Life-History Consequences of Density Dependence and the Evolution of Human Body Size

Previous attempts to explain variation in human growth and development emphasize the energetic constraints imposed by malnutrition and disease. However, this approach does not address the evolutionary effects of mortality risk on ontogenetic variation, a common theme in life-history studies. The conventional approach can be reconciled with life-history theory by considering the effect of mortality on the rates and timing of maturity in subsistence-based human populations. Humans slow down growth and development and demonstrate smaller adult body sizes in high-population-density contexts, presumably because of increased nutritional constraints and disease loads. In addition, there is evidence of mortality-based selection for relatively faster/earlier ontogeny in small-bodied hunter-gatherers living at high densities. This finding may be interpreted as an evolved reaction norm for earlier reproductive maturity and consequent smaller adult body size in high-mortality regimes. In sum, comparative results support density-dependent effects on body size that act through two pathways—nutritional constraints and juvenile mortality—at varying intensities, contributing to a nearly twofold range in body size across human societies.