Sex-Bias in Prenatal Parental Investment in the Eastern Cottontail (Sylvilagus floridanus)

Abstract

We based our research on the premise that if variation in reproductive success for one sex is greater than that of the other sex, and if differential parental investment can enhance the probability of that sex reproducing at a higher rate, selection should favor investment in that sex. We tested for sex-bias in prenatal investment by weighing 75 fetuses of eastern cottontails (Sylvilagus floridanus) and their placentae in 17 gravida gestated ≥18 days. Significantly more male than female fetuses were heavier than the mean mass of fetuses in their respective gravida; placentae of these fetuses did not exhibit the same relationship between sexes. However, total resources devoted to males (fetal and placental masses combined) by maternal females was significantly greater than to females. Also, the slope of the regression line of log10 of placental mass on log10 of fetal mass for male fetuses was significantly greater than for female fetuses, suggesting a direct means by which resources are allocated differentially. Fetal mass was not affected significantly by intrauterine position in relation to fetuses of the same or opposite sex. The greatest opportunity for the maternal female to impart a potential advantage to male offspring is during the prenatal period and any advantage obtained by male neonates during postnatal development probably is a direct result of additional resources obtained before birth.