In utero proximity of female mouse fetuses to males: effect on reproductive performance during later life.

Female mice were delivered by Caesearian section, with females between 2 male fetuses in the uterus (2M females) and females not contiguous to males in the uterus (OM females) being com- pared morphobogically, physiologically and behaviorally during later life. The results reveal that 2M females are masculinized morphologically and behaviorally relative to OM females; 2M females have a larger anogenitab space, are more aggressive in a variety of different situations, are less effective at inhibiting male-induced puberty when group housed and urine mark their environment at higher rates (all masculine traits). On the other hand, OM females are more sexually attractive and arous- ing to males than are 2M females. While no differences were noted between OM and 2M females in terms of their sensitivity to gonadal steroids or in their capacity to produce and raise healthy young in an optimal laboratory environment, the data indicate that in utero contiguity to male fetuses produces a masculine bias in the transmission of and response to reproductively important social cues. Such a bias could alter the probability of mating and of successfully producing healthy offspring in a natural environment.