Life–history theory, fertility and reproductive success in humans

Abstract

According to life-history theory, any organism that maximizes fitness will face a trade-off between female fertility and offspring survivorship. This trade-off has been demonstrated in a variety of species, but explicit tests in humans have found a positive linear relationship between fitness and fertility. The failure to demonstrate a maximum beyond which additional births cease to enhance fitness is potentially at odds with the view that human fertility behaviour is currently adaptive. Here we report, to our knowledge, the first clear evidence for the predicted nonlinear relationship between female fertility and reproductive success in a human population, the Dogon of Mali, West Africa. The predicted maximum reproductive success of 4.1 ± 0.3 surviving offspring was attained at a fertility of 10.5 births. Eighty-three per cent of the women achieved a lifetime fertility level (7-13 births) for which the predicted mean reproductive success was within the confidence limits (3.4 to 4.8) for reproductive success at the optimal fertility level. Child mortality, rather than fertility, was the primary determinant of fitness. Since the Dogon people are farmers, our results do not support the assumptions that: (i) contemporary foragers behave more adaptively than agriculturalists, and (ii) that adaptive fertility behaviour ceased with the Neolithic revolution some 9000 years ago. We also present a new method that avoids common biases in measures of reproductive success.

The effects of kin on female fertility in rural Gambia

“…A similar analysis of the effects of kin on female fertility and child mortality using historical German data found larger effects of the presence of grandmothers on child mortality than on fertility (Voland & Beise, in press). There is growing evidence that, in natural fertility societies, variation in child mortality may be more important than variation in fertility in determining reproductive success (Pennington, 1992;Strassmann & Gillespie, 2002). The analyses of the effects of relatives on child mortality and fertility in this population suggest that any effects kin have on the reproductive success of women is likely to act more strongly through child mortality rates than fertility rates.…”

Parental investment and the optimization of human family size

“…of offspring born, except for Borgerhoff Mulder [33,69] and Gibson [41] [ 36] and were actually associated with higher fertility for males [28,30]. Predicted negative relationships between family size and child survival have been more successfully demonstrated in a number of contemporary African agriculturalist societies ( [31,32]; but see [33]) and historical European and American populations [34 -36]. It should also be noted that a number of related studies have presented evidence of an association between family size and child anthropometric status which probably predicts future survival.…”

What do men want? Re-examining whether men benefit from higher fertility than is optimal for women

“…This inconsistency means the same empirical association of kin presence with higher fertility is often interpreted as evidence of altruism, or of coercion, depending on whether the family member is a woman's mother [57,59] or motherin-law [11,60], respectively. In fact, in most places where anthropologists have tried to measure the relationship between fertility and fitness, including in high fertility contexts with limited effective contraception, these are positively and monotonically related [61][62][63][64]. This does not necessarily mean that one or both arguments must be incorrect: a woman's costs to shorter IBIs can be lowered by supportive social relations, and she can be coerced into a shorter (or longer) IBI by a mate unsure about his future paternity.…”