Sexual Differentiation in Litter-Bearing Mammals: Influence of Sex of Adjacent Fetuses in Utero

Abstract

In rodents and swine, individual differences in a broad range of characteristics correlate with intrauterine position during fetal life. By identifying the intrauterine position of mice at cesarean delivery, we can predict reliably postnatal reproductive traits such as genital morphology, timing of puberty, length of estrous cycles, timing of reproductive senescence, sexual attractiveness, sexual behavior, aggressiveness, daily activity level, body weight and tissue enzyme activity in females; in males we can predict genital and brain morphology, sexual behavior, aggressiveness, daily activity level, body weight, and tissue enzyme activity. In mice, as in all mammals, male fetuses have greater concentrations of testosterone than do females. In addition, female mouse fetuses have greater circulating concentrations of estradiol than do male fetuses, a condition not found in all mammals. A mouse fetus positioned between males has greater concentrations of testosterone than does a fetus of the same sex positioned between females, and a fetus positioned between females has greater concentrations of estradiol than does a fetus of the same sex positioned between males. Gonadal steroids regulate differentiation of secondary sexual characteristics. Studies in which the effects of intrauterine position have been eliminated by exposing fetuses to steroid receptor blockers reveal the critical role of steroids in mediating this phenomenon. The intrauterine position phenomenon provides the only mammalian model for relating postnatal traits to concentrations of endogenous hormones to which individuals are exposed during fetal life. Results from studies using this naturally occurring experimental system in litter-bearing species have given insights concerning the consequences of individual differences in steroid concentrations during sexual differentiation that likely apply to all mammals. One specific hypothesis is that circulating estradiol may interact with testosterone in mediating some aspects of sexual differentiation in rodents and, thus, possibly in other mammals. (

Influence of sex ratio during multiple pregnancies on productive and reproductive parameters of lambs and ewes

“…The first conclusion is in good agreement with earlier reports in rodents (Vom Saal, 1989) and swine (Wise and Christenson, 1992) showing no effect of intra-uterine position on birth weight. Furthermore, in rodents (Vom Saal and Bronson, 1978), as in sheep in the present study, body weight at puberty was also unrelated to intra-uterine environment/position.…”

“…Age of vaginal opening (Vom Saal, 1981 ) and age at puberty in swine (Rohde Parfet et al, 1990) were unaffected by earlier intra-uterine position, a conclusion in good agreement with the identical ages of puberty of all groups in the present study. Because differences in ovarian function related to intrauterine position had not been identified in adult rodents (Vom Saal, 1989) In rodents (Vom Saal, 1981) (Vom Saal, 1989). This however does not appear to be the case in swine (Wise and Christenson, 1992 …”

“…or more limited such as observed with the intra-uterine position effect (Vom Saal, 1989). Freemar-tinism, while documented, is very uncommon in sheep (Matejka et al, 1987).…”

Does having a twin brother make for a bigger brain?

“…Conform our expectations, having shared the uterus with a brother as compared with a sister was related to a larger brain volume. Animal research suggests that the intrauterine presence of a male fetus leads to exposure to higher levels of prenatal testosterone within other fetuses, compared with the intrauterine presence of a female fetus, leading to more masculine brain morphology (14). Furthermore, prenatal exposure to sex steroids is implicated in the development of sex differences in brain structure (8).…”

“…More specifically, a male fetus has a higher blood level of testosterone than a female fetus, but irrespective of its own sex, a fetus located between male fetuses has a higher concentration of testosterone than a fetus positioned between females (14). This phenomenon in turn influences several anatomical and behavioral parameters, such as reproductive organs and aggressive behavior (13).…”