The same kind of differential frailty proposed to underlie the Strehler–Mildvan correlations in Fig. 11.2, and modeled in Fig. 11.3, is relevant to age-specific fertility. As shown in Fig. 11.1, the childbearing years end at the age of ~45 in both humans and chimpanzees. Like other female mammals, humans and chimpanzees build initial oocyte stocks in early life that then deplete with age (vom Saal et al., 1994). Most of the initial stock is lost to atresia, a continuing process of cell death that begins near birth. In women, stocks decline from ~7 million oocytes at 5 months after conception to <2 million at birth and ~400,000 at puberty (Baker, 1963). Only one in a thousand of those remaining when ovarian cycling begins actually ovulate. Numbers continue to fall across young and middle adulthood, reaching thresholds associated first with reduced fecundability, then secondary sterility, and finally menopause ~10 years after last birth. Average ages at these thresholds differ some across populations (Bentley and Muttukrishna, 2007) with substantial variation around the averages (Faddy and Gosden, 1996; O’Connor et al., 2001; Sievert, 2006; Broekmans et al., 2009). The classic counts of human ovarian follicle stocks show that among females of the same age, remaining primordial follicle stocks can vary by two orders of magnitude (Block, 1952; Richardson et al., 1987; Gougeon et al., 1994).
Chimpanzee follicle stocks also vary among individuals of similar age (Jones et al., 2007). Archived ovarian sections taken at necropsy from captive chimpanzees of ages 0–47 years index this variation and the declining numbers with age (Jones et al., 2007). Exponential regressions fit to the age-specific primordial follicle counts on those sections and also to the whole ovary counts across that 0- to 47-year range in the classic human datasets provide a quantitative comparison of follicular loss rate in the two species. The intercepts—the heights—of the two regression lines are necessarily different because the human data represent whole ovaries and only single sections were available for the chimpanzees. [An average section is ~1/2,000 of a human ovary (Block, 1952; Richardson et al., 1987)—likely the same for chimpanzees.] However, the rate of depletion with age measured this way, on these samples, across this age range, is indistinguishable between the two species (Jones et al., 2007). This similarity is consistent with a wider body of findings, including hormone and cycling data from captive chimpanzees (Graham, 1979; Gould et al., 1981; Lacreuse et al., 2008), suggesting they would reach menopause at about the same ages humans do—if they lived long enough (Walker and Herndon, 2008).
As implied by these similarities and noted above, humans and chimpanzees can give birth into their mid-forties but not beyond. However, in spite of this similarity in the end of the childbearing years (Fig. 11.1), the shapes of age-specific fertility curves in the two species are strikingly