The prairie vole, a small North American grassland rodent, breeds year-round, and a group of voles living together consists primarily of an extended family, often including two or more litters  Voles commonly live in large groups from late autumn through winter; from spring through early autumn, however, most voles live in far smaller groups
The seasonal variation in group size can probably be explained by a seasonal variation in mortality among young voles
Prairie vole populations vary dramatically in size from year to year.