Favorable ecological circumstances promote life expectancy in chimpanzees similar to that of human hunter-gatherers

Abstract

Demographic data on wild chimpanzees are crucial for understanding the evolution of chimpanzee and hominin life histories, but most data come from populations affected by disease outbreaks and anthropogenic disturbance. We present survivorship data from a relatively undisturbed and exceptionally large community of eastern chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda. We monitored births, deaths, immigrations, and emigrations in the community between 1995 and 2016. Using known and estimated ages, we calculated survivorship curves for the whole community, for males and females separately, and for individuals ≤2 years old when identified. We used a novel method to address age estimation error by calculating stochastic survivorship curves. We compared Ngogo life expectancy, survivorship, and mortality rates to those from other chimpanzee communities and human hunter-gatherers. Life expectancy at birth for both sexes combined was 32.8 years, far exceeding estimates of chimpanzee life expectancy in other communities, and falling within the range of human hunter-gatherers (i.e., 27-37 years). Overall, the pattern of survivorship at Ngogo was more similar to that of human hunter-gatherers than to other chimpanzee communities. Maximum lifespan for the Ngogo chimpanzees, however, was similar to that reported at other chimpanzee research sites and was less than that of human-hunter gatherers. The absence of predation by large carnivores may contribute to some of the higher survivorship at Ngogo, but this cannot explain the much higher survivorship at Ngogo than at Kanyawara, another chimpanzee community in the same forest, which also lacks large carnivores. Higher survivorship at Ngogo appears to be an adaptive response to a food supply that is more abundant and varies less than that of Kanyawara. Future analyses of hominin life history evolution should take these results into account.

Keywords: Chimpanzees; Demography; Hunter-gatherers; Life-expectancy; Mortality; Ngogo.

Figure 1

Kaplan–Meier survivorship at Ngogo for both sexes combined. Solid line shows expected survival to a given age; dashed lines show 95% confidence intervals.

Figure 2

Kaplan–Meier survivorship curves at Ngogo and Kanyawara for those individuals two years old or younger when first identified.

Figure 3

Kaplan–Meier survivorship curves at Ngogo and Kanyawara, for both sexes combined. Solid lines show expected survival to a given age; dashed lines show 95% confidence intervals.

Figure 4

Simulated survivorship curves for Ngogo and Kanyawara incorporating error into age estimates. The centralcurvefor a given site shows theresults of 100 Kaplan–Meier survivorship analyses, each based on a set of values generated using Equation (1) (see text); the upper and lower curves show the corresponding 95% confidence intervals.

Figure 5

Kaplan–Meier survivorship curves at Ngogo for males and females considered separately.

Figure 6

Differences in yearly survival rates (p_x) between Ngogo and Kanyawara, by sex. The values of p_x are taken from life tables. The red curve is a loess smoothing through the distribution (points not shown) of the maximum improvement of survival that is mathematically possible at each age. The black curve is a loess smoothing though the distribution of the actual survival difference, with points shown in black.

Figure 7

The sensitivity of population growth rates to survival rates of each age. Sensitivity is calculated from the perspective of Kanyawara, the baseline group for this analysis.

Figure 8

Survivorship at Ngogo for both sexes combined compared to that of: a) other wild chimpanzee communities and b) human hunter-gatherer populations. Lines plot the values of l_x as reported in life tables (Kanyawara: Muller and Wrangham, 2014, their Table 1; Taï all periods: Hill et al., 2001; Taï stable period: Boesch and Boesch-Achermann, 2000; Mahale: Nishida et al., 2003, their Table 3; Gombe: Bronikowski et al., 2016; Aché forest period: Hill and Hurtado, 1996; Hadza: Blurton Jones, 2016, their Table SI 8.3; Hiwi: Hill et al., 2007; Dobe !Kung: Howell, 2010).

Figure 9

Difference in mortality profiles (annual probabilities of death, q_x) for Ngogo chimpanzees and Hadza hunter-gatherers. Values give [q_x(Ngogo) q_x(Hadza)] for each age; positive values indicate that mortality was higher at Ngogo, negative values that it was higher for the Hadza. Data are for both sexes combined; Hadza data are from Blurton Jones (2016).

Figure 10

Female survivorship (l_x) in mountain gorillas (data from Bronikowski et al., 2016), muriquis (data from Bronikowski et al., 2016) and Ngogo chimpanzees.