On reproduction in lemmings

Many northern small mammals have restricted breeding seasons, which results in limited opportunities for reproduction by adults and delayed maturation until one year of age. Survival and longevity is enhanced under such condi tions and the potential fbr population growth is low relative to small mamm als in temperate environments. However, some northern small mammals, such a s lemmings, are regarded as having high reproductive rates and a tremendous potential for population growth. Here I review what is known about lemming life histories. Available data indicates that lemmings have the potential for both summer and winter breeding seasons. During the summer, adults are generally restricted to only one or two litters during a short breeding sea son, and only the earliest-born young-of-the-year can mature in the summer of their birth. In most years, very few or no young mature in the summer of their birth, and in some years survival of nestlings is very poor. Surviva l to the winter breeding season is important for the persistence of populat ions under these conditions. Reproduction by lemmings in winter is poorly d ocumented, but years with little or no winter breeding are not uncommon. Su mmers in which no young mature, or reproduction is unsuccessful, followed b y winters in which no breeding occurs would favor lemmings that can success fully reproduce at an advanced age. A complete understanding of the role of age-specific reproduction in the dynamics of lemming populations awaits fu rther study.