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Why so many Agta boys? Explaining ‘extreme’ sex ratios in Philippine foragers

Page, AE; Myers, S; Dyble, M; Migliano, AB; (2019) Why so many Agta boys? Explaining ‘extreme’ sex ratios in Philippine foragers. Evolutionary Human Sciences , 1 , Article e5. 10.1017/ehs.2019.4. Green open access

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Abstract

Male-biased sex ratios have been observed in multiple small-scale societies. Although intentional and systematic female-biased mortality has been posited as an explanation, there is often a lack of ethnographic evidence of systematic female neglect and/or infanticide. The Agta, a foraging population from the Philippines, have a skewed sex ratio of 1.29 (129 males per 100 females) aged 15 years or under. We hypothesised that this skew was not caused by greater female deaths, but due to an adaptive response, where more males were produced at birth in reaction to high male-biased extrinsic mortality. To test this hypothesis we utilise census, childcare and mortality data from 915 Agta. The Agta's sex ratio is significantly male-biased in the <1 (n = 48, 2:1) and 1–5 year (n = 170, 1.39:1) age cohorts; however, we find no evidence of systematic female neglect in patterns of childcare. Furthermore, the sex ratio decreases over cohorts, becoming balanced by the end of the juvenile period, owing to significantly higher male mortality. Taken together, these results are not supportive of female infanticide or neglect, and instead suggest an adaptive mechanism, acting in utero as a response to male-biased juvenile mortality, following Fisherian principles of equalising parental investment.

Type: Article
Title: Why so many Agta boys? Explaining ‘extreme’ sex ratios in Philippine foragers
Open access status: An open access version is available from UCL Discovery
DOI: 10.1017/ehs.2019.4
Publisher version: https://doi.org/10.1017/ehs.2019.4
Language: English
Additional information: This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
Keywords: Biased sex ratios; female-biased infanticide/neglect; male-biased mortality; hunter–gatherers; Agta
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL SLASH
UCL > Provost and Vice Provost Offices > UCL SLASH > Faculty of S&HS
UCL > Provost and Vice Provost Offices > UCL SLASH > Faculty of S&HS > Dept of Anthropology
URI: https://discovery.ucl.ac.uk/id/eprint/10077658
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