Growing Up Shuar: Life History Tradeoffs and Energy Allocation in the Context of Physical Growth Among an Indigenous Amazonian Population

Abstract

Life history theory assumes that organisms allocate energetic resources (i.e., calories) to primary life functions such as maintenance, growth, reproduction, and physical activity in a manner that maximizes fitness. Under this conceptual framework, energy is limited, resource allocation is dynamic, and functional tradeoffs between competing metabolic processes are expected to occur. Life history tradeoffs have been invoked to explain biological variation across a range of species and ecological contexts. However, few studies have examined patterns of energy allocation and tradeoffs during human development, restricting fundamental understandings of human life history, phenotypic plasticity, and health. This dissertation investigates human energy allocation and life history tradeoffs in the context of physical growth among the Shuar, a small-scale indigenous population from Amazonian Ecuador. Mixed-longitudinal anthropometric data were collected from 2,553 Shuar between the ages of 0-29 years. Additional market integration (i.e., production for and consumption from a market-based economy) and immune activity (i.e., finger-prick blood biomarker) data were obtained from a subset of children and adolescents. Analysis was performed to explore variation in physical growth at the population, regional, household, and individual levels. Results demonstrate that, as a population, the Shuar grow significantly more slowly than international references and experience several unique developmental characteristics that may be explained by energetic life history tradeoffs. Between-individual variation in Shuar growth, however, is substantial. A large portion of this variation is explained by household-level market integration factors associated with differences in diet, lifestyle, and pathogen exposure. Among Shuar children, linear growth is negatively related to several diverse biomarkers of immune function, such that growth is reduced by as much as 83% during intermittent periods of elevated immune activity. These tradeoffs occur over timeframes as short as one week and are typically avoided by children with adequate energy reserves (i.e., high levels of subcutaneous body fat). Taken together, these findings provide evidence for an important role of energetic tradeoffs in shaping patterns of human ontogeny and health.