Demography of grandmothering – a case study in Agta foragers

Women who care for multiple highly dependent children require significant support from non-maternal caregivers (allomothers). Theoretically, grandmothers are seen as key allomothers due to low costs and high inclusive fitness returns. Empirically, however, the evidence is inconsistent and the reasons for this variation are not well-understood. Understanding the factors which promote, or hinder, grandmaternal childcare, such as demographic schedules, is an important next step. Here, we explore the demographic predictors of the low levels of grandmothering for 78 Agta children. Due to generational fertility trends (ages of first and last birth) grandmothers still had dependent children until an average age of 52, creating reproductive overlap. Little to no grandmaternal investment after the age of 60 is explained by declining health in older women due to high mortality rates. This means the ‘helping window’ for grandmothering only spans seven years on average, yet the impact of this period is still be limited by multiple dependent grandchildren given high total fertility rates. We suggest then that grandmothering is constrained by 1) generational reproductive overlap and 2) grandchildren competition. Accordingly, we tested how: 1) the number of dependent children and 2) grandchildren associated with grandmothering using Bayesian mixed-effect models. We found moderate to strong evidence that the more children/grandchildren a women had, the lower her investment in each grandchild. Consequently, whether grandmothers help, or not, appears to be a function of demographic schedules, which vary widely. Future formal demographic modelling then will help shed light on the evolution of grandmothering in humans.