The females were considered to be fecund if they were gravid, or if there were placental scars in the horns of the uteri.
Figure 2. Fecundity of Prairie Voles by Months. Adults and Subadults are Considered Together.
Figure 3. Seasonal Changes in the Length of Testes.
Size of Litters
The number of mammae characteristic of a species of vole may be a rough guide to the average size of a litter for that species. The prairie vole has fewer mammae (three pairs) than some other voles in North America, and might, therefore, be expected to have smaller litters. Fifty-eight gravid females of Microtus ochrogaster examined by me had an average of 3.4 embryos each; the number of embryos ranged from one to seven. Hamilton (1936a) gave 5.07 as the average number of young per litter in M. pennsylvanicus. Hatfield (1935) stated that M. californicus has an average of 5.7 young in a litter. Both pennsylvanicus and californicus normally have four pairs of mammae. The expectation as to the size of the litter seems to be realized. In the prairie vole one pair of mammae is pectoral and two pairs are abdominal. Usually a lactating vole showed evidence of only the abdominal mammae having been in use.