Sex ratios of newly recruited and resident members of Microtus ochrogaster and M. pennsylvanicus populations have been analyzed from live trapping data of 1965-1969. The resident trappable population of these voles usually showed a deficiency of males at any given instant, while the total number of recruits obtained over a long time period showed an excess of males. A computer simulation model was used to test the influence of differential recruitment, survival, movement, trappability, and growth on the sex ratios. Male voles survive less well than females but grow more rapidly and move about more. Increased male recruitment lowered population growth, and differential survival reduces the proportion of males in the resident population. Differential growth greatly influenced the sex ratios of newly caught animals in the three age categories. Male movement is probably important in determining the high sex ratio of newly caught M. pennsylvanicus. However, a slight excess of males at birth could also produce the observed sex ratios. In M. ochrogaster there is an apparent association between the transferrin system and sex ratio of progeny in laboratory crosses. Some genotypes produce distorted sex ratios of progeny and do not seem to expend equal effort on the production of males and females. Nevertheless, the population sex ratio seems to be 1:1 as Fisher's hypothesis predicts. This system of distorted sex ratios may have demographic consequences in fluctuating small rodent populations.
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Myers et al. (1971) studied this question.