Katie Hinde: UCLA Anthropology

Lactation represents the greatest post-natal cost of mothering in primates and numerous studies have
established that variation in maternal condition is associated with infant growth, development, health, and
survival. Presumably the effects of maternal condition are mediated through milk output during lactation,
however this relationship remains poorly understood. Here I present the first systematic investigation of the
magnitude, sources, and consequences of individual variation in mother’s milk for an Old World monkey.
Rhesus macaques produce dilute milk typical of the primate order, however there was substantial variation
between mothers, as well as within mother, over lactation in milk composition and yield, and therefore the milk
energy available for the infant. Maternal life history was associated with milk yield and milk energy density was
biased in favor of sons, especially first-born sons. Infants that had higher available milk energy at one month
of age were characterized by higher activity levels and greater confidence at 3.5 months of age suggesting that
mother’s milk may serve as a nutritional cue that calibrates infant behavior and temperament to maternal
and/or environmental conditions. These results, obtained from a well-fed captive population, demonstrate that
small differences between mothers can have important implications for lactational investment and infant
outcomes.