Rodents of the triumvirate

Rodents arrived in Antarctica thanks to Aves ex machina about 5 million years ago and they rapidly flourished in a moltitude of forms. Over 100 species can be found (ex. insular species), most of them located at lower latitudes, where temperatures are higher. Species diversity drastically falls in the southernmost forests, since few lineages had enough time to adapt to these cold environments.
Most of the southern rodents are fairly large and deal with deep snow by moving in the forest canopy, like rattrees (Pluviomys spp.) This murid genus form an interesting species complex*, with interfertile species adapted to different habitats, maintaining a hybrid zone. The genus follows the Bergmann Rule, with smaller species ( around 3 kg) that live in the sub-tropical woodlands of the Antarctic Peninsula and way larger (>10 kg) that dwell in the cold austral forests.

Rattrees are not found in the coldvanna, where the arid conditions in the summer and natural grazing limit tree size/density and food source. These open lands are occupied by another species of rodent, the running vole (Neopseudomys muschiatum): as its name suggests, this cricetid has adapted for speed and possesses long limbs with a semiplantigrade locomotion. Paws are large, avoiding the risk of sinking in the snow. They are mostly solitary during their entire life, but large herds ( up to 1000 individuals)  can often form in some feeding areas. 

While they are highly fast on plains, running voles often avoid steep terrains, where they can fatally stumble. This is probably why they are completely absent from the highlands of Antarctica, where the largest rodent on the continent, the hogster (Magnicricetus longiventri), lives all year. It's probably the most cold-adapted species of antarctic rodent and can reach the size of a large beaver. During winter they grow a dense fur that keeps them warm against freezing temperatures as low as -30°C. A thick layer of fat also helps for isolation, which is maintained by feeding on the incredibly productive vegetation of this alpine ecosystem, rich in highly palatable plants (legumes, rushes, grasses, etc).
Hogsters form large social groups that live at high density compared to other herbivores: they possess in fact a lower basal metabolic rate, 15-30% lower than expected which helps them to spare energy.  The energy intake of the organs is greatly reduced except for the digestive system, which is somewhat complex for a rodent. This metabolic adaptation is common in some arctic fauna of the past, like arctic hares.