Differential grandparental investment when maternal grandmothers are dead vs alive

Grandparents can increase their inclusive fitness by investing time and resources in their grandchildren. However, not all grandparents make such investments equally, and between-grandparent differences in this regard can be predicted based on paternity uncertainty, lineage, and grandparents’ sex. Using population-based data for English and Welsh adolescents (n = 1,430), we examined whether the death of the most important grandparent (in terms of investment), the maternal grandmother (MGM), changes relative support for existing hypotheses predicting differential grandparental-investment patterns. To contrast the predictions of the grandparental-investment hypotheses, we used generalized order-restricted information criterion approximation (GORICA). We consequently found that, when MGMs are alive, the most-supported hypothesis is ‘discriminative grandparental solicitude’, which ranks grandparental investment as MGMs > maternal grandfathers (MGFs) > paternal grandmothers (PGMs) > paternal grandfathers (PGFs). However, when MGMs are deceased, the paternity uncertainty hypothesis (MGFs = PGMs > PGFs) receives the most support; this is due to increased investment by PGMs. Thus, the standard evolutionary hypothesis explaining differential grandparental investment, and paternity uncertainty, receives the most support only when the heaviest investors (i.e., MGMs) are deceased.