Microtus Population Biology; III. Reproductive Changes in Fluctuating Populations of M. ochrogaster and M. pennsylvanicus in Southern Indiana, 1965‐67
Authors: Keller Barry L. , Krebs Charles J. 
Published: April 2006

About 1,200 Microtus pennsylvanicus and 1,000 M. ochrogaster were collected over a 3—year period by sequential trapping of snap—trap lines in 15 grassland areas in southern Indiana. Peak densities were reached in 1966 for all but two of the populations studied. Reproductive data were obtained to determine whether weight at sexual maturity, pregnancy rate, living embryo production, prenatal mortality, and body weight varied in relation to fluctuations in numbers. The rate of winter breeding was high in both species of vole between the cyclic increase and cyclic peak years. Breeding was continued at a reduced level during the fall of the cyclic peak and was greatly reduced during the winter between the cyclic peak and decline. In both species sexual maturity occurred at higher weights in periods of peak population density and lower weights in periods of low density. Only heavy individuals remained fecund during the winter months of November—February. Sexual maturity was reached at the same weight in males and in females. Litter size was correlated with body size. In M. pennsylvanicus litter size was not significantly different in multiparous and primiparous females, and litter size remained constant in the summer breeding seasons of the increase, peak, and decline years. Difference between seasons was significant, the fall—winter—spring periods having a 14% lower litter size than the summer period. In M. ochrogaster living embryo counts were 25% lower during the period of cyclic peak in multiparous females than they were during the years of cyclic increase or decline. This observation was confirmed independently in a population that was out of synchrony. A decline in litter size was not associated with the year of cyclic decline in either species. Corpora lutea counts were correlated with body size. The pattern established for living embryo counts was nearly identical to the ovulation pattern. Prenatal mortality did not change significantly, either seasonally or yearly, and thus was not higher in the decline phase for either species. Testes weight and body weight were correlated. The testes weight of non—fertile Microtus pennsylvanicus differed little among the summers of population increase, peak, or decline. Testes development was inhibited during fall and winter. Fertile males taken during the summer of population peak and the summer of decline had heavier testes than males taken during the summer of increase. A midsummer drop in testes weight was observed during three breeding seasons. Non—fertile M. ochrogaster had only slightly reduced testes weight during the winter between the population increase and peak, but testes weight declined significantly during the winter following the population peak. Testes weights of fertile males changed little during the summer of population increase, but they increased steadily during the summer of population peak. A significant decline in testicular weight occurred during the winter following the population peak. High body weights were associated with peak populations in both species of Microtus. The pattern of change in weight distributions was similar to that described for other cyclic voles and lemmings.