Sex Ratio of Offspring in Peromyscus maniculatus borealis

We examined the effect of environmental conditions and timing of reproduction on sex ratio of offspring in Peromyscus maniculatus borealis. Timing of reproduction within the breeding season had no effect on the sex ratio of offspring at first capture. However, sex ratio at first capture in a wild population was correlated positively with the amount of precipitation during the breeding season. We suggest that environmental conditions affect the relative rate of mortality of male and female offspring during the period between birth and first capture. We introduce a method to test the hypothesis that multiparous females repeat the direction of sex-bias between litters. Our results suggest that females may be more likely to repeat direction of sex-bias than to shift direction of sex-bias between litters in a breeding season. In this population, the microhabitats of breeding females have been shown to affect timing of reproduction and sex ratio of offspring. Thus, microhabitat during the breeding season may be an important determinant of the reproductive success of females.