(Charnov, 1993), remain strongly related to each other even when the effects of body size are removed; that is, constraints of size are not what lead to the correlation among traits, as was often assumed in many studies prior to the mid-1980s (see historical review in Harvey and Purvis, 1999).
Both humans and baboons exhibit slow life histories; those of baboons are basically as expected for a primate of their size, whereas some aspects of human life histories tend to be slower than expected (but see, e.g., Hrdy, 1999, and Hawkes, 2002, regarding human “hyperfertility”). That is, primates in general and anthropoid primates in particular have life histories characteristic of much larger nonprimate mammals. They also have particularly long periods of immaturity. Our human quality-based lifestyle runs deep in our phylogenetic history, and we come from a lineage, a family tree, that has at each branch exaggerated or extended the slow lifestyle—to a considerable extent a trade-off of quantity for quality. What explains these patterns, the differences among mammalian orders and the correlations found among life history variables at higher taxonomic levels? Diverse answers to those questions have been proposed, both historically and currently, and the interested reader is referred to Charnov (1993, 2001), Kozlowski and Weiner (1997), the historical review and perspective provided by Harvey and Purvis (1999), and an application to human life history evolution based on Charnov’s approach in Hawkes (2002) and Hawkes et al. (2003).
Most important, however, from the perspective of the current volume and the topic of this chapter—variability within a species, whether humans or baboons—is that good explanations of life history variability and correlations are not necessarily the same for all taxa or at all levels of investigation. The relationship among life history variables within species or populations often is, and is expected to be, different in direction, as well as strength, from that among orders (see, e.g., Lande, 1979; Harvey and Clutton-Brock, 1985; Emerson and Arnold, 1989; Lee et al., 1991; Worthman, this volume, for humans). For example, as a result of ecological sensitivity within, rather than among, species, large body size is often associated with large litters, early maturation, high reproductive rates, and low adult mortality rates, in striking contrast to the relationship of these variables among species of a given mammalian order. Life history theories that apply at one level cannot simply be extrapolated from that level to another—for example, from across mammalian orders or from differences among species within an order to variability within species (see, e.g., Lande, 1979; Emerson and Arnold, 1989; Kozlowski and Weiner, 1997).