Comparative studies serve to anchor our perception of human or non-human primate traits to our shared biological history and some basic relationships such as those among body size, phylogeny, and life histories. They leave unanswered, however, questions about current dynamic patterns that are shaping behavior, life history variability, ecological responses, and evolutionary potential within species. These require analyses of lifetimes and of the factors that influence them. Here we present two analyses of life history variability in savannah baboons. First, we use matrix demography models to examine the relative strength of selection on different vital rates. In particular, we examine the sensitivity of fitness to comparable changes in infant survival and adult fertility. Second, we evaluate the variability existing in a natural population and the extent to which behavior, particularly choice of habitat and social environment, affects vital rates for both females and males.

Until recently, humans were the only primate species for which the requisite life history data were available for detailed analysis of life history variability (see Blurton-Jones et al., 1999, and Kaplan and Lancaster, this volume, and references in both). However, for a small handful of species, these data are accumulating for at least some life history components, and we provide here one of the first such analyses for the large, sexually dimorphic, predominantly terrestrial and highly social baboon, Papio cynocephalus. Selective omnivores, baboons are widespread throughout Africa and occupy a broad range of habitats from mountain through woodland and savannah to semidesert. The data presented here derive primarily from a study underway for more than three decades of the Amboseli baboon population, which resides in the basin to the north and west of Mount Kilimanjaro.


Baboons live in discrete social groups. Members of a group forage during the day and sleep at night in much closer proximity to each other than to members of other groups, and virtually all social interactions are among members of the group of residence. A female usually spends her whole life in the group into which she was born, whereas a male leaves his natal group around the time he attains full adulthood at 8 years. Although groups are sometimes in close proximity, the boundaries are usually very clear, spatially as well as behaviorally. The amount of time that groups spend in close proximity is of relatively short duration and can be somewhat tense, even in habitats or years in which these encounters are rela-

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