Primate Milk in an Evolutionary Perspective

Lactation is a defining characteristic of mammals. Although all mammals lactate, there are a remarkable number of lactation strategies that have evolved. The lactation strategy of a species is the integration of the duration of lactation, the frequency of suckling, the volume of milk the mother produces per day, and the composition of that milk. Primates in general have relatively long lactations, nurse their infants frequently, and produce large quantities of dilute milk. Within this general pattern there is variation among species that exhibits phylogentic patterns. For example, human milk is much more similar to chimpanzee milk than it is to baboon or macaque milk. Human milk may exhibit some unique characteristics. For example, human milk has very high concentrations of secretory IgA (SIgA); the level of SIgA in mid lactation human milk exceeds that measured for colostrum of rhesus macaques. Several lemur species from Madagascar produce relatively similar and invariant milks. It appears impossible to distinguish between the milks of different females, or even between species. Other primate species appear to produce more variable milks. There is convincing evidence in the rhesus macaque and the common marmoset that female condition has a significant effect on milk composition. Even in these variable milks, however, there appear to be aspects of constancy, which may reflect phylogentic constraints.