# Evolution of longevity, age at last birth and sexual conflict with grandmothering

## TL;DR

This study models how grandmothering influences the evolution of longevity and reproductive age, revealing that it facilitates transitions to higher longevities without extending fertility duration, with male competition also playing a key role.

## Contribution

It extends a PDE model to include evolution in longevity and age at last birth, highlighting grandmothering's role in evolutionary transitions and the impact of male competition.

## Key findings

- Grandmothering enables transition to higher longevities.
- Male competition influences lifespan evolution.
- Grandmothering affects age at last birth but not adult lifespan.

## Abstract

We use a two-sex partial differential equation (PDE) model based on the Grandmother Hypothesis. We build on an earlier model by Kim et al. [27] by allowing for evolution in both longevity and age at last birth, and also assuming that post-fertile females support only their daughters' fertility. Similarly to Kim et al. [27], we find that only two locally stable equilibria exist: one corresponding to great ape-like longevities and the other corresponding to hunter-gatherer longevities. Our results show that grandmothering enables the transition between these two equilibria, without extending the end of fertility. Moreover, sensitivity analyses of the model show that male competition, arising from a skew in the mating sex ratio towards males, plays a significant role in determining whether the transition from great ape-like longevities to higher longevities is possible and the equilibrium value of the average adult lifespan. Whereas grandmothering effects have a significant impact on the equilibrium value of the average age at last birth and enable the transition to higher longevities, they have an insignificant impact on the equilibrium value of the average adult lifespan.