Collaborators: Anushé Hassan (LSHTM), Sylvain Viguier (Graphcore, London), Mark Dyble (UCL), Daniel Smith (Bristol), Andrea Migliano (Zürich), Emily Emmott (UCL), Nikhil Chaudhary (Cambridge), Sarah Myers (UCL & MPI), Susie Lee (MPI), Rebecca Sear (LSHTM)
Funding: Medical Research Council
Non-maternal carers (allomothers) are hypothesized to lighten the mother’s workload, allowing for the specialized human life history including relatively short interbirth intervals and multiple dependent offspring. To date, evolutionary anthropology’s approach to understanding allomothering has largely (but not exclusively) investigated the ultimate explanations of allomothering (i.e. why it evolved) by examining allomother effects on maternal reproductive success (measured by fertility, child health and/or development and child survival). From this perspective, allomothers are assumed to reduce maternal energetic burden, freeing up the mother, allowing her to ‘stack’ offspring. For a holistic understanding of cooperative childrearing, however, ultimate reasons must be complemented with an understanding of how allomothering translates to increased reproductive success. In addition, while a wide range of contemporary and historical cross-cultural studies have highlighted that mothers receive help with childcare from various individuals (allomothers), little exploration has occurred into why we see diversity in allomothers. This project seeks to explore the complexities of childcare in the Agta.
In one paper, we focus on the mechanisms behind how allomothering influences maternal and child outcomes. We asked whether allomothernal childcare substitutes maternal childcare, and whether this is influenced by the type of allomother. By exploring these mechanisms we are better able to hypothesise about the mechanisms behind the evolution of cooperative childrearing and gain insights into human life history. We found that allomother caregiving was associated with reduced maternal childcare, but the substitutive effect varied depending on the source and type of care. Children-only playgroups consistently predicted a decrease in maternal childcare. While grandmothers were rarely available, their presence was negatively associated with maternal presence and childcare, and grandmothers performed similar childcare activities to mothers. These results underscore the importance of allomothering in reducing maternal childcare in the Agta. Our findings suggest that flexibility in childcare sources, including children-only playgroups, may have been the key to human life-history evolution.To find out more, check out our publication on Children are important too, and see my LSHTM Expert Opinion blog on social support for mothers which features this research.
In another paper, my colleague Sarah Myers and I were really interested in understanding why the Agta in Palanan had so many boys. To be exact, there were 131 boys aged 5 or under, compared to 87 girls, that’s a sex ratio of 1.51 or 151 boys to every 100 girls! The equivalent figure for the England and Wales is 1.05. Gendercide (either via selective abortion, infanticide, or neglect) is the primary explanation for why we see male-biased sex ratios in many parts of the world today. Anthropologists have often proposed this is the reason sex ratios are often male-biased in hunter-gatherer populations. Here, we decided to explore what were the possible mechanisms behind the biased sex ratio by looking at sex-specific mortality trends and parental investment. Using detailed data on childcare, we found that parents showed no difference in their interactions with children based on their gender. The graph of sex ratios across age cohorts also suggested the idea that females were more likely to die was wrong, because sex ratios decreased rather than increased as the age of cohorts increased. When we looked at mortality in the under 16’s we found that males are more likely to die across this juvenile period and while disease-related mortality was equally likely in both sexes, females were less likely to die of non-disease causes than males. We hope to have provided sufficient cause for looking more critically at behavioural-based assumptions regarding male-biased sex ratios in foraging populations and, in the absence of evidence, avoid the further invocation of gendercide. To hear more about this research, read more in Sarah’s excellent blog. or see the open access paper in Evolutionary Human Sciences on Why so many Agta boys?.
Teaming up with Anushé Hassan - who has done some excellent work on sex-biased parental care in Tanzania, as well as forthcoming work on the impact of market integration on allomothering - we are currently exploring diversity in allomothering. We aim to shed light onto the documented diversity of allomothering by exploring how it varies by livelihood diversity in a population of mobile and settled hunter-gatherers, the Agta. Specifically, we test if we can successfully predict who are important allomothers following a risk-based hypothesis, since cooperation has been argued to have evolved to deal with unpredictability resulting in shortfalls and loss. To hear about the preliminary results from this analysis please see our AAPA 2021 talk
Who provides childcare, and why this varies, is also likely related to demographic structures. It has long been understood that close kin are essential allomothers, in particular grandmothers who have a vested interest in their grandchildren and an (assumed) reduction in their own caring responsibilities. However, while the importance of grandmothers has been well demonstrated in a number of populations, they also appear to be of little importance in others. Understanding the factors which support, or hinder, grandmaternal childcare, such as demographic schedules, is an important next step. Working with Susie Lee and Rebecca Sear, we are currently exploring how demographic processes (age-specific fertility and mortality trends) impact grandmaternal presence, availability and willing to provide allomaternal care. This research is informed by findings in the Agta that grandmothers were notable in their absence, providing between 0-3% of childcare. Due to high mortality and residential mobility, only 28% of children resided with a grandmother. In the reduced sample of children with grandmothers (n = 34), grandmaternal involvement only increased to 1-5% likely due to reproductive competition. Further, due to early age at first birth and high fertility (TFR 7.7) many grandmothers (95%) also had several competing offspring and grandoffspring. In settled camps in which grandmothers had fewer dependents (1 versus 2 in mobile camps) they provided 152% more childcare, suggesting the importance of generational overlap.
Project link: https://osf.io/6vbj4/