Viviparity as a constraint on sex-ratio evolution.
In polytocous mammals, the sex ratio during gestation can influence a variety of morphological, physiological, and life-history traits because of steroid leakage between fetuses. Similar phenomena have also recently been described for a viviparous lizard. Some of these effects have important fitness consequences by influencing reproductive success later in life. Thus, biasing the sex ratio toward one sex may lead to a decreased fitness for the other sex, and therefore constrain the evolution of skewed sex ratios. By incorporating effects of sex ratio on offspring fitness in a simple sex-allocation model, I show that, under some circumstances (1) skewed sex ratios are predicted to evolve, and (2) this cost can constrain the evolution of skewed sex ratios.