The youngest female known to be gravid was 26 days old and weighed 28 grams. During summer most of the females were gravid before they were six weeks old, although females born in October and after were often more than 15 weeks old before they became gravid. The youngest male known to be fecund was approximately six weeks old. Male fecundity was determined as described by Jameson (1950). Difference in the age of attainment of sexual maturity serves to reduce the mating of litter mates (Hamilton, 1941:7) and has been noticed in various species of the genus Microtus by several authors (Bailey, 1924:529; Hatfield, 1935:264; Hamilton, loc. cit.; Leslie and Ransom, 1940:32).
For 35 females, each of which was caught at least once each month for ten consecutive months or longer, the mean number of litters per year was 4.07. Certain of the more productive members of the group produced 11 litters in 16 months. M. ochrogaster seems to be less prolific than M. pennsylvanicus. Bailey (1924:528) reported that one female meadow vole delivered 17 litters in 12 months. Hamilton (1941:14) considered 17 litters per year to be the maximum and stated that in years when the vole population was low the females produced an average of five to six litters per year. In "mouse years" the average rose to eight to ten litters per year. During this study several females delivered two or more litters in rapid succession. This was noted more frequently in spring and early summer than in other parts of the year. Those females which produced two or three litters in rapid succession in spring and early summer often did not litter again until fall. Post-parous copulation has been observed in M. pennsylvanicus by Bailey (1924:528) and Hamilton (1940:429; 1949:259) and probably occurs also in M. ochrogaster.
The gestation period was approximately 21 days, the same as reported for M. pennsylvanicus (Bailey, loc. cit.; Hamilton, 1941:13) and M. californicus (Hatfield, 1935:264). A more precise study of the breeding habits of M. ochrogaster failed to materialize when the voles refused to breed in captivity. Fisher (1945:437) also reported that M. ochrogaster failed to breed in captivity although M. pennsylvanicus (Bailey, 1924) and M. californicus (Hatfield, 1935) reproduced readily in the laboratory.
[Litter Size and Weight]
In the course of this study 65 litters were observed. The mean number of young per litter was 3.18 ± 0.24 and the median was three ([Fig. 10]). Three litters contained but one individual and the largest litter contained six individuals. Other investigators have reported the number of young per litter in M. ochrogaster as three or four (Lantz, 1907:18) and 3.4 (1-7) (Jameson, 1947:146). M. pennsylvanicus seems to have larger litters. Although Poiley (1949:317) found the mean size of 416 litters to be only 3.72 ± 0.18, both Bailey (1924:528) and Hamilton (1941:15) found five to be the commonest number of young per litter in that species. Leslie and Ransom (1940:29) reported the average number of live births per litter to be 3.61 in the British vole, M. agrestis. Selle (1928:96) reported the average size of five litters of M. californicus to be 4.8. Hatfield (1935:265), working with the same species, found that litter size varied directly with the age of the female producing the litter. He reported litters of young females as two to four young per litter and of older females as five to seven young per litter. In the litters of M. ochrogaster that I examined, young females did not have more than three young and usually had but two. However, older females had litters of one, two and three often enough so that no relationship, as described above, was indicated clearly.
Fig. 10. Distribution of litter size among 65 litters of voles.
No seasonal variation in litter size was noted. The mean size of the litters in 1950, 2.68 ± 0.30, was significantly lower than that found in 1951 (3.76 ± 0.20) but neither differed significantly from the mean size of litters in 1952 (3.35 ± 0.66). The lower mean size of litters was in part coincidental with a high population level and the higher mean of the two later years was in part coincidental with a low population level. Since a sharp break in the curve for population density occurred after the flood in July, 1951, the litters were arranged in pre-flood and post-flood categories for study. Pre-flood litters averaged 3.07 ± 0.28 young per litter whereas post-flood litters averaged 3.34 ± 0.48. This difference was not significant. Increase in litter size, if it had actually occurred, might have been a response to the increasing food supply and lower population density after the flood.