An evolutionary perspective on kin care directed up the generations

Megan Arnot,Ruth Mace

doi:10.1038/s41598-021-93652-4

Abstract

Within evolutionary sciences, care towards younger kin is well understood from an inclusive fitness framework, but why adults would care for older relatives has been less well researched. One existing model has argued that care directed towards elderly parents might be adaptive because of their benefits as carers themselves, with their help freeing up the middle generations’ energy which can then be invested into direct reproduction. However, in this model, elder care is more beneficial to fitness if the carer is fecund. To offer an initial test of this hypothesis, we look at caring behaviour relative to fecundity status in a contemporary dataset from the United Kingdom. If elder care is contingent on possible direct fitness benefits, we would expect women who are still menstruating to care more for their parents than women who can no longer reproduce. Based on this, we also predict that women who are physiologically post-reproductive would invest more in their grandchildren, through whom they can increase their inclusive fitness. After controlling for age and other relevant factors, we find that women who are still menstruating spend more time caring for their parents than those who are not, and the reverse is true when looking at time spent caring for grandchildren. These findings demonstrate that potential inclusive fitness outcomes influence how women allocate care up and down the generations.

Highlights

Within evolutionary sciences, care towards younger kin is well understood from an inclusive fitness framework, but why adults would care for older relatives has been less well researched
Women spent a median of 18 h per month caring for grandchildren and 2 h a week caring for parents (IQR: 0, 5)
We test a hypothesis proposing that upwards intergenerational care will be selected for as long as the actor is able to translate some of the proposed reciprocal benefits of the care into direct fitness, meaning that upwards care being adaptive is somewhat dependent upon the fecundity status of the actor

Summary

Introduction

Care towards younger kin is well understood from an inclusive fitness framework, but why adults would care for older relatives has been less well researched. Though the elderly would already have an incentive to invest in younger kin as it is the only route to increase their inclusive fitness, it is thought that receiving care from adult relatives makes the elderly more able to offer direct care towards younger relatives This additional support from the elderly allows fertile adults to allocate their energy into other activities, such as increasing their direct fitness through reproduction (which would not be possible for the elderly to do) resulting in each individual doing the task they are ‘best’ at[22]. Once a woman is post-menopausal and the benefits to be had from investing in older relatives decline (though do not disappear completely), it may be better from a fitness perspective to invest energy into relatives through whom there is a way to increase inclusive fitness, such as children or grandchildren

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