Numbers of litters, litter size and survival in two species of microtines at two elevations

The number of litters, litter size and survival of Clethrionomys gapperi and Microtus pennsylvanicus were documented at two elevations (1450 m; 2240 m) in the Canadian Rocky Mountains to determine if those life history traits changed with the length of the breeding season. The high elevation was colder, had fewer frost-free days, and more precipitation than the low elevation. The length of the breeding season of C. gapperi and M. pennsylvanicus averaged 8 d and 34 d longer, respectively, at the low elevation than at the high elevation. In both species and at both elevations, most females survived only long enough to produce one or two litters. Litter size was significantly greater at the high elevation than at the low elevation in M. pennsylvanicus , but not different in C. gapperi. Overall, nestling survival was greater at the low elevation than at the high elevation in M. pennsylvanicus , but not different in C. gapperi. Summer and winter survival rates were not different between elevations in either species. Differences in litter size and nestling survival between elevations in M. pennsylvanicus may be related to differences in habitat quality.