The "costly son" hypothesis: Sons exacerbate obstetrical dilemmas in humans and other mammals

Human childbirth has often been considered uniquely difficult among mammals, yet recent evidence suggests that obstructed labor and fetopelvic disproportion are widespread across placental mammals. Here, we propose the "costly son" hypothesis: because male offspring are typically larger at birth, they impose greater obstetric risk on mothers and themselves by increasing the likelihood of fetopelvic disproportion and dystocia. We compiled sex-specific birth weight data for 140 placental mammal species across 11 orders and reviewed published evidence on sex differences in dystocia risk. Male neonates were more commonly larger at birth than females, and male-biased birth size differences were strongly associated with male-biased adult sexual size dimorphism. Species with male-biased adult size dimorphism had sixfold greater odds of exhibiting male-biased birth weight differences than species with female-biased dimorphism. Male-biased birth weight differences existed irrespective of developmental mode (precocial or altricial), indicating that sex differences in fetal growth arise early in gestation. Evidence from humans, livestock, and at least one non-human primate suggests that male fetal sex is associated with elevated dystocia risk, that is, precocial taxa where birth size approaches the mechanical limits of the maternal birth canal. We further hypothesize and discuss how parity and sex ratio adjustment may modulate these risks and associated trade-offs in a Trivers-Willard context. Our findings identify fetal sex as an underappreciated dimension of mammalian obstetrical dilemmas and suggest that the evolutionary benefits of producing larger sons are balanced by increased risks of birth complications and mortality.