In pre-agricultural societies, reproductive rates and metabolism are predictable from body mass, about 60 kilograms. In the wealthiest contemporary nations, each person consumes not only 100 watts in food energy, but also up to 10,000 watts of extra-metabolic energy, the metabolism of a 30,000 kilogram primate. Women in these nations have correspondingly low birth rates, resulting in fewer than 2 children per lifetime.
Ecology Letters