Why do humans have such long childhoods, and live so long after they stop having children?
My paper with Laurent Lehmann, “Life history evolution and uninvadable mortality schedules with and without intergenerational energy transfers”, has been accepted in Theoretical Population Biology.
In many species, resources flow between generations: parents feed their young, and in human societies children consume more than they produce for almost two decades, while grandparents keep giving long after they stop reproducing. Our paper analyses, within a single framework, life history models with and without intergenerational resource sharing.
Main results:
- Resource sharing transforms life histories. It favours longer childhoods, more investment in embodied capital (e.g. expensive brain tissue), higher productivity, longer lifespans and a post-reproductive phase of life.
- We derive the optimal mortality curve. At each age, the optimal mortality rate is the ratio of current fitness gains (from reproduction, investment in the soma and transfers) to the value of life (an individual’s expected future contribution to fitness). This is an arbitrage condition: the gains from current activities balance the expected loss from dying. Without sharing, the value of life is Fisher’s reproductive value. Transfers add to it, so they lower mortality and allow survival beyond the reproductive years.
- Ageing is not inevitable. When the cost of maintaining the soma is low enough, mortality reaches a stable state instead of rising with age.
- Young individuals gain the most from receiving energy. This helps explain why parental care is so widespread in nature.
Preprint: https://doi.org/10.32942/X2036X