Reproduction in laboratory colonies of Clethrionomys gapperi and Microtus pennsylvanicus: a comparison between species and among populations

Abstract

Reproduction in laboratory colonies of Clethrionomys gapperi and Microtus pennsylvanicus was examined by comparing six populations (three per species) to test the general hypothesis that populations subject to the lowest temperatures and the shortest breeding seasons would be at the "fast" end of the "fast–slow" continuum. All colonies were derived from three sites in western Canada from females that were inseminated in the wild. Postpartum mass, mass of adult females during lactation, litter size, litter and neonate masses at birth, litter mass at weaning, age when the eyes opened, and age at weaning as well as two variables describing the energetics of reproduction were examined between species and among populations within species. Three indices of physiological reproductive effort were also compared. Only neonate mass, age when eyes were open, and one index of reproductive effort differed between species. In C. gapperi, litter mass, litter size, age when the eyes opened, age at weaning, and one index of reproductive effort differed among populations. In M. pennsylvanicus, postpartum, litter (at birth and weaning), and neonate mass, age when eyes open, age at weaning, and maintenance costs during lactation were different among populations. In both species, some differences could be attributed to sampling biases, while others were simply a consequence of differences in maternal mass. In both species, the maximum difference in the age at weaning and the age when the eyes opened was less than 2 days. Differences among populations did not appear to be related to meteorological conditions, and populations in either species could not be ranked on a continuum.