Overwinter of small herbivorous mammals inhabiting alpine area [Hibernation des petits mammifères herbivores occupant l'aire alpine].

Résumé

Discussed the wintering strategies of small herbivorous mammals inhabiting alpine area based on the data collected by the authors during 1985 - 1998, in the region of the Research Station of Alpine Meadow Ecosystem, Acedemia Sinica. Observations were concentrated on the five most common species: Himalayan marmot (Marmota himalayana), plateau pika (Ochotona curzoniae), Gansu pika (O. cansus), root vole (Microtus oeconomus), and plateau zokor (Myospalax baileyi). Three main strategies to overwinter adopted by the species were found. Himalayan marmot was the only hibernator. So it needed not to storage food for winter and had a winter sleep from late September to the next middle April. Plateau zokor was the only subterranean rodent in non-hibernators. It usually started to store food individually for winter at the middle of September when juveniles was able to leave their mother to live separately. This activity ended in late October or early November when soils were frozen and food collection by burrowing was impossible. Root voles began to establish their food storage in late September. They stored their winter foods in underground storerooms dug before, so they could avoid the extreme cold exposure while using the foods. Gansu pikas exhibited haying behavior in late September. They harvested plants and cached them above ground at places with distance less than 1 meter from their burrow openings. Both root vole and gansu pika worked in herd when storing winter food. Plateau pika was found never to store food for winter. Why did small herbivorous mammals in the alpine area adopt different wintering strategies? There are four reasons. First, high risk of predation might prevent small mammals from hibernating. Second, because fat deposits correlate to survival during hibernation and during post-hibernation activity that occurs before new food become available in the environment, too small species could not adopt hibernation with the limitation of fat deposits in body. These two factors could explain why Himalayan marmot was the only hibernator. Third, lifestyle and physical conditions in environment could determine animals how to overwinter. For instance, plateau zokor is a non-hibernating subterranean rodent, it lives on the underground parts of plants. If it dose not store enough food for winter, it will die from starvation because foraging by burrowing in frozen soils is impossible. Fourth, animal's tolerance to cold, correlated to body size, determines it's cold exposure limitations and thus it's way to overwinter. This can be used to explain the differences of wintering strategies adopted by plateau pika, Gansu pika and root vole. It can be concluded that wintering strategies of animals shaped in the long process of evolution and should be optimalized by nature selection. One can understand the profound evolution of animals by comparing their different ways to overwinter.