Offspring fertility and grandchild survival enhanced by maternal grandmothers in a pre-industrial human society

Simon N. Chapman, Mirkka Lahdenperä, Jenni E. Pettay, Robert F. Lynch & Virpi Lummaa 

Abstract

Help is directed towards kin in many cooperative species, but its nature and intensity can vary by context. Humans are one of few species in which grandmothers invest in grandchildren, and this may have served as an important driver of our unusual life history. But helping behaviour is hardly uniform, and insight into the importance of grandmothering in human evolution depends on understanding the contextual expression of helping benefits. Here, we use an eighteenth-nineteenth century pre-industrial genealogical dataset from Finland to investigate whether maternal or paternal grandmother presence (lineage relative to focal individuals) differentially affects two key fitness outcomes of descendants: fertility and survival. We found grandmother presence shortened spacing between births, particularly at younger mother ages and earlier birth orders. Maternal grandmother presence increased the likelihood of focal grandchild survival, regardless of whether grandmothers had grandchildren only through daughters, sons, or both. In contrast, paternal grandmother presence was not associated with descendants’ fertility or survival. We discuss these results in terms of current hypotheses for lineage differences in helping outcomes.

Read more: Chapman, S.N. et al. Offspring fertility and grandchild survival enhanced by maternal grandmothers in a pre-industrial human society. Sci Rep 11, 3652 (2021). https://doi.org/10.1038/s41598-021-83353-3

Other News

We are delighted to once again host PhD candidate Silke van Daalen, who will stay with us for most of September.

Laisk T, Tšuiko O, Jatsenko T, Hõrak P, Otala M, Lahdenperä M, Lummaa V, Tuuri T, Salumets A, Tapanainen JS:

Simon's latest work on the demography of grandmothers is now out in PLoS ONE. 

We were delighted to host Professors Martin Daly and Gretchen Perry for a day of excellent talks, with a particular focus on grandmothering and alloparental behaviour.

Robert Lynch is at the Human Behavior and Evolution Society (HBES) conference 2018 in Amsterdam

The manuscript "The transition to modernity and chronic disease: mismatch and natural selection" by Stephen Corbett, Alexandre Courtiol, Virpi Lummaa, Jacob Moorad and Stephen Stea

Two papers out now from Simon's PhD project!

1) Changes in the Length of Grandparenthood in Finland 1790-1959, published in the Finnish Yearbook of Population Reasarch. In this paper, the team investigated how the shared time between grandparents and grandchildren changed across the demographic transition and with industrialisation. This shared time was low and stable before these major events, and began to increase rapidly after they began.

2) Limited support for the X-linked grandmother hypothesis in pre-industrial Finland, published in Biology Letters. Here, we tested whether slight differences in relatedness via the X-chromosome might lead to differences the survival of male and female grandchildren with maternal or paternal grandmothers. Though two of three predictions were supported, we concluded that the X-linked grandmother hypothesis cannot account for lineage differences by itself. 

Our latest paper shows that early-life environment is associated with sex differences in adult mortality and expected lifespan. Out now in Ecology Letters:
http://doi.wiley.com/10.1111/ele.12888

Figure 3a+b, from Griffin et al. 2017

Our review of the contribution of human studies to evolutionary biology is out now in Proceedings of the Royal Society B:
http://rspb.royalsocietypublishing.org/content/284/1866/20171164