Grandparenting and the Evolution of Post-Reproductive Lifespan

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Humans have very long lifespans, a significant portion of which is spent in a non-reproductive state, in comparison to other primates. Although lifespan has increased over the past century, there is good evidence that the pattern of a long, post-reproductive lifespan is not novel, and that recent increases in longevity are just extreme manifestations of the general human pattern. This research project attempts to test several hypotheses for this extension of the human lifespan, focusing on support by mothers of their own offspring; support by grandmothers of grandchildren; the support by fathers of their offspring; and a model of support focusing on embodied capital. Several of these hypotheses emphasize the quantity and quality of help that grandparents provide for their descendants, which can be interpreted as a form of indirect reproduction. The researchers will examine behavioral pathways by which older individuals impact their descendants' well-being, the impacts grandparents have on the growth, health, and morbidity of grandchildren, and the effects such assistance may have on fitness outcomes such as child fertility and grandchild survivorship. The project focuses on the Tsimane, a group of forager-horticulturalists in Amazonian Bolivia. The researcher will work with anthropologists and physicians from Bolivia and the U.S., and study over 2,500 Tsimane living in 22 villages. Methods include demographic and life-historical interviews; analysis of extant data from missionary and public sources; biomedical assessments of health and morbidity; anthropometrical measures; time allocation and resource production observations and interviews; and food consumption and resource transfer observations and interviews. The project represents the first extensive tests of theories concerning longevity that examine the overall fitness effects among kin and the proximate means by which such effects are realized in a traditional small-scale population with a sufficiently large sample size. Such small-scale foraging-horticultural populations are the closest contemporary analogues to our human ancestors, so the new knowledge to be gained will help to advance understanding of our human history. The opportunity to do research with these sorts of populations is fast disappearing due to development and urbanization. The broader significance includes the value of the empirical data and ethnography of aging for a variety of health and welfare planners. The project will actively involve community members in data collection and train local people in primary health care, as well as provide some primary care assistance. In addition, the researchers will organize data on the health status of children and elderly, and the role of inter-generational support networks for the purpose of planning future local health initiatives.