Original Article Low serum vitamin A mothers breastfeed daughters more often than sons in drought-ridden northern Kenya: a test of the Trivers–Willard hypothesis Masako Fujita a, ⁎ , Eric A. Roth b , Yun-Jia Lo c , Carolyn Hurst a , Jennifer Vollner a , Ashley Kendell a a Department of Anthropology, Michigan State University, MI b Department of Anthropology, University of Victoria, Canada c Center for Statistical Training and Consulting, and the Department of Counseling, Educational Psychology and Special Education, Michigan State University, MI Initial receipt 27 May 2011; final revision received 10 November 2011 Abstract The Trivers–Willard hypothesis predicts that natural selection should favor unequal parental investment between daughters and sons based upon maternal condition and offspring reproductive potential. Specifically, it predicts that mothers in good condition should increase investment toward sons, while mothers in poor condition should favor daughters. Previous tests of the hypothesis in human populations overwhelmingly focused on economic resources as maternal condition indicators. We test the Trivers–Willard hypothesis using maternal nutrition—energy and vitamin A status representing macro- and micronutrition, respectively—as the indicator for maternal condition, with breastfeeding frequency recalls serving as the indicator for parental investment. Data from exclusively breastfeeding mothers (n=83) in drought-ridden Ariaal agropastoral villages of northern Kenya were used to test the hypothesis that mothers in poor condition will breastfeed daughters more frequently than sons. Poor condition was defined as having a body mass index b18.5 or serum retinol (vitamin A) concentration b1.05 μmol/l. A linear regression model was applied using breastfeeding frequency as the dependent variable and respective maternal condition, infant's sex, and the maternal condition–infant's sex interaction as the predictors, controlling for covariates. Results supported the hypothesis only in the vitamin A model which predicts that low-vitamin-A mothers breastfeed daughters significantly more frequently than sons (11 vs. 6 times/day), while vitamin-A-sufficient mothers breastfeed daughters and sons equivalently (9 times). These results indicate that maternal nutritional status, particularly micronutrient status, can contribute to the investigation of the evolutionary hypothesis of sex-biased parental investment. Published by Elsevier Inc. Keywords: Parental investment; Trivers–Willard effect; Nutrition; Vitamin A; BMI; Breastfeeding; Kenya 1. Introduction The Trivers–Willard hypothesis predicts that, under certain conditions of the mother and the reproductive potential of her offspring, natural selection should favor unequal parental investment between daughters and sons. Specifically, the hypothesis predicts that mothers in good condition should increase investment toward sons, while mothers in poor condition should favor daughters (Trivers & Willard, 1973). This prediction is based on the observation that males tend to have higher reproductive variance than females in polygynous mammalian species, including humans. Different variances arise due to the biologically based differential parental investment of mothers and fathers, with females spending disproportionately more time and energy for each offspring from conception to birth and beyond (Roth, 2004). This difference may be amplified in polygynous societies where males can have a large number of mates/spouses, further increasing their reproductive success relative to males with an exclusive single partner (Roth, 2004). Given this assumption of sex-based differential in reproductive potential, when offspring reproduction is affected by maternal condition, mothers in good condition who can provide more resources to the offspring potentially achieve higher reproductive fitness through preferential Evolution and Human Behavior 33 (2012) 357 – 364 ⁎ Corresponding author. Masako Fujita, Dept of Anthropology, Michigan State University, 328 Baker Hall, East Lansing, MI 48824. E-mail address: masakof@msu.edu (M. Fujita). 1090-5138/$ – see front matter. Published by Elsevier Inc. doi:10.1016/j.evolhumbehav.2011.11.006