Nutrition Physical Work and Natural Fertility

The effects of hard physical work and nutrition on reproductive ability suggest that differences in the fertility of populations, historically and today, may be explained by a direct pathway from food intake to fertility (Figure 6), in addition to the classic Malthusian pathway through mortality. Charles Darwin described this commonsense direct relationship between food supplies and fertility, observing the following:

1. Domestic animals that have regular, plentiful food without working to get it are more fertile than the corresponding wild animals.

2. ''Hard living retards the period at which animals conceive.''

3. The amount of food affects the fertility of the same individual.

4. It is difficult to fatten a cow that is lactating.

All of Darwin's dicta apply to human beings.

THE CURVE OF REPRODUCTIVE ABILITY (MAXIMUM FERTILITY RATE = 100)

20-24

' nubility

Age 18 nubility Stage I

25-29 > Peak / nubility Age 22 nubility Stage I

Adolescent subfecundity

Adolescent subfecundity

25-29 > Peak / nubility Age 22 nubility Stage I

growth to

12-13 / Menarche / 15-16 Menarche

10 15

Premenopausal subfecundity 41

birth Menopause Menopause

Age (years)

Figure 6 The mid-nineteenth century curve of female reproductive ability (variation of the rate of childbearing with age) compared with that of the well-nourished, modern Hutterites who do not use contraception. The Hutterite fertility curve (broken line) results in an average of 10-12 children; the 1850-1870 fertility curve (solid line) in approximately 6-8 children. (From Frisch (1978) Population, food intake and fertility. Science 199: 22-30, with permission from Science.)

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