Testing the Energetics of Gestation and Growth Hypothesis for Human Secondary Altriciality.

The pronounced neurological immaturity, yet increased birthweight of human neonates relative to other primates, known as secondary altriciality, is traditionally attributed to obstetric constraints arising from pelvic adaptations for bipedal locomotion and childbirth-the so-called obstetrical dilemma. The Energetics of Gestation and Growth (EGG) hypothesis instead attributes it to a constrained maternal metabolism. We tested the assumptions of the EGG hypothesis with hypothetical maternofetal dyads using empirical data on maternal metabolic scope and fetal growth. Our results revealed no consistent relationship between fetal and maternal patterns with birth timing. Moreover, there is no evidence that cortisol, the presumed proximate birth mechanism under the EGG hypothesis, is involved in birth timing in humans, thus questioning that metabolic limitations can serve as its trigger. Finally, we propose that secondary altriciality does not result from simple shifts in birth timing but from heterochronic growth patterns related to obstetric constraints.