Singleton female mouse fetuses are subsequently unresponsive to the aggression-activating property of testosterone.

Eight-day pregnant mice were surgically prepared so that they carried a single fetus which was delivered by cesarean section on Day 18, one day prior to term. Other animals were permitted to carry their entire complement of fetuses which also were delivered surgically. Their position within a uterine horn relative to one another was noted. Adult ovariectomized testosterone-treated females that had been situated in utero contiguous to no male fetuses attacked an olfactory bulbectomized adult stimulus male. In contrast, females that resided singly in the uterus were unresponsive to the hormone; they displayed no aggressive behavior. Masculinization, therefore, appears to occur in utero in the absence of contiguity to male fetuses.