Sex allocation

Sex allocation is allocation of resources to male versus female reproduction. This single body of evolutionary theory unifies questions such as: (a) When and how should individuals adjust the sex of their offspring in response to environmental conditions? (e.g. Nishimura & Jahn 1996) (b) When and in what direction are individuals selected to change sex? There is a huge theoretical and experimental literature on this topic, providing some of the clearest support for evolutionary theory.

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Condition-dependent sex allocation

Sex allocation theory is branch of parental investment theory dealing with the relative expenditure of parental effort in male and female offspring. One influential hypothesis for sex-biased parental investment posits that mothers in poor “condition” (usually measured in socioeconomic or status terms among humans) may bias investment toward daughters while mothers in good condition bias investment towards sons (Trivers & Willard 1973, see also Dickemann 1979). This prediction is logical if males have more reproductive variance than females and if offspring reproduction is sensitive to parental resources. Parents in good condition provide more resources, leading to potentially higher fitness through sons, and parents in poor condition provide fewer resources, leading to better fitness through daughters. Among humans, males often have higher reproductive variance than do females and correlates of reproductive success (wealth, social competence etc.) appear to be sensitive to parental influence.

Recent studies using the “Trivers-Willard” (TW) hypothesis to examine sex-biased PI among humans show conflicting results. Studies of non-human primates (Brown 2001) and other mammals (Hewison & Gaillard 1999) also find conflicting results for tests of TW. Among the relatively low-status Mukogodo of Kenya, girls were received more investment than did boys, a finding consistent with the hypothesis (Cronk 2000). Compared with ethnic Hungarians, low-status Gypsies favored girls over boys (Bereczkei & Dunbar 1997). In Poland, Koziel & Ulijaszek (2001) found a weak paternal status by child’s sex interaction effect on parental care, but no maternal status by sex interaction. Other results are not consistent with TW: Research in the contemporary United States found no status by child’s sex interaction effect on parental investment (Keller, Nesse & Hofferth 2001), and a study of Hutterites concludes that female-biased parental care cannot be attributed to a Trivers-Willard effect (Margulis, Altmann, & Ober 1993). Finally, three tests of the Trivers-Willard hypothesis failed to support predictions concerning sex-biased breastfeeding in the Commonwealth of Dominica (Quinlan et al. 2003).

Failure of Trivers-Willard to explain sex-biased breastfeeding in some populations may be due to local factors that alter sex-specific costs of children (Brown 2001; Sieff 1990). If girls help parents (or relatives) more than boys do, then parents may be willing to bias investment toward daughters, because expenditures in them are less costly to other components of fitness than are expenditures in sons. (Conversely, if sons are more helpful, then we expect male-biased care.) Indeed, among foragers, sex differences in subsistence contributions are associated with juvenile sex ratio, indicating sex-biased PI (Hewlett 1991:23-8). Male-biased PI among the Inuit, for example, appears to be related to hunting and male contributions to family wellbeing (Smith & Smith 1994). Assuming reproduction is constrained by demands on maternal labor, investment biased toward the more helpful sex may increase parental fitness. Among Hungarian Gypsies, for example, girls take on childcare duties, apparently increasing their mothers’ fertility; and they are weaned later than boys (Bereczkei & Dunbar 2002). Similarly, in rural Dominica girls spend more time in household work than do boys, which may offset costs of investing in girls (Quinlan et al. 2005). This sex-specific repayment of parental care has been dubbed “local resource enhancement” (Gowaty & Lenartz 1985).

References:

Bereczkei, T. and R.I.M. Dunbar. 1997. Female biased reproductive strategies in a Hungarian Gypsy population. Proceedings of the Royal Society of London, Series B: Biological Sciences, 264:17-22.

Brown, G.R. 2001. Sex-biased investment in nonhuman primates: can Trivers and Willard’s theory be tested? Animal Behavior 61: 683-694.

Cronk, Lee. 2000. "Female-biased parental investment and growth performance among the Mukogodo," in Adaptation and Human Behavior: an Anthropological Perspective Edited by L. Cronk, N. Chagnon, W. Irons, pp. 203-221. New York: Aldine de Gruyter.

Dickemann, Mildred. 1979. "Female infanticide, reproductive strategies, and social stratification: A preliminary model," in. Evolutionary Biology and Human Social Behavior: An Anthropological Perspective. Edited by N.A. Chagnon and W. Irons, pp. 321-367. North Scituate, Ma: Duxbury.

Gowaty, Patricia A. and M.R. Lenartz. 1985. Sex ratios of nestling and fledgling red-cockaded woodpeckers (Picoides borealis) favor males. American Naturalist,126: 347-353.

Hewison, A.J. and J.M. Gaillard. 1999. Successful sons or advantaged daughters? The Trivers-Willard model and sex-biased maternal investment in ungulates. Trends in Ecology and Evolution, 14(6):229-234.

Hewlett, Barry S. 1991. Demography and childcare in preindustrial societies. Journal of Anthropological Research. 47(1):1-37.

Keller, M.C., R.M. Nesse and S. Hofferth. 2001. The Trivers-Willard hypothesis of parental investment: No effect in the contemporary United States. Evolution and Human Behavior, 22:343-360.

Koziel, S. and S.J. Ulijaszek. 2001. Waiting for Trivers and Willard: Do the rich really favor sons? American Journal of Physical Anthropology 115:71-79.

Margulis, S.W., J. Altmann and C. Ober. 1993. Sex-biased lactational duration in a human population and its reproductive costs. Behavioral Ecology and Sociobiology, 32:41-45.

Nishimura, K. & G. C. Jahn 1996. Sex allocation of three solitary ectoparasitic wasp species on bean weevil larvae: sex ratio change with host quality and local mate competition. Journal of Ethology 14 (1): 27-34.

Quinlan, R.J., Quinlan, M.B. & Flinn, M.V. 2005. Local resource enhancement & sex-biased breastfeeding in a Caribbean community. Current Anthropology. 46(3):471-480. [1]

Quinlan, Robert J., Marsha B. Quinlan and Mark V. Flinn. 2003. Parental investment and age at weaning in a Caribbean village. Evolution and Human Behavior 24:1-16. [2]

Sieff, Daniela F. 1990. Explaining Biased Sex Ratios in Human Populations: A Critique of Recent Studies. Current Anthropology 31: 25-35.

Smith, Eric Alden and S.Abagail Smith. 1994. Inuit sex-ratio variation. Current Anthropology 35(5): 595-624.

Trivers, Robert L. and Dan E. Willard. 1973. Natural selection of paternal ability to vary the sex-ratio of offspring. Science 179: 90-92