When my colleague Dr Abbey Page was conducting her fieldwork with the Agta, a Philippine hunter-gatherer population, she spent a lot of time watching children play – for two reasons: 1) she was interested in how children in a population with no formal school system interact and learn from each other and 2) there are a lot of Agta children around (Agta mothers, on average, have 7.7 children, one of the highest recorded total fertility rates among hunter-gatherers). Whilst doing so, she started to notice something – there were a lot of 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 UK is 1.05. This begs the question why?
The Agta live in the Northern Sierra Madre Natural Park of the Philippines, where they make their home in the mountainous rainforest and coastal beaches looking out over the Pacific Ocean. They practice spearfishing, the spoils of which they trade for rice with local farmers, supplemented by the gathering of wild foods and some cultivation. Currently, roughly 1000 Agta live in the area of Palanan where our fieldwork was conducted and my colleagues collected data from 915 of them (500 of whom were males, that’s a sex ratio of 1.21!).
As a first step to figuring out why there are so many more males in a population, it is important to look at how sex ratios may vary across different age groups in the population – for example, the reasons why there are more young boys may differ from why there are more old men. The graph below shows what happens when we breakdown Agta sex ratios by age cohort. This graph shows that the male-bias at the population level (which is 1.21), is being driven by skew towards males at younger ages – while it looks like there is also a dramatic skew towards men in the over 55’s, this is probably just an artefact of the small number of individuals (only 41) surviving to this age.
So why so many young boys?
Many of you might be thinking the obvious answer is that girls are dying – indeed 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 of often male-biased in hunter-gatherer populations.
Fieldwork by my colleagues and others lead us to believe this was unlikely to be the case in the Agta for two reasons: 1) infanticide has never been documented in the Agta and 2) the Agta, verbally at least, do not express any preference for sons. However, you can’t always trust what people say, particularly when it comes to sensitive subjects; you have to look at what they do. 35 Agta children were observed for 9 hours each; during this time the form childcare they were receiving and who was providing it were recorded every 20 seconds. Using this highly detailed data on childcare, we found that parents also show 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 (hazard ratio = 1.557, p = 0.011) and while disease-related mortality was equally likely in both sexes, females were less likely to die of non-disease causes than males.
This leaves us with an intriguing alternative explanation – the Agta give birth to more males…
It is generally assumed that the sex ratio at birth is equal, i.e. 1.00 (well actually it is assumed that the sex ratio at conception is equal and that it is “natural” for a few more females to die in utero, though it is rarely asked why this should be!). However, for simplicity, if the sex ratio at birth is equal and more females are dying, then sex ratios should become progressively male skewed (the yellow, green, and light blue lines in the graph below), and vice versa if more males are dying as they are in the Agta (the dark blue line in the graph below). But if the sex ratio at birth is not equal and instead is male-biased, after which more males die, then the pattern of sex ratios across age cohorts will start high and then decline (the orange line in the graph below). If we look back at the sex ratios across age cohorts in the Agta (above), we see a pattern most similar to this orange line – the sex ratio starts high and then falls.
Can the Agta really be having more male infants? My colleagues witnessed 16 births while they were conducting fieldwork, of these 12 were males (i.e. a sex ratio of 3.00). This is far too small a sample size to trust, but when we look at the reported reproductive histories of the Agta women censused they appear more likely to have given birth to more boys (graph A below). The sex ratio of births per year also seem more likely to be male-biased (graph B below).
Putting this altogether we suggest that it is plausible that the Agta sex ratio at birth is substantially male biased as a result of a biological mechanism adjusting the sex ratio at conception and/or the sex ratio at birth in response to higher male juvenile mortality. It is beyond the current scope to go into how they might be doing this, but if you’re interested then we discuss in our open access paper.
Why is this important?
We do not claim that this is the explanation for all male sex ratio biases seen in hunter-gatherer populations; however, 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. A priori assumptions of female infanticide and/or neglect not only unnecessarily stigmatise groups, but also mean that much needed interventions aimed at improving child outcomes in these high mortality populations are potentially misdirected.
This blog is based on data published in Page, A.E., Myers, S., Dyble, M. and Migliano, A.B., 2019. Why so many Agta boys? Explaining ‘extreme’ sex ratios in Philippine foragers. Evolutionary Human Sciences, 1. COPYRIGHT: © The Author(s) 2019. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.