Variation in infanticide and parental behavior in male mice due to prior intrauterine proximity to female fetuses: elimination by prenatal stress.

When sexually-naive male mice are placed together with newborn young, some males will commit infanticide (kill the young) while others will behave parentally (retrieve the young to a nest and keep them warm). The intrauterine position of male mouse fetuses, which is recorded at Cesarean delivery, has previously been found to influence the titers of estradiol that male fetuses are exposed to in utero. In adulthood, most male mice that developed in utero between male fetuses (2 M males) behaved parentally toward young, while most males that developed in utero between female fetuses (0 M males) committed infanticide. When 0 M and 2 M males were castrated at birth and tested with young in adulthood, few males committed infanticide. But, when these same males were tested with young after 25 days of treatment with testosterone, more 0 M than 2 M males committed infanticide and more 2 M than 0 M males behaved parentally. In contrast to the above findings, more 2 M than 0 M males that were castrated when 28 days old behaved parentally without treatment with testosterone; few of the non-parental males committed infanticide, but, instead, ignored the young. Finally, stressing pregnant mice by placing them under bright lights during the last third of pregnancy eliminated the effect of developing next to female fetuses in the male offspring, and all prenatally-stressed males resembled 2 M males in their behavior toward young: most prenatally-stressed males were parental rather than infanticidal when tested with young. The significance of these findings to models of hormonal effects on sexual differentiation is discussed.