Further evidence for masculinization of female rats by males located caudally in utero.
The morphology and behavior of female rodents is partially masculinized as a result of residence near males in the same uterine horn (Clemens effect). Two hypothetical mechanisms have been proposed to account for this effect. In the first hypothesis ("contiguity") androgens secreted by males in utero are proposed to diffuse across the amniotic membrane, reaching adjacent fetuses. In the second hypothesis ("caudal male") androgens are transported via the cervical-to-ovarian blood flow and may diffuse directly between closely apposed uterine veins and arteries. This study was designed to test directly which of these mechanisms appears more influential in masculinizing the morphology of female rats. Pregnant Sprague-Dawley rats were decapitated early on Day 22 of gestation and pups were Caesarean delivered. Their anogenital distance and body weight were recorded, location in utero coded by means of footpad tatooing, and each litter fostered to a maternal female. Measurements were taken again when the animals were weaned. Statistical analysis revealed that the presence of one or more males caudal to a female in the uterine horn has a more critical influence on that female's morphology than contiguity per se. Such a mechanism may result in partial masculinization of dimorphic behaviors later in life.