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 inseminate d 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 pri mi- or multiparous females. No differences were found in litter size a mong years, among the different collection methods, or between overwin tered and young-of-the-year in months in which they co-occurred. Howev er, litter size was significantly influenced by both month and parity. Litter size of multiparous females was relatively constant over the s eason, but it continuously increased from May to August in primiparous females. The seasonal change in litter size was atypical of other mic rotine populations, which usually show a decline late in the season. T he determinants of litter size among Clethrionomys appear to be numero us, but are also inconsistant among species or populations.