Life history trade-offs in growth and immune function: The behavioral and immunological ecology of the Shuar of Amazonian Ecuador, an indigenous population in the midst of rapid economic and ecological change
Blackwell, Aaron D., 1978-
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Blackwell, Aaron D., 1978-
Life history theory examines the allocation of resources among competing demands, including growth, immune function, and reproduction. Immune function can itself be divided into innate, cell mediated, and humoral responses. For humans, factors like economic condition, disease exposure. and social milieu are all hypothesized to affect life history allocations. For the Shuar of Amazonian Ecuador these factors are rapidly changing as traditional subsistence hunting and horticulture give way to wage labor and Western medicine. This dissertation presents fieldwork conducted amongst the Shuar between 2005 and 2009. It is among the first studies to test for life history trade-offs between different branches of immunity and growth across market conditions. Shuar data include anthropometrics (n=1,547), biomarkers (n=163), and household compositions (n=292). Comparison samples include the Shiwiar of Ecuador (n=42), non-indigenous Ecuadorian colono children (n=570), the Tsimane of Bolivia (n=329), and the 2005-2006 U.S. NHANES (n=8,336). The dissertation finds significant differences between both populations and Shuar villages in growth and immunity. Increasing market integration is associated with poorer growth, but household factors mediate these changes. Adult males have positive effects on child growth in acculturated areas with wage labor and in distant areas where fishing and hunting remain important but not in intermediate areas. Children have consistent negative effects on one another's growth, suggesting competition for resources. Poorer growth is also associated with higher levels of immunoglobulin E (IgE), a humoral response to helminths. In contrast, C-reactive protein, an inflammatory marker, has a positive association with growth. This divergence between humoral and innate immunity is consistent with a lasting reallocation of immune resources towards a T H 2 response in helminth infected individuals. The age-profile of IgE also varies across market conditions: comparing the Shuar with samples from the U.S. and Bolivia, the age of peak IgE is correlated with the level of peak IgE in each population, providing some of the first evidence for a "peak shift" in immune response. Overall, these results support the hypothesis that local conditions lead to the adaptive "tuning" of trade-offs between branches of immunity and growth. This dissertation includes previously published and unpublished co-authored material.