The Demography of the Gombe Chimpanzees: An Analysis of 42 Years of Vital-Event Data

I will present an analysis, carried out collaboratively with researchers from the Jane Goodall Institutes's Center for Primate Studies at the University of Minnesota, of the demography of the chimpanzees of Gombe National Park, Tanzania. The analysis is based on 42 years of observation of the Kasekela community of chimpanzees. Using an increment-decrement life table methodology, we found strong evidence for sex differences in mortality, with females having lower mortality at all but the youngest ages. Cox regressions fit with an individual-level frailty term on non-first birth intervals indicate that mother's age had no statistically significant effect on birth interval length. This lack of age effects on birth interval length is surprising and is suggestive of a lack of reproductive senescence in free-ranging chimpanzees. In contrast, there was an extremely strong interaction between mother's rank and loss of the previous infant. Lower-ranking females were more likely to lose infants and therefore closed birth intervals more rapidly. In addition, the frailty term, sex of previous infant, and loss of previous infant before age 5 were all significant covariates affecting the duration of birth intervals. We constructed and analyzed an age-structured projection matrix. The asymptotic annual rate of increase of the female dominant Leslie matrix is r=-0.0018, indicating that Kasekela is a declining population. Fitness elasticities show that the growth rate of the population is strongly sensitive to changes in pre-reproductive mortality. We discuss the results in light of conservation efforts. We further discuss the implications of chimpanzee demography for understanding human evolution, focusing particularly on distributional differences in schedules of net maternity and their implications for life-history evolution.