Dakota E McCoy, Jennifer Kotler, Brianna Weir, Arvid Ågren

In their recent Nature Ecology and Evolution paper, Webb and colleagues show that chimpanzee pelvises present a tight fit for newborn infants, just like in humans. Their detailed 3D characterization shows that the degree of the squeeze is comparable between humans and chimpanzees, and that both have sexually dimorphic pelvises. The authors challenge the so-called “obstetric dilemma” – the long-standing hypothesis that pregnancy and childbirth are particularly dangerous in humans as a consequence of being both bipedal and large-brained. If cephalopelvic proportions are an insufficient explanation for complications in human pregnancy, new perspectives are required to address this dilemma. Webb and colleagues suggest that humans sit at one end of a spectrum of complicated primate births shaped by a gradual series of obstetric compromises, exemplified by the metric of birth canal contortion. Here, we offer an additional, complementary explanation for difficult human childbirth. Maternal-fetal genetic conflict over resource allocation explains many of the most severe complications of pregnancy and childbirth. Two of the most common complications are hypertension and hemorrhage, both of which result directly from fetal manipulation of maternal vasculature to increase resources flowing to the placenta. We suggest that maternal-fetal conflict is more severe in humans than in other apes as a result of cooperative breeding. Humans have more cooperative child care than other apes, so relatives can help compensate for relatively poorer infant outcomes if the mother is marginally more weakened in pregnancy.