Rhesus macaque milk: magnitude, sources, and consequences of individual variation over lactation

Abstract

Lactation represents the greatest postnatal energetic expenditure for mammalian mothers, and a mother's ability to sustain the costs of lactation is influenced by her physical condition. Mothers in good condition may produce infants who weigh more, grow faster, and are more likely to survive than the infants of mothers in poor condition. These effects may be partially mediated through the quantity and quality of milk that mothers produce during lactation. However, we know relatively little about the relationships between maternal condition, milk composition, milk yield, and infant outcomes. Here, we present the first systematic investigation of the magnitude, sources, and consequences of individual variation in milk for an Old World monkey. Rhesus macaques produce dilute milk typical of the primate order, but there was substantial variation among mothers in the composition and amount of milk they produced and thus in the milk energy available to infants. Relative milk yield value (MYV), the grams of milk obtained by mammary evacuation after 3.5-4 h of maternal-infant separation, increased with maternal parity and was positively associated with infant weight. Both milk gross energy (GE) and MYV increased during lactation as infants aged. There was, however, a trade-off; those mothers with greater increases in GE had smaller increases in MYV, and their infants grew more slowly. These results from a well-fed captive population demonstrate that differences between mothers can have important implications for milk synthesis and infant outcome.

Fig. 1

On a per calorie basis, infants nursing from mothers that produced low-energy milk received more grams of sugar and protein and fewer grams of fat than did infants nursing from mothers that produced high-energy milk at 1 month (a) and 3.5 months (b) of infant age. Results remained significant (P < 0.01) even after removing the high GE outlier. *P = 0.001, **P < 0.0001.

Fig. 2

Milk yield value was associated with maternal parity at 3.5 months of infant age even accounting for infant age, sex, and maternal weight.

Fig. 3

Infant growth rate (grams per day) was positively associated with the increase in milk yield value (MYV) from 1 month to 3.5 months of infant age after controlling for infant age and baseline MYV at 1 month of infant age.