Mortality and fertility rates in humans and chimpanzees: How within‐species variation complicates cross‐species comparisons

Abstract

A grandmother hypothesis may explain why humans evolved greater longevity while continuing to end female fertility at about the same age as do the other great apes. With that grandmother hypothesis in mind, we sought to compare age-specific mortality and fertility rates between humans and chimpanzees, our closest living relatives, and found two puzzles. First, we expected that lower adult mortality in humans would be associated with slower senescence, but the rate of chimpanzee demographic aging falls within the human range. Second, we expected declines in age-specific fertility to be similar in the two species but instead of falling in the thirties as it does in women, fertility remains high into the forties in some chimpanzee populations. We report these puzzles using data from nine human populations and both wild and captive chimpanzees, and suggest that systematic differences in the heterogeneity of surviving adults may explain them.

Menopause occurs late in life in the captive chimpanzee (Pan troglodytes)

“…The horizontal dotted lines show the standard error of the K-M estimate. The curved dashed line shows the prevalence of menopause for women, drawn after Hawkes et al (2009). Chimpanzees include those in the present study as well as those in Lacreuse et al (2008a) humans as well (Hawkes and Smith 2010).…”

“…6. For comparison, the percentage of women past their final menstrual cycle is presented as a dashed line, which represents the graphically determined median of the 4 populations of women presented by Hawkes et al (2009). In the terminology of survival analysis, menopause is considered the event of interest, and data from animals that died before menopause were considered "rightcensored."…”

How grandmother effects plus individual variation in frailty shape fertility and mortality: Guidance from human–chimpanzee comparisons

“…21, p. 12), but MRDTs vary at least 2-fold across human populations (28). That variation among populations is correlated with variation in the initial mortality rate.…”

“…Following Finch (61) the models consider age-specific mortality risk from ages 30 to 80 (see discussion in ref. 28). The log of A, the hazard of death at age 30 (representing the IMR) is on the horizontal axis, and G, the slope of the log of the Gompertz curve is on the vertical axis.…”

“…More survival to older ages makes senescence-measured as the pace of increase in age-specific mortality-appear to be faster. Heterogeneity of frailty within populations may explain these apparent contradictions (28).…”