Aging and fertility patterns in wild chimpanzees provide insights into the evolution of menopause

Abstract

Human menopause is remarkable in that reproductive senescence is markedly accelerated relative to somatic aging, leaving an extended postreproductive period for a large proportion of women. Functional explanations for this are debated, in part because comparative data from closely related species are inadequate. Existing studies of chimpanzees are based on very small samples and have not provided clear conclusions about the reproductive function of aging females. These studies have not examined whether reproductive senescence in chimpanzees exceeds the pace of general aging, as in humans, or occurs in parallel with declines in overall health, as in many other animals. In order to remedy these problems, we examined fertility and mortality patterns in six free-living chimpanzee populations. Chimpanzee and human birth rates show similar patterns of decline beginning in the fourth decade, suggesting that the physiology of reproductive senescence was relatively conserved in human evolution. However, in contrast to humans, chimpanzee fertility declines are consistent with declines in survivorship, and healthy females maintain high birth rates late into life. Thus, in contrast to recent claims, we find no evidence that menopause is a typical characteristic of chimpanzee life histories.

Fig 1

Comparison of chimpanzee and human age-specific fertility and mortality patterns. (a) chimpanzee age-specific fertility (mean ± standard error of 6 populations) and female probability of surviving (l_x) to end of each age class (5 wild populations only); (b) Dobe ! Kung hunter-gatherers, 1963-1973 [22]; (c) Ache hunter-gatherers, Forest Period [1]; (d) comparison plot of chimpanzee and human hunter-gatherer age-specific fertility. Age-specific fertility was calculated as the number of births as a fraction of risk years in each 5-year age interval. Intervals with ≤2 risk years in any population were excluded. All data are derived from true fertility and mortality rates rather than from model-fitted data.

Fig 2

In females at or above the age of 25, healthy individuals had significantly higher fertility than females who died within five years of the birth or risk year considered. We used data for the two sufficiently-sampled long-term populations, Gombe and Mahale, and indicate the mean ± standard error of the two populations. Other populations have small samples but similar trends.

Fig 3

Hazard for chimpanzee and human interbirth intervals according to the age of mother at the start of the interval. Lines reflect the probability of bearing a new infant at each time point when the previous infant has survived, (a) all wild chimpanzee birth intervals; (b) wild chimpanzee birth intervals, excluding intervals that terminated with the mother's death; (c) birth intervals of human hunter-gatherers in the Dobe !Kung population, adapted from Howell, 1979 [22].