Multiple factors determine the sex ratio of postpartum-conceived Norway rat litters

When Norway rat litters are nursed communally, postpartum-conceived litters born asynchronously are female biased at birth. To determine if one or more mechanisms produced this sex ratio bias, we studied litters conceived at a postpartum estrus, and systematically varied the presence or absence of a litter suckling during gestation. Four different factors biased the sex ratio of postpartum-conceived litters. Postpartum condition, implantation in a single uterine horn, and very large litter size favored the birth of female pups. The presence of suckling pups during gestation, on the other hand, brought the sex ratio up to parity. Each factor operated at or before implantation, and apparently did so asymmetrically in the two horns of the uterus. Thus, a litter's sex ratio at birth, even a sex ratio of parity, can be the result of multiple mechanisms of bias, some working in opposition to each other. Theories of sex ratio biasing must incorporate multiple mechanisms rather than focusing on single mechanisms within a given species.