\doc\web\99\17\sexratio.txt From: "Peter Frost" To: , Date sent: Thu, 2 Dec 1999 09:30:46 -0500 Subject: [h-bd] African sex ratios and polygyny [ Double-click this line for list subscription options ] The following is the first half of an article on the interrelationships in sub-Saharan Africa between low sex ratios, generalized polygyny, and agriculture. This is still a preliminary version, so I would certainly welcome criticism and comments. Peter Frost ---------------------------------------------------------------------------- Low Sex Ratios in Sub-Saharan Africa: An Adaptive Response to Generalized Polygyny? Peter Frost Sex ratios at birth are low throughout sub-Saharan Africa (Romaniuk 1968:278-281, 334; van de Walle 1968:38-43). They are also low in African diaspora populations from the West Indies (Visaria 1967), Britain (James 1984), Latin America (Feitosa & Krieger 1993), and the U.S. (Ciocco 1938; Erickson 1976; Strandskov 1945; Teitelbaum 1970; Teitelbaum 1972). Between Black and White Americans, the sex ratio difference remains significant even when birth order, socioeconomic status, paternal age, and paternal education are taken into account (Erickson 1976; Teitelbaum 1972). Recent attempts to explain this phenomenon have focused on polygyny. A study of seven different Kenyan ethnic groups has found significantly lower sex ratios in the children of polygynous parents than in those of monogamous parents (Whiting 1995). Whiting (1995) and Martin (1994) have suggested that women bear more daughters when they experience less frequent sexual intercourse, as appears to be the case in polygynous relationships.1 Low African sex ratios may thus reflect the "generalized" polygyny (>20% of all sexual unions) that prevails in 85% of sub-Saharan societies (Goody 1973:177-178). Why would lower sex ratios be adaptive in a polygynous population? It may be that more daughters are born to offset the "wife shortage" created by polygyny. No such compensatory effect, however, has been found in non-human polygynous species. Although the subordinate females in such species usually bear more daughters, the dominant females bear more sons, so the overall sex ratio remains more or less equal (Clutton-Brock & Iason 1986). Apparently, the wasted reproductive potential of unmated males is allowed to go underutilized because it is confined mainly to subordinate individuals with limited reproductive value. The benefit of bearing a daughter does not outweigh that of bearing a son-who may become a dominant male with better chances of reproductive success. In sub-Saharan Africa, however, polygynous individuals differ from non-polygynous ones primarily in age and not reproductive quality: Inequality between old and young men was general in African lineage systems. While a young man might often work harder than his father or other elders, access to wives was determined not by current earnings but by access to prestige goods. The young man knew, however, that some day he would inherit his father's wealth, take more wives, and assume authority over his sons in turn. (Curtin et al. 1978:160-161). Young men had to put off marriage until they could save up enough to pay the bridewealth (van den Berghe 1979:66). Young warriors were often completely barred from marriage (Gluckman 1940:26; Whiting 1995:440). It was thus age, and not lifetime reproductive value, that distinguished single males from their married counterparts. In fact, because single males were younger and could expect to live longer, they may actually have been worth more to a prospective mate Because polygyny leads to fewer available women and celibate young males, natural selection would tend to compensate by lowering the sex ratio (i.e., more daughters, fewer sons). The actual mechanism seems to be a maternal effect mediated by the frequency of sexual relations experienced by the mother. African Americans How do we account, then, for low sex ratios in the African diaspora, notably in the U.S.? Young African American males are not barred from sexual relations, at least not as they are in traditional African societies. Moreover, the evidence does not point to a maternal effect. A study of mixed-race couples found that when the mother was white and the father black, the sex ratio at birth was the same as for children born to two black parents (Khoury et al 1984). This would seem to indicate a paternal effect, possibly mediated by the proportion of Y-bearing sperm in the father's semen. Conceivably, the same selection pressures that produced one mechanism could have produced another. Hence, the sex ratio may have initially compensated for the effects of polygyny through a flexible mechanism; in this case, a maternal effect mediated by coital frequency. If the polygyny rate remained consistently high, natural selection would, over time, have also favored heritable traits that lower the sex ratio. Although it is notoriously difficult to raise or lower the sex ratio by selective breeding, small but significant heritable differences have been achieved in bulls, pigs, and albino rats (Clutton-Brock & Iason 1986:345-346; Watson 1992). Sustained selection, in the order of 25 generations, appears to be required (Watson 1992). Evidence for Antiquity of Generalized Polygyny For such selection to have taken place, generalized polygyny must have prevailed among sub-Saharan Africans for a long time. Several lines of evidence seem to bear this out. Genetics. Sub-Saharan Africans display much lower Y chromosome/X chromosome variability than do other populations, apparently because proportionately fewer men have contributed to the sub-Saharan gene pool (Torroni et al. 1990; Spurdle et al. 1994; Scozzari et al. 1997). Linguistics. Reconstruction of proto-Bantu, spoken approximately 3,000 years ago, has uncovered a specific term for "taking a second wife" (Polome 1977). Physical anthropology. Over time, too many men competing for too few women should favor the evolution of physical robustness. Such male-male competition may be reflected in the increased sexual dimorphism of African Americans for weight, chest size, arm girth, and leg girth (Todd & Lindala 1928; Wolff & Steggerda 1943). In contrast, a small, gracile, and almost childlike body form characterizes Khoisans and Pygmies, the only sub-Saharan populations to have a low incidence of polygyny. Origins of Generalized polygyny in Sub-Saharan Africa According to mtDNA and Y-chromosome dendrograms, Khoisans are the oldest living population in sub-Saharan Africa, followed by Pygmies (Holden 1999; Penny et al. 1995; Spurdle et al 1994; Watson et al. 1996). Only 6% of males in one Khoisan people, the !Kung, practice polygny (Howell 1979:234-235). The sex ratio at birth, 105 males per 100 females, is comparable to that of non-African populations (Howell 1979:247). Thus, Africa's high polygyny rates and low sex ratios are probably not an ancestral condition. "True" Black Africans appear as a recent adaptive radiation in the above dendrograms, apparently branching off from an ancestral Pygmy population-a line of ancestry also indicated by osteological data (Coon 1962:651-656; Watson et al. 1996). This radiation seems to have occurred somewhere in West Africa. Before the Bantu expansion about 3,000 years ago, true Black Africans were absent from the continent's central, eastern, and southern regions (Cavalli-Sforza 1986:361-362; Oliver 1966). They were also absent from the middle Nile until about 4,000 years ago, at which time they begin to appear in paintings from Pharaonic Egypt and in skeletal remains from Nubia (Junker 1921). Murdock (1959:44, 64-68) attributes this expansion out of West Africa to development of the Sudanic food complex some 6,000-7,000 years ago, near the Niger's headwaters. There, a wide range of cultivated plants (sorghum, pearl millet, cow pea, etc.) were developed independently of the Southwest Asian food complex. Other authors, like Shaw (1980), postulate a larger area of origin in West Africa. Full development of this complex seems to have followed a long period of "proto-agriculture" during which hunter-gatherers protected fields of wild grains and created clearings for wild yams and oil palms (Davies 1968; Shaw 1980:111-114). Tending of wild edible species is suggested by unusually abundant Canarium (pili nut) leaf impressions from a southern Ghanaian site dated to 8000-9000 B.P. (Posnansky 1984:149). Some form of agriculture is also apparent in reconstructed words of proto-Niger-Congo, probably spoken ca. 10,000 B.P. (Ehret 1984). At first glance, a West African origin seems inconsistent with genetic evidence for Black Africans and Pygmies sharing a common ancestor, since the latter now live only in central Africa. It is likely, however, that they once inhabited the entire rain forest zone, including the Guinea coast of West Africa, as indicated by finds of Sangoan artifacts-widely considered to be produced by ancestral Pygmies (Murdock 1959:48-49). Since Sangoan sites are confined to the rain forest zone and attest to a hunting and gathering lifestyle much like that of present-day Pygmies, the lineage from the Guinea coast Sangoans to present-day Black Africans must have involved a number of major physical and cultural changes. >From the outset, this ancestral Guinea coast population may have tended towards some reproductive isolation, and hence genetic differentiation, because of the Dahomey Gap-a mosaic of savanna and woodland separating the rain forest on the Guinea coast from that of central Africa. The thinning of Africa's rain forests during the dry conditions of the last ice age may have increased this partial isolation and, more importantly, made it easier to manage food production from wild yams and oil palms (Maley 1995:45-46; Posnansky 1984:150). Indicative of a shift in subsistence is the appearance of hoe-like implements at Guinea coast sites as early as 12,000 B.P. (Stahl 1995:262). With the end of the ice age, the return of a less open forest environment by 9000 B.P. may have compelled these proto-agriculturalists to move out into mosaic environments to the north and east (Maley 1995:46; Posnansky 1984:150). Such a migration may correspond to the breakup of proto-Niger-Congo, estimated at 10,000 B.P. (Ehret 1984). The first to branch off was proto-Mande (Blench 1984:128-129); its descendent languages occupy an area centered on the Niger's headwaters-the same area that Murdock sees as the cradle of Sudanic food crops. The Sudanic food complex developed primarily out of female gathering and only secondarily out of male hunting.2 It thus greatly enhanced women's contribution to food provisioning, the corollary being a reduction in the costs of polygyny to men (van den Berghe 1979:65). As polygyny became more frequent, male-male competition would have increased for the shrinking pool of potential mates, the result being an intensification of sexual selection for larger, stronger, and more muscular males, as is the case in non-human polygynous species.3 Such a scenario leaves surprisingly little time for the morphogenesis of true Black Africans. The beginnings of proto-agriculture cannot be pushed back much further than 12,000 B.P. A tall, clearly Negroid skeleton (Asselar Man) has been dated to 6500 B.P. (Camp 1974:241; Coon 1962:649-650). This leaves a window of not much more than six thousand years for the changes that differentiate Black Africans from Pygmies, i.e., a shift from a gracile, almost childlike body to a much more robust one, with attendant increases in stature, weight, and muscle mass. As development of the Sudanic food complex allowed these agriculturalists to expand out of the mosaic environments and into the savanna, the ratio of female to male participation in food provisioning should have declined. The savanna is more demanding on women's time, particularly for collection of water and firewood, so successful penetration of this environment would have required greater male involvement in agriculture (Goody 1973:185-186). In the savanna regions of Ghana, "women planted grain and helped with the harvest, but they were not concerned with yam cultivation, and did not carry out the many hoeing activities that were connected with cereal agriculture"- yet surprisingly polygyny rates were as high as in the mosaic and rain forest environments further south, a fact leading Goody (1973:185) to conclude: "While hoe agriculture, female farming and polygyny are clearly associated in a general way, there seems little evidence directly to connect variations in rates of polygyny with differences in the role of women in farming or in trade." High rates of polygyny in Africa may thus reflect not so much existing conditions as pre-existing ones whose adaptations have been maintained through culture lag, notably the retention of a large sex difference in the age of first marriage (Goody 1973:184-185). In addition, natural selection may have favored an increased predisposition to polygyny that persists even when the adaptive landscape has changed. ------------------------------------------------------------------------How to contribute to H-Bd: 1. To reply privately to just the sender of this message, click the "Reply" button on your email package. 2. To reply publicly to the entire H-Bd list, click the "Reply All" (or equivalent) button o n your email package. 3. To start a thread, email your message to h-bd@egroups.com ------------------------------------------------------------------------ -- Create a poll/survey for your group! -- http://www.egroups.com/vote?listname=h-bd&m=1 Date sent: Thu, 02 Dec 1999 12:06:57 -0500 From: "J. P. Rushton" [ Double-click this line for list subscription options ] Phil Rushton responding to Peter Frost's post on the low sex ratio (more females) in Africa. The value (once again) of not making theories for Africans separate from the East Asian-European-African gradient. East Asians have a sex ratio opposite (once again) to Europeans with more males, and not duie to the Chinese one child policy because it is true in Japan and of east Asians in the USA. So whatever the cause, it is not something specif to Africa vs the rest of the world but is fine grained going from East Asia to Europe to Africa. r-K selection again? But I'm not sure how. More sex hormones in Africa (they show the three way gradient). Just better food and economic circumstances? (In other animals leading to more male births). An interesting puzzle and lots of interesting data reviewed. ------------------------------------------------------------------------How to contribute to H-Bd: 1. To reply privately to just the sender of this message, click the "Reply" button on your email package. 2. To reply publicly to the entire H-Bd list, click the "Reply All" (or equivalent) button o n your email package. 3. To start a thread, email your message to h-bd@egroups.com ------------------------------------------------------------------------ -- Check out your group's private Chat room -- http://www.egroups.com/ChatPage?listName=h-bd&m=1 reply.... In Japan, 105.4 boys were born last year for every 100 girls. The ratio has not changed since 1899 and is only a bit above the world average. Much higher figures (118) have been reported from China, but this probably reflects unreported female infanticide as well as the sex ratio bias that results when couples stop at their first boy child. The highest sex ratios at birth come from Eastern Europe (Hungary, Ukraine, Poland), where values of 107-109 are the norm.