1Department of Archaeology, Simon Fraser University, 2Department of Anthropology, University of Toronto, 3Faculty of Health Sciences, Simon Fraser University, 4Department of Anthropology, University of Missouri
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Mothers in different human populations vary substantially in when they begin introducing non-breastmilk liquids and solids to their infants’ diets, but we do not fully understand why the variation evolved.
One hypothesis holds that mothers adjust their parental investment strategy in response to level of extrinsic risk. This hypothesis assumes mothers focus on offspring quality in low risk environments but concentrate on offspring quantity in higher risk environments and that weaning represents reduced maternal investment in an individual child. It predicts an inverse quadratic relationship between age at introduction of "transitional" foods and level of risk: mothers introduce transitional foods later as risk increases from low to moderate levels and then earlier as risk increases from moderate to high levels.
We tested the risk hypothesis using ethnohistoric data from 38 natural fertility farming and herding populations. We regressed age at introduction of transitional foods on three proxies of risk: subsistence risk, pathogenesis, and infant mortality. We found that introduction of transitional foods is positively rather than inversely quadratically correlated with subsistence risk. The relationship between transitional foods and pathogen risk is negative and linear. Transitional foods is not correlated with infant mortality. Thus, none of our analyses supports the risk hypothesis as currently formulated.
Although the relationships do not follow the predictions of the risk hypothesis, our findings suggest age at introduction of transitional foods covaries with extrinsic risks. These results raise questions about whether introducing non-breastmilk foods represents a shift in form rather than a reduction in maternal investment.
Research funded by SSHRC Joseph-Bombardier Canada Graduate Scholarships (award numbers: 766-2008-1083 and 767-2011-2333), the Canada Research Chairs Program, the Canada Foundation for Innovation, the British Columbia Knowledge Development Fund, and Simon Fraser University.