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.Keywords milk composition and yield; parity; infant growth; maternal effects; life history For mammalian females, lactation is the most costly form of parental investment. Mothers vary in their ability to meet the costs of lactation, and maternal effects on infant outcomes are common among mammals. As such, maternal influences on infant phenotype contribute to the variation on which natural selection acts (Mosseau and Fox, 1998;Räsänen and Kruuk, 2007). In nonhuman primates, infant growth and survival have been linked to variation in maternal weight, parity, age, and social rank as well as the timing of birth (in Chlorocebus aethiops; age and weight Fairbanks and McGuire, 1995; in Papio sp maternal age; Cheney et al., 2004; rank and parity;Altmann and Alberts, 2005; in Macaca mulatta maternal age and adiposity; Johnson and Kapsalis, 1995; birth-timing, Drickamer, 1974; Fairbanks, 1996 andMaestripieri, 2009). It is likely that these maternal effects are at least partially mediated through the quantity or quality of milk that mothers provide their infants during lactation, but currently little is known about interindividual differences in milk synthesis.To develop a full understanding of parent-offspring conflict and life-history theory in primates, it is important to identify the sources, magnitude, and consequences of variation in milk composition and output (Trivers, 1974;Bowman and Lee, 1995;Johnson and Kapsalis, 1995;Lee, 1996;Johnson, 2003). Although variation in...

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