Mound-building mice (Mus spicilegus) construct large mounds of soil and plant material in autumn, where juvenile animals overwinter in groups without reproducing. The mounds have several functions, including food storage, thermoregulation, and protection from predators. We examined whether these mounds have food storage or thermoregulatory function. Mice used mainly seeds and ears of certain plants (Echinochloa sp., Chenopodium spp., and Setaria spp., depending on availability) as building materials, but microhistological analysis of mice feces revealed that the cached plants are not represented in their actual diet. By comparing the features of soil under the mound to neighboring random points we found that the mounds have water-insulating and thermoregulatory properties. We also found a positive correlation between the size of the mound, its vegetal content, and its effectiveness to lessen the impact of the harsh conditions of the outside environment. Mound size was dependent on the number of inhabitants, indicating that larger mounds are constructed by and shelter bigger groups. The existence of communal mound building thus can contribute greatly to successful overwintering and the relatively low fluctuation in population size in this species.