Richer milk for sons but more milk for daughters: Sex‐biased investment during lactation varies with maternal life history in rhesus macaques

Lactation represents the greatest postnatal energetic expenditure for human and non-human primate females, and the ability to sustain the costs of lactation is influenced by a mother's physical condition. This is especially true for young mothers that initiate reproduction shortly after adolescence. These mothers have fewer bodily reserves available for lactation and face tradeoffs between reproduction and their own growth. Milk synthesis among captive rhesus macaques (Macaca mulatta) was investigated at the California National Primate Research Center from 2005 to 2007 (N = 114). Rhesus macaques produced low energy density milk typical of the primate order, but there was substantial individual variation among mothers in both milk energy density and yield. As a consequence, the available milk energy (AME), the product of milk energy density and milk yield, to support infant growth, development, and activity, varied tenfold among mothers. Primiparous mothers (N = 40) had fewer bodily resources, as measured by mass and body mass index, available for lactation than did multiparous mothers (N = 74) and showed poorer lactational performance. Mothers of sons produced milk of higher energy density, especially primiparous mothers, but lower milk yield, such that AME was the same for sons and daughters. Although AME from the mother was the same for sons and daughters, there was significant sexual dimorphism in infant mass. These data indicate that selection has likely favored sex-specific regulation of growth and development that is not necessarily contingent on greater maternal investment.