Reproduction in the Island Beach Vole, Microtus breweri, and the Mainland Meadow Vole, Microtus pennsylvanicus, in Southeastern Massachusetts

The beach vole, Microtus breweri, was trapped monthly on Muskeget Island, Massachusetts, from May 1972 until June 1975. Populations of the meadow vole, Microtus pennsylvanicus, were trapped in Barnstable and Middleborough counties, Massachusetts, during the same time. Three of the four meadow vole populations showed typical cycles, whereas the beach vole showed only annual changes in density. The beach vole had a sex ratio weighted towards males (1.2 : 1), whereas the meadow vole showed a 1:1 sex ratio. Microtus pennsylvanicus, as compared to M. breweri, had a larger litter size (4.5 versus 3.4), a higher pregnancy rate (0.60 versus 0.46), winter pregnancy, a 30 percent lower weight at sexual maturity, and lower pre-implantation (0.3 ova versus 0.5) and post-implantation (0.1 embryos versus 0.3) mortality. These parameters, plus the sex ratio, indicate that M. breweri is K-selected. This is a result of their remaining at high densities virtually all the time.