Comparative survival analyses among captive chimpanzees (Pan troglodytes) in America and Japan

Abstract

Detailed, long-term datasets on the life histories of long-lived species such as great apes are necessary to understand their survival patterns but are relatively rare. Such information requires prolonged and consistent record-keeping over many generations, so for chimpanzees (Pan troglodytes), this equates to many decades of input. As life history variables can be altered by differences in environmental influences (whether natural or artificial), there is substantial value to being able to compare across populations. Here, we present the first comparative analysis of life history data for two ex situ chimpanzee populations residing in North America (1975-2020; n = 730) and Japan (1980-2020; n = 660). Overall, survival patterns were similar between regions, and the median life expectancy from birth is estimated at 35.7 (95% CI = [32.4-40.0]) years for females and 30.1 (27.3-34.3) years for males across both populations. Females who survive to their first birthday are estimated to survive 42.4 (40.0-46.3) years and males 35.5 (32.6-38.0) years. We found that birth type (wild-born or captive-born) did not influence survival patterns in either population, but there were differential effects of sex on longevity. In the America population, males had higher mortality rates than females, whereas in the Japan population we found no differences between the sexes. First year mortality did not differ between populations for males (18-20%), but for females it was lower in America (15%) compared to Japan (25%). Survival patterns of chimpanzees in the present study will be useful for future investigation into potential causes of regional differences and cross-species comparisons.

Keywords: America; Captivity; Chimpanzee; Japan; Life history; Life table; Longevity; Mortality; Survival analyses.

Figure 1. Age-specific survival rates for females and males, from life tables for a wild population and the AZA and Japan captive populations of chimpanzees from birth (top) and from age 1 (bottom).

Wild population data is from Gombe, Tanzania (Bronikowski et al., 2016).

Figure 2. Observed distribution of ages at death by sex, time period, and birth type in (A) the AZA and (B) the Japan captive chimpanzee populations.

Only deaths occurring on or after age one are included. The “early” time period is 1975–2000 for AZA and 1980–2000 for Japan, and the “recent” period is 2001–2020 for both. Sample sizes for the number of observed deaths for females and males are included in parentheses for each group. Note that the y-axis range differs between panels.

Figure 3. Median age and total population size of the AZA and Japan captive chimpanzee populations over time.

Ages were tabulated starting at age one and censused at Dec. 31 of each year (except for the last year, which was censused on the last data compilation date of Feb. 1, 2020). Individuals who had not yet reached age one or had an unknown birth location (30 individuals in the Japan population and 6 in the AZA population as of Feb. 2020) were not included in this figure.

Figure 4. Numbers of deaths in the AZA (top panel) and the Japan (bottom panel) captive chimpanzee populations by month (AZA: 1975–2020, Japan: 1980–2020).

Seasons are grouped in alternating colors: winter = Dec–Feb (green); spring = Mar–May (gray); summer = Jun–Aug (green); Fall = Sep–Nov (gray). Darker bars include all observed deaths, and lighter bars include only deaths on or after age one.