Blastocysts prepare for the race to be male.

Recent findings in different mammalian species have demonstrated that XY embryos grow faster than XX embryos before the gonads are differentiated. In mice and cattle, accelerated development is already evident in XY blastocysts, while in the rat and in human fetuses a quantitative sex difference has been shown to be present before testicular differentiation has occurred. These data demonstrate that in these species the histological differentiation of the testis, which occurs early and rapidly, is preceded by an increased growth rate of the embryo. This may be expected to increase the probability of the gonad reaching the threshold for testis development, since it is known that developmental delay can result in ovarian differentiation. It is postulated that the fast development of the male may be an adaptation to the reproductive biology of eutherian mammals, in which development of both sexes occurs in the hormonal environment of the uterus. The question is raised as to a possible connection between sex-related growth and other sex differences, such as longevity.