Science & Education, The Field Museum, Chicago
March 27, 2015 1:00, Grand Ballroom A/B
Average adult relative brain size in higher in primates than in other mammals, but considerable overlap exists. Although human relative brain size is largest, dolphins have the next largest values. Nevertheless, throughout fetal development, and hence as neonates, primates have relatively larger brains than all other mammals. This distinctive feature highlights the importance of gestation for brain development. Along with similar scaling of brain size and basal metabolic rate, this suggests a link with maternal energy turnover. Starting from a negative correlation between brain and gut size in primates, the alternative “Expensive Tissue Hypothesis” infers that a trade-off operates. But analysis of a large dataset for mammals revealed no significant relationship between brain and gut size after eliminating confounding factors. By contrast, analyses allowing for confounders supported the competing Maternal Energy Hypothesis. Human brain development shows several unique features. Neonates are relatively large with correspondingly large brains, yet the brain quadruples in size after birth rather than doubling as in other primates. Nevertheless the human brain is so big at birth that it poses an “obstetric dilemma”: The fetal head only just passes through the pelvic canal after complex rotation. In a recent challenge, it was proposed that the primary constraint is posed by maternal energy costs reaching a maximal level by birth. But “genetic pruning” clearly selects against overlarge neonatal heads and undersized pelvic canals. The pelvic constraint in human birth is highlighted by the lack of any constraint on birth of large-brained neonates in pelvis-less dolphins.