Pregnancy and lactation

In this chapter, we draw on life history theory to create a comparative framework for exploring pregnancy and lactation as coevolved elements of the primate reproductive strategy, broadly defined. We draw on the physiological evidence to create a picture of how pregnancy and lactation represent a highly integrative reproductive strategy. Life history theory offers several advantages for understanding this integrative primate reproductive strategy. Firstly, by comparing life history characteristics across nonhuman and human primates we can examine benefits and costs to this energetically expensive strategy and how natural selection may have tailored aspects of the larger strategy toward more species specific ends. Secondly, life history theory emphasizes trade-offs between survival and current reproduction, current and future reproduction, and number, size, and sex of offspring among many others (Stearns, 1992). We take the position that these trade-offs must be examined from the dual vantage point of the mother and the fetus/infant since the two will not always have completely overlapping interests. The chapter unfolds across four sections. In the first section, we explore the primate reproductive strategy by comparing life history parameters across broad phylogenetic groupings, making note of similarities and differences. The next section examines gestation from the perspective of maternal constraint, maternal investment in reproduction, and physiological mechanisms that might shift the cost of any given pregnancy. Mechanisms that might enhance fetal survival and offer developmental cues are also explored.