Masako Fujita of Michigan State University and colleagues tested the breast milk of 83 mothers in northern Kenya, where men can have multiple wives.
The study, published in the journal Nature and the American Journal of Physical Anthropology, found poor mothers produced fattier milk for their daughters than for their sons, while mothers who were better off financially produced fattier milk for sons.
Men who have multiple wives need wealth to support a larger family -- leaving poor males less competitive for marriage.
Fujita said mothers with less land and fewer livestock provided richer milk to their daughters than to their sons -- because daughters stand a better chance than sons of increasing mothers' status through marriage.
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