The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Rethinking lifetime reproductive effort in humans: does early weaning provide the fuel to extend the human lifespan?


1Department of Anthropology, Northwestern University, 2School of Sport, Exercise & Health Sciences, Loughborough University

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Large brains, prolonged development, and other derived traits that characterize Homo sapiens entail a significant increase in energetic demands. This may be especially true for human females, who are confronted with a substantial portion of the metabolic costs of our lineages’ intensive reproductive strategy. While previous work has suggested that allocare provided by a wide array of kin and non-kin may help to ameliorate the burden of increased costs of reproduction for females, it is less clear whether this subsidy is so great as to enable surplus allocation to other domains of the life history, such as increased longevity. One approach to clarifying this question is calculating female lifetime reproductive effort (LRE), which reflects the metabolic energy devoted to reproduction over the lifecycle. A recent estimate of LRE among women from a sample of small scale societies (Burger et al. 2010 Proc R Soc B) finds an average LRE close to the predicted value for all mammals. That analysis, however, does not consider the substantial energy savings owing to the provisioning of human infants specially-prepared foods, which reduces the costs of lactation to human mothers. Adjusting for this “complementary feeding,” we find that the average LRE of human females in these societies is well below the invariant value predicted by theory. We conclude that early weaning may have contributed not only to our species’ characteristically high fertility rate, but may have also freed up excess energy, allowing increased allocation to somatic maintenance and thereby contributing to our unusually lengthy lifespan.

JMB is supported by a National Science Foundation Graduate Research Fellowship.

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