The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

In poor families mothers’ milk is richer for daughters than sons: an example of Trivers-Willard effects on mother’s milk in northern Kenya


1Department of Anthropology, Michigan State University, 2Department of Anthropology, University of Victoria, 3Center for Statistical Training and Consulting, Michigan State University, 4Department of Counseling, Educational Psychology and Special Education, Michigan State University

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The Trivers-Willard hypothesis predicts unequal parental investment between daughters and sons depending on maternal condition and offspring reproductive potential. Specifically, it predicts that given higher reproductive variance in males, mothers in good condition should invest more in sons than daughters while mothers in poor condition should invest more in daughters than sons. For human parental investment, milk synthesis is energetically costly, yet variations in milk synthesis in relation to the offspring’s sex are poorly understood. This study tests the Trivers-Willard hypothesis on human milkfat concentrations. Data from exclusively breastfeeding mothers (n=72) in Ariaal agro-pastoral villages of northern Kenya were used to test the hypothesis that economically sufficient mothers will produce higher milkfat for sons than daughters while poor mothers will produce higher milkfat for daughters than sons. A linear regression model was applied, using log transformed milkfat as the dependent variable, and offspring’s sex (Son=1/Daughter=0), wealth status (Poor=1/Not poor=0), and the sex-wealth interaction as the predictors. The model controlled for maternal age, parity, postpartum time, body mass index, dietary fat intake, breastfeeding frequency, and household composition. Results supported the hypothesis: offspring’s sex and wealth interacted (p=0.002) with milkfat. The model estimates that economically sufficient mothers produce richer milk for sons than daughters (2.6 vs. 1.5 g/dL) while poor mothers produce richer milk for daughters than sons (2.3 vs. 2.0 g/dL). Further research on the variation in milkfat in relation to the offspring’s sex is warranted.

Sponsor: NSF Dissertation Improvement Grant #0622358; the Wenner-Gren Foundation; the Micronutrient Initiative

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