tively more frequent (e.g., Cheney and Seyfarth, 1977; Shopland and Altmann, 1987). Within groups, adult females form clear dominance hierarchies that are predominantly stable both within and across generations as juvenile daughters assume the “family rank” about a year before menarche. Dominance rank in males, in contrast, is much more highly dependent on size and strength and is highly age dependent and unstable (Alberts et al., in press; Packer, 1979; Packer et al., 2000).

The larger African carnivores—leopards, lions, and hyenas—prey on baboons and are a particular a risk at night. In each habitat where they are found, members of a baboon group sleep close together either on cliff edges or high in those trees in their habitat that would be the most difficult for a predator to climb. Of the two major tree species in Amboseli, for example, baboons prefer fever trees, Acacia xanthopholea, to umbrella trees, A. tortilis; fever tree branches are higher off the ground, smoother, and more vertical. For baboons the distribution of sparsely scattered nighttime roosts, as well as of potable water and food resources, affects patterns of encounters between groups, daily travel, and seasonal variability in these patterns. In Amboseli, baboons of the fully wild-foraging groups awaken and descend from their sleeping trees shortly after dawn. For the next 11 to 12 hours, they spend almost 75 percent of their time foraging—feeding or traveling to food—across their short-grassland savannah habitat, approximately 10 percent socializing, and the remainder resting, often in a midday siesta.

Baboon infants weigh a little less than 1 kg at birth. In the first few months of life the infant clings to its mother’s ventrum and thereby obtains continuous nipple access and transportation during the 8 to 10 km of daily travel. Gradual nutritional and locomotor independence develops during the next year until the infant’s mother conceives again when the infant is about 18 months old and weighs approximately 3 to 4 kg. Although adult male baboons are approximately double the body mass of adult females, infant and juvenile females are very close in size to their male age peers, and almost all of the sexual dimorphism in body size arises during an adolescent growth spurt in males after females reach menarche between 4 and 5 years of age.

Life History Patterns in Amboseli

To analyze baboon life histories and life history variability, we used data collected from 1971 to 1999 for approximately 600 individuals living in completely wild-foraging groups of baboons (Alberts and Altmann, 2003). We constructed life tables with 1-year age classes and the corresponding survivorship and fertility entries of a population projection matrix, shown for females in Table 6-1 and males in Table 6-2. The tables also

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