Early Reproductive Maturity Among Pumé Foragers: Implications Of A Pooled Energy Model To Fast Life Histories

Life history theory places central importance on relationships between ontogeny, reproduction, and mortality. Fast human life histories have been theoretically and empirically associated with high mortality regimes. This relationship, however, poses an unanswered question about energy allocation. In epidemiologically stressful envi- ronments, a greater proportion of energy is allocated to immune function. If growth and maintenance are competing energetic expenditures, less energy should be available for growth, and the mechanism to sustain rapid maturation remains unclear. The human pattern of extended juvenile provisioning and resource sharing may provide an important source of variation in energy availability not predicted by tradeoff models that assume independence at weaning. We consider a group of South American foragers to evaluate the effects that pooled energy budgets may have on early reproduction. Despite growing up in an environment with distinct seasonal under-nutrition, harsh epidemiological con- ditions, and no health care, Pumegirls mature quickly and initiate childbearing in their midteens. Pooled energy budg- ets compensate for the low productivity of girls not only through direct food transfers but importantly by reducing energy they would otherwise expend in foraging activities to meet metabolic requirements. We suggest that pooled energy budgets affect energy availability at both extrinsic and intrinsic levels. Because energy budgets are pooled, Pumegirls and young women are buffered from environmental downturns and can maximize energy allocated to growth completion and initiate reproduction earlier than a traditional bound-energy model would predict. Am. J. Hum. Biol. 21:430-437, 2009. ' 2009 Wiley-Liss, Inc. Life history models provide important frameworks for understanding many aspects of human ontogeny and reproduction. These models are robust tools for predicting differences in how long an individual should invest in growth before initiating reproduction. This fundamental life history tradeoff builds on the principle that metabolic investments are made among alternate expenditures rela- tive to fitness payoffs. An individual expends energy to support basic maintenance (BMR, organ function, cellular repair, and immune responses) and activity functions. Energy balance in excess of these essential metabolic expenditures can be spent on production. In an immature individual, production is allocated to growth and becomes available for reproduction at sexual maturity (Charnov, 1991; Stearns, 1992). Underlying this shift in energy allo- cation is the assumption that the energy available for growth and reproduction is constrained by an individual's metabolic budget (Fig. 1a). For most mammals, maternal support ends at weaning. Following weaning, an individu- al's energetic budget is limited by the ability to provide its own resource needs, and growth rates are constrained by a juvenile's own foraging efforts. However, the assumption that an individual's metabolic budget is bound is inconsis- tent with human patterns of extended juvenile provision- ing and resource sharing. These human patterns provide an important source of variation in the energy available for growth and reproduction.