FACTORS AFFECTING LITTER SIZE IN CLETHRIONOMYS-GAPPERI

Litter size of 207 parous Clethrionomys gapperi in the front range of the Canadian Rocky Mountains, was examined by means of embryo counts, births in live-traps and births in the laboratory from wild inseminated females. The overall mean was 5.5, with a mode of 6 and a range of 1-10. Litter size and maternal weight were not correlated in either primi- or multiparous females. No differences were found in litter size among years, among the different collection methods, or between overwintered and young-of-the-year in months in which they co-occurred. However, litter size was significantly influenced by both month and parity. Litter size of multiparous females was relatively constant over the season, but it continuously increased from May to August in primiparous females. The seasonal change in litter size was atypical of other microtine populations, which usually show a decline late in the season. The determinants of litter size among Clethrionomys appear to be numerous, but are also inconsistant among species or populations.