The hands and feet are dark gray above, and the tail is dusky gray and only slightly bicolored. The whiskers are short, scarcely reaching to the ears, and the feet are well haired. The dark phase of H. wynnei, unusual among animals of this kind, may be a natural result of retaining the darker coat of the immature form, owing to the fact that the animal has adopted a subterranean life away from light. H. wynnei differs from H. fertilis in being larger in size and in having longer, more lax, more luxuriant pelage. The general appearance of Hyperacrius is similar to that of Microtus and Alticola, the principal means of differentiation being in cranial and dental structure.
Females have four inguinal mammae.
H. wynnei is highly fossorial, excavating extensive networks of shallow feeding tunnels, about 32-75 mm deep, as well as much deeper burrows descending almost vertically into the ground that are used for sleeping and breeding. During the winter this species constructs tunnels near or on the surface, through the snow, and lines them with soil and vegetation. It does not hibernate and is active at any time of the day or night. H. fertilis is not so fossorial and will forage above the ground. Both species are completely herbivorous, eating grass, stems, and roots.