Life-history evolution and uninvadable mortality schedules with and without intergenerational energy transfers

Piret Avila, Laurent Lehmann

Intergenerational energy transfers are widespread in nature, yet most life history theory assumes that organisms balance energy production and consumption at each age, leaving the evolutionary consequences of transfers underexplored. We develop a life history model under two energy budget constraints: (i) no transfers, where production equals consumption at each age, and (ii) transfers, where energy is balanced over the lifetime. Using optimal control theory, we derive the necessary conditions for uninvadable life histories in age-structured populations with development, yielding several general results. In particular, at any age in the uninvadable life history, current fitness gains (from current reproduction, somatic investments, and transfers) are exactly offset by the loss in the value of life due to mortality, where this marginal value reflects future fitness contributions from reproduction and energy transfers. We show that the resulting uninvadable mortality schedule is not necessarily senescent and how transfers lower it even after reproduction ceases, enabling longer lifespans and a post-reproductive phase. We apply our results to human life history evolution by extending the model of Kaplan and Robson (2009) to compare scenarios with and without transfers. We find that the shift to transfers extends the juvenile growth phase, characterised by substantial early-life energy deficits, which are compensated by enhanced lifetime productivity through somatic capital accumulation and longer lifespans. Post-reproductive survival emerges when declining reproductive efficiency makes resource transfers to younger individuals optimal. Our findings show that transitioning from individual energy balance to intergenerational transfers is sufficient to account for several major hallmarks of human life history.