ }\mathrm{\times}\mathrm{\ }\frac{a}{2}\mathrm{\ }\mathrm{\times}\mathrm{\ }\frac{b}{2}\)(1)
(3) First tunnel length: We used a measuring tape to measure the length
from the entrance to the first corner of the tunnel.
(4) Den volume:
We
used the equal volume method to measure the tunnel volume based on the
pile of dirt beside each den. The volume of the truncated cone-shaped
pile is approximately equal to the tunnel volume:
\(\mathrm{V}\mathrm{\ }\mathrm{\approx}\mathrm{\ }\frac{\mathrm{1}}{\mathrm{3}}\mathrm{\ }\mathrm{\times}\mathrm{\ }\mathrm{H}\mathrm{\ }\mathrm{\times}\mathrm{\ }\mathrm{\pi}\mathrm{\ }\mathrm{\times}\mathrm{\ }\left(\mathrm{R}^{\mathrm{2}}\mathrm{\ }\mathrm{+}\mathrm{\ }\mathrm{R}\mathrm{\ }\mathrm{\times}\mathrm{\ }\mathrm{r}\mathrm{\ }\mathrm{+}\mathrm{\ }\mathrm{r}^{\mathrm{2}}\right)\)(2)
Where V is the tunnel volume; H is the height of the mound; and R and r
represent the upper and lower radius of the mound, respectively.
(5) Den orientation and angle of den entrance: We used a rangefinder
(Aicevoos Z5, China) to measure the orientation and angle of the
entrance. Here we divided the den orientation into eight directions: N
(0°, at the top (12 o’clock) position), NE (1°−89°), E (90°), SE
(91°−179°), S (180°), SW (181°−269°), W (270°), and NW (271°−360°).
(6) Path density near the den entrance: We determined the path density
according to the trampling situation of the vegetation around the den
entrance.
(7) Vegetation characteristics near the den entrance: For each den, to
avoid any influence of the mound, we selected a quadrat (0.5 m × 0.5 m)
in the opposite direction of the mound and 30 cm away from the den
entrance (referred to as the near entrance quadrat). At the same time,
we analyzed a control quadrat (CK) at a distance of 30 m (referred to as
activity area) away from the den entrance. Individual plant species
(referring to species richness) and the height of each species
(referring to an average height per species) were recorded in each
quadrat, and aboveground vegetation was collected and dried to a
constant weight at 65 °C.