Skip to main content Accessibility help

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

  • Abigail E. Page (a1) (a2), Sarah Myers (a1), Mark Dyble (a3) and Andrea Bamberg Migliano (a1) (a4)


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.

  • View HTML
    • Send article to Kindle

      To send this article to your Kindle, first ensure is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle. Find out more about sending to your Kindle.

      Note you can select to send to either the or variations. ‘’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

      Find out more about the Kindle Personal Document Service.

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

      Send article to Dropbox

      To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Dropbox.

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

      Send article to Google Drive

      To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Google Drive.

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


This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (, which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.

Corresponding author

*Corresponding author. London School of Hygiene & Tropical MedicineKeppel Street London WC1E 7HT. E-mail:


Hide All

These authors contributed equally to the paper.



Hide All
Austad, SN and Fischer, KE (2016) Sex differences in lifespan. Cell Metabolism 23(6), 10221033;
Bailey, RC (1988) The significance of hypergyny for understanding subsistence behavior among contemporary hunters and gatherers. In Kennedy, BV and LeMoine, GM (eds), Diet and Subsistence: Current Archaeological Perspectives (pp. 5765). Calgary: University of Calgary.
Balikci, A (1970) The Netsilik. Garden City: The Natural History Press.
Bazer, FW, Spencer, TE and Ott, TL (1997) Interferon tau: A novel pregnancy recognition signal. American Journal of Reproductive Immunology 37, 412420.
Beckerman, S, Lizarralde, M, Peluso, D, Yvinec, C, Harris, N, Parker, D, Walker, R and Hill, K (2017) Partible paternity, the secondary sex ratio, and a possible Trivers–Willard effect. Current Anthropology 58(4), 540543;
Bereczkei, T and Dunbar, RIM (1997) Female-biased reproductive strategies in a Hungarian Gypsy population. Proceedings of the Royal Society B: Biological Sciences 264, 1722.
Blurton Jones, NG (2016) Demography and Evolutionary Ecology of Hadza Hunter–Gatherers. Cambridge: Cambridge University Press.
Cameron, EZ (2004) Facultative adjustment of mammalian sex ratios in support of the Trivers – Willard hypothesis: evidence for a mechanism. Proceedings of the Royal Society B: Biological Sciences, 271, 17231728;
Catalano, R, Yorifuji, T and Kawachi, I (2013) Natural selection in utero: evidence from the Great East Japan Earthquake. American Journal of Human Biology: The Official Journal of the Human Biology Council 25(4), 555559;
CIA (2018) World Factbook.
Clutton-Brock, TH and Iason, GR (1986) Sex ratio variation in mammals. The Quarterly Review of Biology 61(3), 339374;
Clutton-Brock, TH, Guinness, FE and Albon, SD (1982) Red Deer: the Behaviour and Ecology of two Sexes. Chicago, IL: University of Chicago Press.
Crittenden, AN and Marlowe, FW (2008) Allomaternal care among the Hadza of Tanzania. Human Nature 19(July), 249262;
Cronk, L (1991) Preferential parental investment in daughters over sons. Human Nature 2(4), 387417;
Cronk, L (2007) Boy or girl: gender preferences from a Darwinian point of view. Reproductive BioMedicine Online;
Diekmann, Y, Smith, D, Gerbault, P, Dyble, M, Page, AE, Chaudhary, N, Bamberg Migliano, A and Thomas, MG (2017) Accurate age estimation in small-scale societies. Proceedings of the National Academy of Sciences, 1619583114;
Drevenstedt, G, Crimmins, E, Vasunilashorn, S and Finch, C (2008) The rise and fall of excess male infant mortality. Proceedings of the National Academy of Sciences of the United States of America 105(13), 50165021;
Dyble, M, Salali, GD, Chaudhary, N, Page, A, Smith, D, Thompson, J, Vinicius, L, Mace, R and Migliano, AB (2015) Sex equality can explain the unique social structure of hunter–gatherer bands. Science 348(6236), 796798;
Dyble, M, Thompson, J, Smith, D, Salali, GD, Chaudhary, N, Page, AE, Vinicuis, L, Mace, R and Migliano, AB (2016) Networks of food sharing reveal the functional significance of multilevel sociality in two hunter–gatherer groups. Current Biology 26, 20172021;
Early, JD and Headland, TN (1998) Population Dynamics of a Philippine Rain Forest People: The San Ildefonso Agta. Gainesville, FL: University Press of Florida.
Eder, JF (1987) On the Road to Tribal Extinction: Depopulation, Deculturation, and Maladaptation among the Batak of the Philippines. Berkeley, CA: University of California Press.
Ellison, GT, de Wet, T, Matshidze, KP and Cooper, P (2000) The reliability and validity of self-reported reproductive history and obstetric morbidity amongst Birth to Ten mothers in Soweto. Curationis 23(4), 7680.
Emlen, ST, Merritt Emlen, J and Levin, SA (1986) Sex-ratio selection in species with helpers-at-the-nest. The American Naturalist 127, 18;
Fisher, RA (1930) The Genetical Theory of Natural Selection. Oxford: Clarendon Press.
Flint, APF, Albon, SD and Jafar, SI (1997) Blastocyst development and conceptus sex selection in red deer Cervus elaphus: studies of a free-living population on the Isle of Rum. General and Comparative Endocrinology 383, 374383.
Gardner, PM (1988) The paliyans. In Bicchieri, MG (ed.), Hunters and Gatherers Today (pp. 404450). Prospect Heights: Waveland Press.
Gellatly, C and Petrie, M (2017) Prenatal sex selection and female infant mortality are more common in India after first-born and second-born daughters. Journal of Epidemiology and Community Health 71(3), 269274;
Gurven, M and Kaplan, H (2007) Longevity among hunter–gatherers: a cross-cultural examination. Population and Development Review 33(June), 321365.
Hamilton, WD (1967) Extraordinary sex ratios. Science 156(3774), 477488;
Headland, TN (1988) Ecosystemic change in a Philippine tropical rainforest and its effects on a Negrito foraging society. Tropical Ecology 29, 121135.
Headland, TN (1989) Population decline in a Philippine Negrito hunter–gatherer society. American Journal of Human Biology 1(1), 5972.
Headland, TN, Headland, JD and Uehara, RT (2011) Agta Demographic Database: Chronicle of a Hunter-Gatherer Community in Transition. SIL Language and Culture Documentation and Description 2;
Hesketh, T and Xing, ZW (2006) Abnormal sex ratios in human populations: causes and consequences. Proceedings of the National Academy of Sciences 103(36), 1327113275;
Hewlett, BS (1991) Demography and Childcare in preindustrial societies. Journal of Anthropological Research 42(1), 137.
Hewlett, BS (1993) Intimate Fathers: the Nature and Context of Aka Pygmy Paternal Infant Care. Ann Arbor, MI: University of Michigan Press.
Hewlett, BS, Lamb, ME, Leyendecker, B and Schölmerich, A (2000) Parental investment strategies among Aka foragers, Ngandu farmers and Euro-American urban-industrialists. In Adaptation and Human Behaviour: an Anthropological Perspective (pp. 155177). New York: Aldine de Gruyter.
Hill, K and Hurtado, AM (1996) Aché Life History: The Ecology and Demography of a Foraging People. New Haven, CT: Aldine de Gruyter;
Hill, KR and Kaplan, HS (1988) Tradeoffs in male and female reproductive strategies among the Ache, part 1. In Betzig, L, Turke, P and Borgerhoff Mulder, M (eds), Human Reproductive Behavior (pp. 277290). Cambridge: Cambridge University Press.
Hill, K, Hurtado, AM and Walker, RS (2007) High adult mortality among Hiwi hunter–gatherers: implications for human evolution. Journal of Human Evolution 52, 443454;
Hurtado, AM and Hill, KR (1987) Early dry season subsistence ecology of Cuiva (Hiwi) foragers of Venezuela. Human Ecology 15(2), 163187;
Irwin, C (1989) Lords of the Arctic: wards of the state. The growing Inuit population. Northern Perspectives 17(1), 212.
James, WH (1986) Hormonal control of sex ratio. Journal of Theoretical Biology 118(4), 427441;
Kelly, RL (2013) The Lifeways of Hunter–Gatherers: The Foraging Specturm (2nd edn) Cambridge: Cambridge University Press.
Komdeur, J, Daan, S, Tinbergen, J and Mateman, C (1997) Extreme adaptive modification in sex ratio of the Seychelles warbler's eggs. Nature 385(6616), 522525;
Kramer, KL (2010) Cooperative breeding and its significance to the demographic success of humans. Annual Review of Anthropology 39(1), 417436;
Labov, JB, William Huck, U, Vaswani, P and Lisk, RD (1986) Sex ratio manipulation and decreased growth of male offspring of undernourished golden hamsters (Mesocricetus auratus). Behavioral Ecology and Sociobiology 18(4), 241249;
Larson, MA, Kimura, K, Kubisch, HM and Roberts, RM (2001) Sexual dimorphism among bovine embryos in their ability to make the transition to expanded blastocyst and in the expression of the signaling molecule IFN-t. Proceedings of the National Academy of Sciences of the United States of America 98(17), 96779682.
Lazarus, J (2002) Human sex ratios: adaptations and mechanisms, problems and prospects. In ICW Hardy (ed.), Sex Ratios: Concepts and Research Methods (pp. 287311). Cambridge: Cambridge University Press;
Mace, R and Jordan, F (2005) The evolution of human sex ratio at birth: a bio-cultural analysis. In Mace, R, Holden, C and Shennan, S (eds), The Evolution of Cultural Diversity: a Phylogenetic Approach (pp. 207216). London: UCL Press.
Maynard Smith, J (1980) A new theory of sexual investment. Behavioral Ecology and Sociobiology 7(3), 247251;
McClure, PA (1981) Sex-biased litter reduction in food-restricted wood rats (Neotoma floridana). Science, 211(4486), 10581060;
McElreath, R and Boyd, R (2007) Mathematical Models of Social Evolution: a Guide for the Perplexed. London: University of Chicago Press.
Meehan, CL (2005) The effects of residential locality on parental and alloparental investment among the Aka foragers of the central African Republic. Human Nature 16(1), 5880;
Minter, T (2010) The Agta of the Northern Sierra Madre: Livelihood Strategies and Resilience Among Philippine Hunter–Gatherers. Leiden: Leiden University.
Morris, B (1982) Economy, affinity and inter-cultural pressure: notes around hill Pandaram group structure. Man 17(3), 452461.
Navara, KJ (2013) Hormone-mediated adjustment of sex ratio in vertebrates. Integrative and Comparative Biology 53(6), 877887.
Orzack, HS, Stubblefield, JW, Akmaev, VR, Colls, P, Munné, S, Scholl, T, Steinsaltz, D and Zuckerman, JE (2015) The human sex ratio from conception to birth. Proceedings of the National Academy of Sciences of the United States of America 112(16), E2102E2111;
Page, AE (2016) On Trade offs and Communal Breeding: the Behavioural Ecology of Agta Foragers. London: University College London.
Page, AE, Viguier, S, Dyble, M, Smith, D, Chaudhary, N, Salali, GD, Thompson, J, Vinicius, L, Mace, R and Migliano, AB (2016) Reproductive trade-offs in extant hunter–gatherers suggest adaptive mechanism for the Neolithic expansion. Proceedings of the National Academy of Sciences of the United States of America 113(17), 46944699.
Peterson, JT (1978) The Ecology of Social Boundaries: Agta Foragers of the Philippines. London: University of Illinois Press.
Promislow, DEL (1992) Costs of sexual selection in natural populations of mammals. Proceedings of the Royal Society B: Biological Sciences 247(1320), 203210.
Rasmussen, K (1931) The Netsilik Eskimos, ed. Gyldeddalske Boghandel., NF Copenhagen.
R Core Team (2012) R: a Language and Environment for Statistical Computing. Vienna: R Foundation for Statistical Computing;
Reza, A, Mercy, JA and Krug, E (2001) Epidemiology of violent deaths in the world. Injury Prevention 7(2), 104111;
Rose, FGG (1960) Clasification of Kin, Age Structure and Marriage Amongst the Groote Eylandt Aboriginies. Berlin: Akademie-Verlag.
Ross, CT, Borgerhoff, M, Winterhalder, B, Uehara, R, Headland, J and Headland, T (2016) Evidence for quantity–quality trade-offs, sex-specific parental investment, and variance compensation in colonized Agta foragers undergoing demographic transition. Evolution and Human Behavior 37(5), 350365;
Sharp, RL (1940) An Australian Aboriginal population. Human Biology 12(4), 481507.
Shenk, MK, Towner, MC, Starkweather, K, Atkisson, CJ and Alam, N (2014) The evolutionary demography of sex ratios in rural Bangladesh. In MA Gibson and D Lawson (eds), Applied Evolutionary Anthropology: Darwinian Approaches to Contemporary World Issues (pp. 141173). London: Springer.
Sieff, DF, Betzig, L, Cronk, L, Fix, AG, Flinn, M, Sattenspiel, L, Gibson, K, Ann Herring, D, Howell, N, Johansson, SR, Pavlík, Z, Sheets, JW, Smith, EA, Voland, E and Siegelkow, E (1990) Explaining biased sex ratios in human populations: a critique of recent studies. [And Comments and Reply.] Current Anthropology 31(1), 2548;
Silk, JB (1983) Local resource competition and facultative adjustment of sex ratios in relation to competitive abilities. The American Naturalist 121(1), 5666;
Skogland, T (1986) Sex ratio variation in relation to maternal condition and parental investment in wild reindeer. Oikos 46, 417419.
Smith, EA and Smith, SA (1994) Inuit sex-ratio variation. Current Anthropology 35(5), 595614.
Trivers, RL and Willard, DE (1973) Natural selection of parental ability to vary the sex ratio of offspring. Science;
Turnball, C (1961) The Forest People. London: Bodley Head.
Uggla, C and Mace, R (2016) Parental investment in child health in sub-Saharan Africa: a cross-national study of health-seeking behaviour. Royal Society Open Science 3(2), 150460;
United Nations (2017) World Population Prospects 2017;
Watts, C and Zimmerman, C (2002) Violence against women: global scope and magnitude. Lancet;
Wells, JCK (2000) Natural selection and sex differences in morbidity and mortality in early life. Journal of Theoretical Biology 202(1), 6576;
West, S, Reece, SE and Sheldon, BC (2002) Sex ratios. Heredity 88, 117124;
Weyer, EM (1932) The Eskimos: their Environment and Folkways. New Haven, CT: Yale University Press.
Yengoyan, AA (1970) Demographic factors in Pitjandjara social organisation. In Berndt, RM (ed.), Australian Aboriginal Anthropology (pp. 7091). Nedlands: University of Western Australia.


Type Description Title
Supplementary materials

Page et al. supplementary material
Page et al. supplementary material 1

 PDF (209 KB)
209 KB


Altmetric attention score

Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed