Increased Grandchild Survival
Child survival is one of the most important fitness indicators in evolutionary studies. In our evolutionary past, grandparents may have increased their inclusive fitness by helping to keep their grandchildren alive.
Humans have been often defined as a cooperative breading species, meaning that other people in addition to the biological mother of the child take part in child-rearing (Hrdy 2009). These “alloparents” are typically closely related to the child and may include the child’s father, older siblings, aunts, uncles, and grandparents. Previous studies indicate that grandparents have often been highly involved in their grandchildren’s lives in both traditional and historical populations and in contemporary societies (Coall and Hertwig 2010). By investing resources in their grandchildren, grandparents can significantly increase a grandchild’s survival (Sear and Coall 2011).
Sear and Mace (2008) reviewed 45 studies that detected associations between kin presence in the same household or village and child survival. In these studies, child survival was indicated among children of different ages, ranging from infants to adolescents. The populations included in the review were mostly natural fertility and natural mortality societies, meaning that modern medical care and welfare services were lacking. These populations included several different types of livelihoods, for instance, historical agricultural societies and hunter-gatherer communities from different countries and continents, providing a comprehensive view of kin effects on child survival.
Based on the review by Sear and Mace (2008), the presence of a maternal grandmother correlated with increased survival rates among grandchildren in 69% of the studies (9/13) and the presence of paternal grandmothers in 53% of the studies (9/17). The effect of grandfathers seems to be much lower compared to that of grandmothers. The presence of maternal grandfathers was related to increased survival rates among grandchildren in 17% of the cases (2/12) and the presence of paternal grandfathers in 25% of the cases (3/12). Moreover, the presence of paternal grandfathers was related to decreased survival rates in 25% of the cases (3/12).
Even though grandparents (and maternal grandmothers in particular) can often have a positive impact on a grandchild’s survival, the effect may vary between ecological and other conditions. Sometimes, grandparental presence can be associated with a decreased survival probability among grandchildren. For instance, a previous study found that the presence of paternal grandmothers was associated with increased mortality rates among grandchildren (Strassman 2011). This could result from resource competition between grandparents and grandchildren, meaning that when the grandparents are old or their health is substantially deteriorated, they may compete with the grandchildren over limited household resources. Even the presence of maternal grandmothers may not always benefit the grandchildren (Sear 2008).
Empirical evidence from traditional and historical populations indicates that grandparental presence in the same household or village has often, but not always, been related to improved grandchild survival rates. These studies have also indicated that the presence of grandmothers tends to benefit the grandchildren more than the presence of grandfathers, with maternal grandmothers often playing the most important role. Therefore, in traditional and historical populations, grandmothers may have increased their own inclusive fitness by helping to keep their grandchildren alive.
- Hrdy, S. B. (2009). Mothers and others. The evolutionary origins of mutual understanding. Cambridge, MA: The Belknap Press of Harvard University Press.Google Scholar