Fertility Theory: Embodied-capital Theory of Life History Evolution

Abstract

The embodied-capital theory generalizes existing life history theory in evolutionary biology by treating the processes of growth, development and maintenance as investments in stocks of somatic or embodied-capital. In a physical sense, embodied capital is organized somatic tissue—muscles, digestive organs, brains, etc. In a functional sense, embodied capital includes strength, immune function, coordination, skill, knowledge, and other abilities. Since such stocks tend to depreciate with time, allocations to maintenance can also be seen as investments in embodied capital. There are two trade-offs affecting natural selection on fertility. The first is the trade-off between present and future reproduction. By growing, an organism can increase its energy capture rates in the future and thus increase its future fertility. This can be understood in terms of optimal investments in own embodied capital vs. reproduction. The second trade-off is between quantity and quality of offspring, where quality is a function of parental investment in offspring and reflects its ability to survive and reproduce. This can be understood in terms of investments in the embodied-capital of offspring. Natural selection has resulted in physiological and psychological mechanisms by which individuals adjust fertility onset and fertility rates in relation to changing environmental conditions. Embodied-capital theory links several existing theories of demographic transition and, in so doing, provides a new perspective on each one. It rationalizes the shift from natural fertility to parity-specific fertility in terms of a changing quality/quantity trade-off, as a transition from high fertility/low parental investment to low fertility/high parental investment. It also shows why the shift to lower mortality rates can lead to lower fertility, but for very different reasons than traditional demographic transition theory proposes.