Abstract Demographic data from 848 Gabbra house- holds are used to examine the relationships between herd size and reproductive success in relation to sex,.
© Springer-Verlag 1996
Behav Ecol Sociobiol (1996) 38 : 75–81
Ruth Mace
Biased parental investment and reproductive success in Gabbra pastoralists
Received: 30 June 1995 / Accepted after revision: 23 October 1995
Abstract Demographic data from 848 Gabbra households are used to examine the relationships between herd size and reproductive success in relation to sex, in a traditional, pastoralist population. The number of camels in the household herd has a significant positive effect on the reproductive success of both men and women, although the effect of wealth is greater for men, as predicted from evolutionary theory. The greater the number of elder brothers a man has, the lower his reproductive success, as a result of a smaller initial herd and a later age at marriage. This is not true for women – number of elder sisters does not have a measurable effect on a woman’s fertility, although it does have a small, negative effect on the size of her dowry. These results are interpreted as competition between same-sex siblings for parental investment, in the form of their father’s herd, which is more intense between sons than daughters as parental investments are greatest in males. Key words Gabbra · Wealth · Reproductive success · Birth order · Parental investment
Introduction It is a central presumption of most studies of human biocultural adaptations that people should spend their wealth on maximizing reproductive success. There is considerable evidence accumulating that wealth and reproductive success are correlated in traditional societies (e.g. Irons 1979; Turke and Betzig 1985; Borgerhoff-Mulder 1987; Voland 1990; Casimir and Rao 1995). Here I shall examine the precise nature of
R. Mace Department of Anthropology, University College London, London WC1E 6BT, UK
that relationship for each sex in a traditional, pastoralist population, and investigate the implications that such relationships have for understanding strategies of parental investment.
Materials and methods People and environment The Gabbra are a group of camel-herding, nomadic pastoralists who inhabit an arid area to the east of Lake Turkana in northern Kenya. The Gabbra are some of the most nomadic peoples in Africa. In a large sector of the community, the whole family migrates, transporting a hut for each married woman by camel. Camels, goats and sometimes sheep are milked, and sheep and goats are regularly slaughtered for meat or sold to markets. Herds of sheep, goats and camels may be herded far from the family huts, which is known as being on fora. This occurs when greater flexibility is required, such as when it is important to migrate rapidly to good grass, possibly in insecure areas, leaving a small section of the herd behind with the rest of the family. Herding strategies are discussed in more detail elsewhere (Mace 1993a). The society is patrilocal and patrilineal. Polygyny is limited – 88 % of women were monogamously married – although it is not uncommon for an older man to marry a second wife when the first wife approaches menopause, should he want more children. In only two cases did a man have three living wives. Virtually all women are married between the ages of 15 and 25. The great majority of men also marry, although some may marry very late. Divorce is very rare. Girls are circumcised and infibulated as young children, and sex before marriage for women is strictly forbidden, with dire consequences (such as expulsion from society) for both the man and the girl should an unmarried girl become pregnant. No cases of pregnancy prior to marriage were reported to us in our survey. Widows are not permitted to remarry but can continue to bear children. Children born to widows are considered by the society as sons and daughters of the dead husband. There are few medical or educational facilities available to those Gabbra living as nomads. Droughts occur regularly, but unpredictably; the most recent were in 1984 and in 1992. On both occasions food aid had to be brought into the area to avert starvation in the human population. These droughts may have been unusually severe, or, more likely, may simply have had unusually harsh effects due to the rapid growth of the population.
76 Data collection Data on 848 rural Gabbra families were collected between July and December 1993. Many of these families were still settled around small towns to which they had migrated during the drought in 1992, in order to collect food aid. Most stated their intention to return to nomadic life when circumstances allowed. Interviews on household composition and herds owned were conducted by four local Gabbra assistants (three women and one man). A household was relatively easily defined as that unit that owned a camel herd. Sons frequently shared a herd with their father, in anticipation of inheriting it on his death. Brothers frequently shared a herd, with the eldest brother as head of household. Such groups were considered one household. Brothers or sons that had separated out their own camel herds were counted as heads of their own households. Information on the age, sex and marital status of every member of each household, including those that were present on the day of the interview and any who normally lived with them but were absent at the time, was collected from the head of household (nearly always male) or the next most senior person if he was absent. The head of household also gave his own marriage history. Every married woman was then asked for a complete birth history. Generally women were interviewed by the women assistants.
Results Fertility The fertility (number of children ever born alive) of married women as a function of age is shown in Fig.1a. This includes all children reported either alive or now dead. Fertility for married men by age is shown in Fig. 1b. This includes reported children born to all wives, including any former wives, that had since died. I have no doubt that a proportion of children that died in infancy were not reported, although it is difficult to estimate the extent of this under-reporting of mortality. Female fertility plateaus around age 45, due to menopause. Male fertility continues to increase right on into old age, although it is of course possible that some of those children assigned to these men may in fact be the genetic progeny of others. I have calculated the fertility of a man or woman relative to the average fertility of a man or woman of the same age in the sample. The average fertility of women over 45 is taken as a single measure of all the women over 45 in the sample. The average fertility of all men over 70 is taken as one measure, due to rapidly declining sample size after this age. Other average fertilities are taken as the five year averages, as shown in Fig. 1a and b. I have called observed minus average age-specific fertility ‘‘residual fertility’’, which gives a measure of the number of children a person has had relative to others in the population of the same age and the same sex. I use this value as the measure of reproductive success of an individual in all the analyses. This measure of residual fertility has the advantage that it takes into account the non-linearity of the sex-specific
Fig. 1 a The effect of age on fertility for ever-married male heads of household. b The effect of age on fertility for ever-married women. Numbers above data points are sample sizes
relationship between age and fertility (whereas including ‘‘age’’ as a variable in linear regression would calculate residuals from a linear fit). Wealth and reproductive success Figure 2a shows the effect of the number of female camels in the herd on the residual fertility of evermarried male heads of household. Fig. 2b shows the residual fertility of ever-married women as a function of the number of female camels in the household herd. No attempt is made to divide the herd between women, because, whilst women may have their own goats and sheep, camels are owned by men; and where there is more than one man in the household, the camels are considered jointly owned. Both functions are nonlinear. Figure 2c shows the best-fit regression curves [residual fertility regressed against log wealth (no. of adult female camels in the herd + 1)] for Fig. 2a, b. Results are presented with fertility corrected for age because wealth correlated positively with age, and therefore a presentation of wealth and fertility alone would be highly confounded with age effects. It is clear that wealth has a positive effect on the reproductive success of both men and women, independent of age, although the effect is greatest for men. These effects could be due to wealth causing any or all of higher fertility, lower mortality in their children and earlier age at marriage in both sexes. The effect of the
77 Table 1 Components of residual fertility related to wealth Coefficient
t-ratio
P
Men: Age at marriage Log wealth r2 = 22.0%
[0.123 0.364
[11.23 4.13
< 0.001 < 0.001
Women: Age at marriage Log wealth r2 = 2.3%
[0.0137 0.150
[4.80 2.57
< 0.001 0.01
poor women, although in both sexes there was still a highly significant effect of wealth after removing the effects of age at marriage (Table 1), suggesting that other factors such as lower fertility and /or higher mortality (which cannot be directly measured due to the under-reporting of child deaths) must also be correlated with wealth. It should also be noted that older families with many children may have reduced their wealth by investing in their children in various ways, such as paying for their marriages. Thus the total amount of wealth that was available to enhance reproduction is not captured by current wealth and these relationships probably under-estimate the benefical effects of wealth on reproductive success. Sex-biased parental investment
Fig. 2 a The effect of the number of adult female camels in the household herd on ever-married male head of household residual fertility. The best fit regression equation is: residual fertility = [0.714 + 0.516 log (no.camels + 1), r2 = 4.6%, P < 0.001. b The effect of the number of adult female camels in the household herd on ever-married women’s residual fertility. The best fit regression equation is: residual fertility =[0.292 + 0.198 log (no.camels + 1), r2 = 0.8%, P = 0.002. c The best fit regression curves of log (no. of female camels + 1) for the data in a (male heads of household) and b (women)
wealth of the household (i.e. of the husband) on female fertility is steep when households are relatively poor but the additional benefits become less the wealthier the household. This is probably due to both a biological upper limit on the fertility of women and to many such large herds being shared between more than one wife. Poverty has adverse effects on both men and women, but very poor men are more disadvantaged, relative to their age-mates, than are very poor women. This may be due to later marriage by poor men as well as lower fertility or higher mortality in their children. Age at marriage was significantly later in poor men and
That male reproductive success is more variable than female reproductive success, and thus that resources are likely to have a more marked influence on male than on female reproductive success, is predicted by evolutionary theory, especially in polygynous species or societies (Fisher 1958; Orians 1969; Trivers 1972). This leads to the prediction that we will find malebiased inheritance in such societies, which is found here in the Gabbra, and it has been shown across cultures that male-biased inheritance is more common the higher the degree of polygyny (Hartung 1982; Mace and Pagel 1996). Trivers and Willard (1973) have argued that the greater variance in male reproductive success favours sex biases in parental investment according to parental resources, which can lead to biased sex ratios. The presumption of a relationship such as that shown in Fig. 2 has been used to explain a number of cases of sex-biased parental investment in humans, where sex ratios are thought to have been modified by the passive or active infanticide of either girls (e.g. Dickemann 1979; Smith and Smith 1994) or boys (e.g. Cronk 1991; Low 1991; Pennington and Harpending 1993) – whichever is the least favoured sex. Such an argument might predict that poor Gabbra families, especially those with fewer than three female camels (see Fig. 2c), might favour daughters, and that richer families might favour sons. There was, however,
78 Table 2 Total number of living sons and daughters in households with fewer than three female camels, and household with three or more female camels
Total living sons
Total living daughters
Total living children
Households < 3 camels
Observed = 591 Expected 589.1
Observed = 559 Expected 560.9
1150
Households > 3 camels
Observed = 875 Expected 876.9
Observed = 837 Expected 835.1
1712
Total
1466
1396
2862
χ2 = 0.022, not significant
no evidence of such a switch in investment leading to a shift in sex ratio with wealth, both groups showing a slightly male-biased sex ratio ( Table 2). However, active or passive infanticide is only one, rather extreme, outcome of biased parental investment. I came across no evidence that infanticide was practised among the Gabbra (with the possible exception of first-born twins). Nor would I expect it to be common. In pastoralist or agriculturalist societies, children’s labour may be useful to adults from quite an early age. Both Gabbra girls and boys help with herding from the age of 4 – 5, and may spend long periods away from home with fora herds from the age of 9 or 10. The costs of feeding children may well be balanced by their usefulness as herders. At marriage, girls will bring in a brideprice to their natal family. This is traditionally set at three camels by the Gabbra, of which one camel goes to a maternal uncle. The bride may be given some stock, nearly always sheep and goats, to take with her at marriage (which I have called dowry) but this is not universal. The average dowry size reported was 16.5 sheep units, ranging from 0 to 90 (where one camel is assumed to be of the value of 10 sheep or goats). Thus the net parental investment in girls at marriage (brideprice / dowry) is likely to be low or even negative, although it should be noted that marriage does entail various other obligations to in-laws throughout their lives, which are rather hard to quantify. Sons are more expensive to marry off. Parents need to pay the brideprice, and also provide their son with animals with which to support the bride and their future children – which may be a greater cost than the brideprice. Heads of household reported that the average size of their camel herd at the formation of their household was 6.75 camels. This is about 67 sheep units, i.e. over 4 times the average gift to daughters at their marriage. When the brideprice is also taken into account, each son costs an average of nearly 10 camel units to marry off. However, if parents cannot afford to marry off all their sons, then unmarried adult sons are useful herd managers. Thus if parents wish to bias investment in their children, infanticide or neglect is not necessary or advantageous; investment can be biased by the number of animals passed on to a child at their marriage.
Birth order and reproductive success Figure 3a shows the influence of elder brothers on the reproductive success on married male heads of household. Figure 3b shows the effect of number of elder sisters on the reproductive success of married women. It is clear that whilst boys are disadvantaged by elder brothers, girls are not disadvantaged by elder sisters. The magnitude of the disadvantage of elder brothers to boys is probably greatly underplayed by this figure. First, it excludes men who are not married at all, and excludes men who are married but are not heads of household, and it excludes men who may have left the area altogether – men in all these groups with lowered reproductive success are likely to be lower down the birth order. Fratkin (1991) describes how lower birthorder sons in the Rendille (another camel-keeping group with primogeniture) are more likely to migrate
Fig. 3 a The effect of elder brothers on the residual fertility of male heads of household. b The effect of elder sisters on the residual fertility of ever-married women. Error bars are SEs
79
out into another ethnic group, the Ariaal. Second, birth order includes elder brothers that may have died young, so some of those heads of households who were laterborn sons may in fact have been the first son to marry or inherit family wealth. Even so, a detrimental effect of elder brothers on male reproductive success is still clear. Total number of sisters in their natal family actually has a significant positive effect on the reproductive success of male heads of household. The regression equation is: residual fertility = [0.031 [0.259 elder brothers + 0.094 sisters (r2 = 3.5 %, P < 0.001). Very similar effects of elder brothers causing a negative effect and sisters causing a small positive effect were found for the male head of household’s wealth now and his wealth at the time that he became head of household. This suggests that sisters might be a net asset in terms of wealth creation, although their influence appears very small. I interpret this result as a consequence of the competition between boys for their father’s herd. It is the responsibility of a father to make arrangements for the division of his herd amongst his sons. Should he die without making any arrangements, his eldest son will inherit the whole herd (although he will also inherit the responsibilities of the head of the family, such as arranging the marriages of his unmarried siblings). Given that male reproductive success is sensitive to wealth, it may be in a father’s best interest, in terms of maximizing grandchildren, to make sure that a small number of his sons are well provided for, rather than necessarily giving equal investment to all his sons if that would render all of them poor and uncompetitive in reproductive terms. Both theoretical (Mace 1993b) and empirical (Mace 1989) studies suggest that very small herds are not viable for pure pastoralists, unless families have recourse to an additional source of food or income. If only a small number of sons could be given herds above some critical minimum size, in such a case it would make sense to favour the earlier over the later-born sons: if maximizing fitness over future generations, there is a disadvantage to waiting. Also, an older son may be able to control younger brothers and co-opt their help with herd management for as long as possible, before they exert overwhelming pressure to be married themselves. Conflicts of interest between younger sons and fathers or elder brothers regarding dates of marriages were apparent on many occasions. Such difficult decisions do not normally arise with daughters, or if they do arise, the competition is less intense because the levels of wealth involved are much lower. Figure 4 shows that the number of elder sisters a woman has does have a small effect on the size of her dowry. As wealth does influence reproductive success in women (Fig. 2b) it seems likely that this might have a small effect on fertility, but it was not measurable (Fig. 3b). The number of brothers a girl has, had
Fig. 4 The effect of elder sisters on the size of dowry given to women at their marriage. Error bars are SEs
a small additional positive effect on the size of her dowry, suggesting that whilst competition exists between sisters, it does not between sisters and brothers. The regression equation is: dowry = 15.8[0.70 elder sisters + 0.55 brothers (r2 = 1.5 %, P = 0.014). This is the parallel and opposite case to that described for the parental investment in boys at household formation. Thus competition between siblings for parental investment at household formation appears to be only with those siblings of the same sex. That the small decline in dowry represents reduced parental investment in late-born daughters is not the only possible interpretation. It could be that the fathers of late-born daughters are more likely to be dead at the time those daughters marry. Those girls might then be reliant on a brother or uncle to provide the dowry, and find themselves in competition with that man’s own children for stock. This effect could also contribute to the decline down the birth order in the investment in males at household formation, although father being dead is unlikely to fully explain the bias towards firstborns, as cases of favouring first-born sons are apparent in many families where the father is still living. The effect of birth order for men appears to occur both through lower birth-order men having fewer stock, and through their marrying later. Herd size at household formation, included in a regression of residual fertility against number of elder brothers, was a significant factor but still left a highly significant influence of elder brothers, indicating that the size of their endowment at household formation is one but not the only factor contributing to the lower reproductive success of lateborn sons. If age at marriage was also included, then only a slight and non-significant effect of elder brothers remained, indicating that these two factors together largely explain how it is that late-born sons are disadvantaged. The full regression equation is: residual fertility = 3.63 + 0.31 (log wealth at household formation) [0.128 (age when married)[0.031 (elder brothers) (r2 = 22.4 %, P < 0.001). The magnitude of these effects is shown in Fig. 5a and b. First-born sons have a marked advantage over all other sons in terms of herd size at household formation. Camel pastoralists are
80
Discussion
Fig. 5 a The effect of the number of a man’s elder brothers on the number of camels he had at the time of the formation of his own household. b The effect of the number of elder brothers on a man’s age when first married. Error bars are SEs
generally described as showing primogeniture; however primogeniture is clearly not exclusive here, with those later-born sons that manage to establish households doing so with inital herds about two-thirds the size of their first-born brothers. But Fig. 5b shows that they may have to wait several years for this opportunity. This could be due partly to fathers and elder brothers exploiting the labour of their younger sons and brothers as herders, but could also be due to needing to give the family herd enough time to grow to enable each son to be endowed with enough stock at marriage to make a successful household of his own. It may be in the interest of parents and their sons to make each son wait, possibly many years, for marriage, rather than allowing them to start married life with too small a herd to raise a family successfully. These results report the size of gifts at marriage and household formation of all currently living married men, and hence reflect a period from about 50 years ago to the present. It should also be noted that both reported dowry (a measure of parental investment in daughters) and the reported size of the herd at the time of becoming head of household (a measure of parental investment in sons) has declined significantly over that 50 years. This is likely to be due to a general decrease in numbers of livestock per person as the human population has been expanding at a rate faster than their herds.
A number of studies have found an effect of birth order on child mortality in historical European populations (Cohen 1975; Wrigley and Schofield 1981; Voland and Dunbar 1995). However it is known that there is frequently great heterogeneity in the child mortality suffered by mothers (e.g. Pennington and Harpending 1993; Low 1991); those mothers experiencing high child mortality will have more births, making late-birth-order children appear to have higher than average mortality without necessarily indicating any bias in parental investment against them. Voland and Dunbar (1995) show an effect of birth order on migration patterns prior to marriage in a historical German population. Low (1991) has shown that total number of siblings and specifically elder brothers correlates with the reproductive success of married adult men, as reported here for the Gabbra. Birth-order effects in these historical European populations have been interpreted as the results of competition for heritable resources. Gabbra pastoralists may be suffering competition with samesex siblings for limited herds, not dissimilar to the competition experienced in historical populations in Europe for farmland. The relationships between wealth and reproductive success in relation to sex in the Gabbra fit with predictions from evolutionary theory. First, wealth translates into reproductive success, and second, the effect of wealth on male reproductive success is much greater than on female reproductive success. Such characteristics predict greater parental investment in males than in females, which is found in gifts at marriage and household formation. The greater influence of wealth on male than on female reproductive success might be expected to lead to manipulations of the sex ratio dependent on wealth (Trivers and Willard 1973). This shift in sex-ratio is not found here, despite clear evidence that poverty is especially damaging to male reproductive success, which is because sex ratio manipulation in favour of one sex is not necessarily expected in human societies with heritable wealth. In such societies, the much more widespread method of investment bias within families is probably the division of wealth – a much finer tool with which parents can maximize their long-term fitness. The concept of a ‘‘preferred sex’’ ignores the influence of birth order – sons may be preferred when they are few, but daughters may be preferred when the family already contains a number of sons. I have developed a formal model of the influence of marriage costs, and their interaction with mortality risk on Gabbra reproductive decision-making and discussed the implications for sex preference elsewhere (Mace 1996a and b). With respect to biases in inheritance, primogeniture is the most obvious example of parental investment biased towards first-born sons (see Hrdy and Judge 1993 for a review). An interaction between sex and
81
birth order has been found in infanticide; firstborn daughters are more likely to survive in societies where female infanticide is common (Dickemann 1979, DasGupta 1987). Early-born children of either sex may be favoured in marriage also (e.g. Johansson 1987). The division of resources through inheritance and marriage gifts, taking account of wealth, sex and birth order, appears to be the means by which parental investment is biased to maximize grandchildren in the Gabbra. Competition for that parental investment appears to be greatest between same-sex siblings, especially between brothers. This is likely to be the case over a range of societies with heritable wealth. Acknowledgements This research was funded by the E.C. (DGXII) through a grant to the International Institute of Environment and Development (IIED) and by the Royal Society. Data was collected with the assistance of Bonaya Abiyo, Halima Adan, Galgallo Bonaya, Kame Kutu and Sororo Orto. Rebecca Sear helped greatly with data analysis. Monique Borgerhoff-Mulder, Sarah Hrdy and Dan Sellen made very helpful comments on the manuscript. The Government of Kenya kindly gave permission for this research to be carried out.
References Borgerhoff-Mulder M (1987) On cultural and biological success: Kipsigis evidence. Am Anthropol 89:619–634 Casimir M, Rao A (1995) Prestige, possessions and progeny: cultural goals and reproductive success among the Bakkarwal. Human Nat 6: 241–272 Cohen JE (1975) Childhood mortality, family size and birth order in pre-industrial Europe. Demography 12:35–55 Cronk L (1991) Wealth status and reproductive success among the Mukogodo of Kenya. Am Anthropol 932:345–360 DasGupta M (1987) Selective discrimination against female children in rural Punjab, India. Popul Dev Rev 13: 77–100 Dickemann M (1979) Female infanticide, reproductive strategies and social stratification. In: Chagnon NA, Irons W (eds) Evolutionary biology and human social behaviour: an anthropological perspective. Duxbury, North Scituate Fisher RA (1958) The genetical theory of natural selection. Dover, New York Fratkin E (1991) Surviving drought and development: Ariaal pastoralists of northern Kenya. Westview Press, Boulder Hartung J (1982) Polygyny and the inheritance of wealth. Curr Anthropol 23:1–12 Hrdy SB, Judge DS (1993) Darwin and the puzzle of primogeniture: an essay on biases in parental investment after death. Human Nat 4: 1–45
Irons W (1979) Cultural and biological success. In: Chagnon N, Irons W (eds) Evolutionary biology and human social behaviour: an anthropological perspective. Duxbury, North Scituate, pp 257–272 Johansson SR (1987) Status anxiety and demographic contraction of privileged populations. Popul Dev Rev 13 : 439– 470 Low B (1991) Reproductive life in 19th century Sweden: an evolutionary perspective on demographic phenomena. Ethol Sociobiol 12 : 411– 448 Mace R (1989) Gambling with goats: variability in herd growth among restocked pastoralists in Kenya (ODI pastoral development network paper 28a). Overseas Development Institute , London Mace R (1993a) Nomadic pastoralists adopt subsistence strategies that maximize long-term household survival. Behav Ecol Sociobiol 33 : 363–382 Mace R (1993b) Transitions between cultivation and pastoralism in sub-Saharan Africa. Curr Anthropol 34 : 363–382 Mace R, Pagel M (1996) Tips, branches and nodes: seeking biocultural adaptations through comparative studies. In: Betzig L (ed) Human nature: a critical reader. Oxford University Press, New York, in press Mace R (1996a) When to have another baby: a model of reproductive decision-making and evidence from Gabbra pastoralists. Ethol Sociobiol, in press Mace R (1996b) Reproductive decisions by pastoralists in the face of demographic risks. Nomadic Peoples, in press Orians GH (1969) On the evolution of mating systems in birds and mammals. Am Nat 103 : 589– 603 Pennington R, Harpending H (1993) The structure of an African pastoralist community: demography, history and ecology of the Ngamiland Herero. Oxford University Press, Oxford Smith EA, Smith SA (1994) Inuit sex-ratio variation: population control, ethnographic error, or parental manipulation? Curr Anthropol 35 : 595– 624 Trivers RL, (1972) Parental investment and sexual selection. In: Campbell B (ed) Sexual selection and the descent of man: 1871–1971. Aldine, Chicago Trivers RL, Willard DE (1973) Natural selection of parental ability to vary the sex ratio of offspring. Science 179 : 90–92 Turke P, Betzig L (1985) Those who can do: wealth status and reproductive success on Ifaluk. Ethol Sociobiol 6 : 79–87 Voland E (1990) Differential reproductive success within the Krummhorn population (Germany 18th and 19th centuries). Behav Ecol Sociobiol 26 : 65–72 Voland E, Dunbar R (1995) Resource competition and reproduction: the relationship between economic and parental strategies in the Krummhorn population (1720–1874). Human Nat 6 : 33– 49 Wrigley EA, Schofield RS (1981) The population history of England, 1541–1871: a reconstruction. Arnold, London
Communicated by M. Borgerhoff-Mulder
