Womb Neighbors Shape Brain Development

In litter-bearing mammals, a fetus’s position in the uterus relative to same- or opposite-sex siblings influences its development, behavior, reproductive success, and survival into adulthood. A study by Prof. Lee Koren and colleagues, published in Biology of Sex Differences, used wild nutria (Myocastor coypus), a large rodent with natural variation and large litters, to explore how uterine position affects neuroendocrine pathways in the fetal brain. The researchers examined the expression of neuronal components in three brain regions of free-ranging nutria fetuses and found higher expression of certain receptors and enzymes in females compared to males. Uterine position further influenced sex-dependent expression patterns. Coordination between hormone systems was found to be strongest early in gestation, with females showing greater coordination in the second trimester and males in the third. These findings uncover fundamental mechanisms by which fetal environment and sex interact to shape lifelong neuroendocrine function and developmental outcomes.