Although worldwide childhood mortality rates are declining, there continue to be disparities in child mortality related to household income and sex of the infant, particularly in places where gender discrimination and malnutrition are common. One factor that may contribute to these differences is the level of protection that children receive from their mothers' milk. This project will test evolutionary theories about parental investment by analyzing breast milk samples, to see whether the beneficial contents of breast milk differ for mothers of sons versus daughters, by maternal nutritional status, and by a combination of the two. The findings will inform efforts in modern society to reduce the mortality of vulnerable children. The project will provide opportunities for women and minority students to be trained and mentored in laboratory methods and to participate in research with global health implications.
The Trivers-Willard hypothesis (TWH) predicts unequal parental investment between daughters and sons in polygynous populations where somatic or economic conditions of males determine their marriageability. Specifically, it predicts that mothers in good condition will invest more in sons while mothers in poor condition will invest more in daughters because these strategies may enhance their reproductive success. Previous studies on sex bias in human milk quality, particularly milk nutrient levels, have reported mixed results. These studies focused on energy-yielding nutrients (e.g. fat, sugar), but not on other nutritive and non-nutritive components of milk that are indispensable for infants to survive and thrive. This project contributes to this body of research by examining the TWH using not only measures of breast milk energy, but also vitamins (B9 or folate) and antibodies (secretory immunoglobulin A or sIgA) as indicators, of maternal investment. A cross-sectional random sample of breast milk specimens from 220 mothers of the Ariaal tribe, a polygynous population of rural Kenya, will be analyzed. The resulting data will be used to test the following predictions: 1) maternal nutrition (e.g. adiposity, micronutrient status) will positively predict folate/sIgA, 2) this effect will be moderated by the sex of the infant, and 3) milk of under-nourished mothers will exhibit preference toward daughters and milk of other mothers toward sons. The findings will facilitate a multi-faceted view on possible sex bias in human milk components in a specific sociocultural and ecological context. This will contribute intellectually to our understanding of conditional variation in human parental investment, which has important implications for sex differences in infant growth and health.
