Breast-feeding: matching supply with demand in human lactation

The potentially conflicting nutritional demands of parent and offspring during lactation may be considered across animal species in terms of cost-benefit analysis of parental investment. Based on Trivers' (1974) kinship theory, Peaker (1989) proposed that parents invest in their young to an extent which increases survival of the present offspring, but not necessarily to an extent which would decrease the parent's ability to invest in other offspring, including those not yet born. When the cost to the mother potentially compromises the parent's ability to produce future offspring, then she may choose to discontinue that investment. At one level, this investment is determined genetically: there is ample evidence that the time-course of lactation and the upper limit to milk production is set by the mother. The system, on the other hand, must be able to exploit unexpected natural advantages or cope with modest set-backs. This is certainly the case: the rate of milk secretion and duration of lactation varies with nutritional state. Thus, when nutrition is not limiting, and breeding does not have to be fitted into a tight seasonal window, the young will benefit accordingly. When food is in short supply, the outcome of the investment analysis is more complicated. Some species, such as rodents, may kill some or all of their litter, choosing instead to invest in the next pregnancy and lactation. In other species with long gestation periods, and a long-term commitment to the offspring, the mother may continue to favour the offspring despite the short-term disadvantage she may incur.