Mortality rates among Kanyawara chimpanzees

Demographic data from wild chimpanzees are of considerable interest for understanding the evolution of the human life history. Published mortality data, however, come primarily from chimpanzee populations that have recently suffered dramatic, human-induced declines, and exhibit rates of reproduction well below replacement. Here we present a life table for chimpanzees living in the Kanyawara community of Kibale National Park, comprising 1129 individual risk years and 56 deaths. This community has shown modest growth over the past 25 years, avoiding some of the worst impacts of human contact. Sex differences in mortality at Kanyawara appeared similar to those reported from other sites. However, overall mortality rates were significantly lower than those reported from the long-term study sites of Gombe, Taï and Mahale. Kanyawara chimpanzees in this sample had a life expectancy at birth of 19 years, and individuals living to age 14 could expect to live for another 24 years. Life table data from Kanyawara indicate a mean mortality rate of 3.9% per year over the ages of 10-35, substantially less than the equivalent figure of 6.8% from a sample of other long-term chimpanzee study sites. The comparable adult mortality rate from a range of human foraging societies is ∼2%. The Kanyawara data thus suggest an important downward revision in adult mortality rates for wild chimpanzees, but they do not challenge the existence of an important difference in adult mortality between humans and chimpanzees.