The maternal energy hypothesis of brain evolution: an update

Abstract

Bivariate scaling analyses can reveal interesting correlations between individual biological variables, but inference of actual causal links in complex networks requires multiple tests to satisfy the criterion of isolation. Mammalian brain tissue has high energy demands, so energy supply is inevitably a key issue in evolution of the primate brain, especially for large-brained hominids. Various hypotheses have proposed a direct link between brain size and metabolic turnover in adults, but the author’s Maternal Energy Hypothesis (MEH) instead focuses on energy supplied by the mother during brain development up to weaning. This hypothesis is supported by various empirical findings, but it has also been challenged, particularly on the basis that these findings do not survive tests conducted to eliminate effects of phylogenetic inertia. New comparative analyses of brain size in mammals with improved datasets have, however, confirmed links to both basal metabolic rate and gestation period, complying with core predictions of MEH. The evidence now available in support of MEH is reviewed and some implications for brain evolution are explored. A widely recognized general trend towards increase in average relative brain size during mammalian evolution has recently been challenged by a study of brain size in bats that inferred, exclusively through analysis of data from extant species, that brain size has actually undergone reduction in numerous lineages. It is shown that the statistical test used to test for directionality of evolution was inappropriate. A review of fossil evidence for brain evolution in primates, cetaceans and carnivores confirms the generally accepted trend towards increased average brain size through the Tertiary. Progressive increase in mammalian relative brain size over time is at least partially attributable to increases in the level and efficiency of maternal investment. Energetic aspects, including those invoked in the MEH, are of special importance for outstandingly large-brained mammals such as hominids.