Fitness and fertility among Kalahari !Kung

In this paper we develop a model that examines fertility and childhood mortality patterns and their relationship to environmental variables. Interactions among environmental variables can account for different fertility patterns and different mixes of these variables can produce similar patterns of fertility. Our model attempts to quantify the idea that there is a trade-off between producing a few children likely to survive to reproductive age and producing a greater number of children with lower chances for survival. The optimum mix of these strategies depends on environmental characteristics. We use the model to make predictions about fertility and mortality patterns among two Bushmen populations of southern Africa--the Ghanzi and Ngamiland !Kung--using data collected by Harpending in 1967-1968. The results do not support explanations of the low fertilities observed among !Kung Bushmen women, in whom it is thought that fitness is maximized by limiting fertility, and show no relationship between mortality and family size in either !Kung population. Instead, the number of offspring reaching reproductive age in both populations increases as their completed family size increases. We examine the effects of sex, birth order, and paternal investment on mortality. No sex ratio differences and no differences in mortality by sex or birth order are present. Infant mortality among women who married more than once is significantly higher than among women who married once, suggesting that paternal care has a significant effect.     

Hunter-gatherer energetics and fertility: A reassessment of the !Kung San

This paper examines the question of why the total fertility rate of the !Kung San hunter-gatherers of the Northern Kalahari desert is as low as 4.69 births. When the intermediate variables involved are examined through the employment of a reproductive equation, it becomes clear that low fecundity is a major issue. Arguments offered previously to explain the low fertility of !Kung women, depending on factors such as nutrition, health status, and lactational practices are insufficient. Drawing upon recent data from sports medicine and endocrinology, I suggest that the pattern of San female energetics in their gathering and subsistence routine has a direct effect upon their fecundity. Such a correlation between activity patterns, endocrine function, and reproductive capacity may also be important for understanding the fertility of other mobile hunter-gatherer groups.     

Did food increase fertility? Evaluation of !Kung and Herero history.

Low fertility among nomadic !Kung foragers of the northern Kalahari Desert of Botswana has been hypothesized to be an adaptation to scarcity of food. However, a comparison of !Kung fertility before and after a transition to a more sedentary lifestyle indicates that more food did not increase fertility. An examination of the fertility of neighboring sedentary Bantu-speaking Herero pastoralists during the same period also indicates that low female reproductive rates in this region are not unique to the !Kung. Herero fertility has increased dramatically in recent decades, probably in response to the control of sexually transmitted diseases in northwestern Botswana. More food appears to have substantially increased !Kung reproductive success by reducing infant and child mortality rates to levels observed among the Herero. These findings suggest that low !Kung fertility and mortality reflect contact with the Herero, who began expanding in large numbers into !Kung territory in the 1950s. This study emphasizes the need for a more rigorous comparative perspective in anthropology to understand better the significance of findings from restricted populations, and I suggest that evolutionary ecology would benefit from shifting some of its current focus on fertility to mortality.     

Who Responds to Crying?

!Kung San (Bushman) hunter-gatherers have unusually high levels of mother-infant contact and represent one of the environments of human evolutionary adaptedness (EEAs). Studies among the !Kung show that levels of crying—the most basic sign of mammalian infant distress—are low, and response to crying is high, and some suggest that responses are overwhelmingly maternal. We show that although !Kung mothers respond to crying most often, one-third of crying bouts are managed solely by someone else. Mothers responded to all bouts lasting ≥30 s, but in half of these responses they were joined by one or more others. Mothers are the most consistent responders, but multiple caregiving is common. The mother is rarely alone when her baby cries; others often substitute or join her in interventions. This social support may facilitate the high levels of maternal responsiveness characteristic of the !Kung, and of hunter-gatherers generally, but it is also consistent with recent theory emphasizing nonmaternal care (allocare) and cooperative breeding.     

Population genetics study of the differential fertility in urban populations

In an urban population with widespread birth control practice the distribution of the number of pregnancies, births and abortions was studied in a cohort of women of completed fertility. The mean number of pregnancies per woman was 4.03 +/- 0.08 (sigma = = 2.98); the mean number of births - 1.12 +/- 0.02 (sigma = 0.77). 7.4% of women which had completed their reproductive performance had no pregnancies and 19.5% - no births. The Crow's Index of the Opportunity for Selection and its components connected with differential fertility and differential mortality were estimated. In the population under study two components of selection - selection at the prenatal stages and selection associated with infertility - are shown to be still significant. Such type of selection is exemplified by investigation of couples suffering from repeated spontaneous abortions.     

Premarital childbearing in northwest Kenya: challenging the concept of illegitimacy

A number of African countries, including Kenya, have experienced a marked rise in births among unmarried women. In Western countries, reproduction outside of marriage is assumed to be illegitimate and a social problem. One hypothesis used to explain the increasing incidence of premarital fertility in Africa is a breakdown of traditional social controls by the extended family over the sexual behavior of adolescents. A competing hypothesis suggests that unmarried women use sexual relations to achieve goals such as marriage. Among Turkana pastoralists of northwest Kenya, we find a pattern of premarital birth that fits either hypothesis only loosely. Premarital fertility among the Turkana is both widespread and culturally acceptable, with over 30 per cent of women having at least one child prior to marriage. Although women with premarital births initiate childbearing on average one year earlier than women with only marital births, women's marital status does not influence the length of the interval between first and second births. Marriage among the Turkana is not a social trigger for the onset and continuation of reproduction or a means to legitimate reproduction. Marital status of the parents simply determines the custody of a child. In a premarital birth, the father pays a set fee to the mother's family, and the custody of the child remains permanently with the mother's family. If the parents later marry, the father must purchase custody of the child by another fee at that time. Since the Turkana have experienced few effects of modernization, the existence of such a practice suggests that cultural factors must be taken into account before assessing premarital fertility in Africa as a social problem.     

Demographic and health status risk factors in childbearing among Indian women. Evidence from hospital data for the later stages of fertility decline

Indirect estimates of maternal mortality in India indicate that fertility decline has reduced maternal deaths by reducing the frequency of pregnancy and childbirth. The earlier stages of fertility decline are also likely to have lowered maternal mortality by reducing the risk of pregnancy and childbirth as the proportion of births among risky multiparous, older women declines. However, further fertility decline may well be associated with some increase in risk. Risk will also remain high if the health status of Indian girls and women remains poor. This study uses a sample of maternal deaths and deliveries among patients who survived which occurred in Civil Hospital, Ahmedabad, Gujarat during 1982-1993 to investigate these issues further. The women in the sample have relatively low fertility and represent a fairly late stage of fertility decline. They also have persistently poor health status. Logit regression analysis reveals that although fertility decline is associated with some increase in risk, poor health status is the more important maternal mortality risk factor. Without attention to female health, even childbearing among expectant mothers with low fertility continues to be hazardous.     

Fertility decline driven by poverty: the case of Addis Ababa, Ethiopia

Demographic transition theory states that fertility declines in response to development, thus wealth and fertility are negatively correlated. Evolutionary theory, however, suggests a positive relationship between wealth and fertility. Fertility transition as a result of industrialization and economic development started in the late 19th and early 20th centuries in Western Europe; and it extended to some of the Asian and Latin American countries later on. However, economic crises since the 1980s have been co-incident with fertility decline in sub-Sahara Africa and other developing countries like Thailand, Nepal and Bangladesh in the last decade of the 20th century. A very low level of fertility is observed in Addis Ababa (TFR=1.9) where contraceptive prevalence rate is modest and recurrent famine as well as drought have been major causes of economic crisis in the country for more than three consecutive decades, which is surprising given the high rural fertility. Detailed socioeconomic and demographic characteristics of 2976 women of reproductive age (i.e. 15-49 years) residing in Addis Ababa were collected during the first quarter of 2003 using an event history calendar and individual women questionnaire. Controlling for the confounding effects of maternal birth cohort, education, marital status and accessible income level, the poor (those who have access to less than a dollar per day or 250 birr a month) were observed to elongate the timing of having first and second births, while relatively better-off women were found to have shorter birth intervals. Results were also the same among the ever-married women only model. More than 50% of women currently in their 20s are also predicted to fail to reproduce as most of the unmarried men and women are 'retreating from marriage' due to economic stress. Qualitative information collected through focus group discussions and in-depth interviews also supports the statistical findings that poverty is at the root of this collapse in fertility. Whilst across countries wealth and fertility have been negatively correlated, this study shows that within one uniform population the relationship is clearly positive.     

Socioecological aspects of human reproduction in Sub-Saharan Africa

The purpose of this study was to compare fertility outcome of two populations of northern Namibia, following different ways of subsistence. The total number of offspring, but also the number of dead and surviving offspring was compared between 236 !Kung San (91 females, 145 males) hunter gatherers and 248 Kavango (87 females, 161 males) horticultural pastoralists and a small number of Kavango people living in the urban center of Rundu. While no typical differences in fertility outcome between the study populations could be observed in males, marked differences were found for the female sample. As to be expected traditional Kavango women had given birth to a higher number of children and these children had a higher chance to survive in comparison to those of !Kung San women. On the other hand Kavango females living in urban centers reported a significantly lower number of offspring. It can be concluded that even in recent populations fertility differences according to subsistence patterns are observable.     

Marital fertility in polygynous unions in Nigeria

The effect of polygyny on fertility, by age cohort, was examined in 4 regions of Nigeria through use of data from the 1981-82 Nigerian Fertility Survey. Simple tabulation of numbers of live births by maternal age and type of marriage union indicated that, with the exception of the 30-39-year age group, fertility in polygynous unions tends to be higher than that in monogamous marriages. Overall, such tabulations reveal an average of 3.90 births among polygynous women and 3.47 births among monogamous women. However, when age-specific fertility rates were compared, except for women under 20 and over 40 years of age, rates were higher in monogamous unions (7.286 overall) than for women in polygynous households (7.200 overall). Mean completed fertility, taking into account marriage duration as well, shows a tendency for women in polygynous unions to be more fertile (with the exception of women aged 25-44 years who had been married 10-19 years). The absolute difference in fertility between the 2 types of marital unions ranged from 0.32 children in the northwest to 0.64 in the southeast. With adjustment for independent variables such as education, the absolute mean differences were reduced, from between 0.28 children in the southeast to 0.42 in the northeast, but the direction of high fertility was still in favor of women in polygynous unions. But when the covariates (e.g., age at marriage) were taken into account as well as the independent variables, there was a dramatic reduction in the mean difference between the fertility of these 2 groups of women. These findings suggest that some changes in reproductive behavior are taking place in Nigeria that are restricting the fertility of women in polygynous unions. These changes are hypothesized to reflect the spread of formal education in Nigeria, with the expectation that women will contribute to the costs involved in educating their children.     

A relative cohort size forecasting model of Canadian total live births

Easterlin and Condran suggest that Canadian fertility is dictated by a relative cohort size (or relative income) model of fertility which points to a resurgence of fertility rather than a decline in the next 2 decades. Samuelson (1976) formalized Easterlin's model and shows that if the Easterlin effect is sufficiently strong, cycles in fertility will result. Ahlburg (1983) demonstrates that this model fits the US postwar fertility data very well and predicts the revival of fertility in the 1980s and 1990s. This model is applied to Canadian data as a check on Easterlin and Condran's earlier analysis and to determine whether the model also predicts a revival of Canadian fertility. According to the model, increases in the size of a birth cohort relative to preceding cohorts reduce that cohort's income relative to that of smaller cohorts and consequently reduce the fertility of the large cohort. In this manner, the size of a particular cohort is a function of the size of previous birth cohorts. The model assumes that immigration is zero, or, at least, relatively small. From the statistics, it does not appear that emigration or immigration was sufficiently large to alter significantly the age-structure of the Canadian population. Additionally, foreign-born women have much lower fertility than native-born women, particularly among postwar immigrants. Thus, the model outlined may be a useful approximation to the Canadian experience. Table 1 reports the results of estimating the model for Canadian data for the 1941-79 period. The results are consistent with the relative cohort size model that is outlined. The variables were correctly signed and significant at the 0.05 level in 1-tailed tests. The overall equation was significant at the 0.91 level. The forecasts with 20-year lags from the relative cohort model lie between the low and medium forecasts of Denton et al. until the end of this decade and between the medium and high forecast for the remainder of the 20th century. The model based on 25-year generational lags predicts 20% fewer births in the year 2000 than does the basic model, although it predicts births to increase significantly relative to the levels of the early 1960s. Policy decisions based on the unchallenged assumption of a continuance of low fertility may be ill-advised.     

Correlates and implications of breast-feeding practices in Bas Zaire

A sample of 1871 women having a child under 3 years old in Bas Zaire was studied to determine the correlates of breastfeeding practices and to examine the interrelationships among breastfeeding, contraceptive practices and desire for pregnancy. The methods of analysis applied were life table analysis and its multivariate extensions. Most of the findings in this analysis are consistent with current literature on the correlates of the duration of breastfeeding. Maternal education, economic status, age, parity, urban residence, pregnancy, and sex of the index child were significantly related to the length of breastfeeding. Among non-pregnant women, current desire for pregnancy was also related to breastfeeding status when the length of time since birth of the last child was taken into consideration. Rural women were reportedly ready for another pregnancy sooner after the birth of their last child than were urban women. Breastfeeding appears to be the most important means of contraceptive protection in the study population. The effective traditional method of extended postpartum abstinence is not widely prevalent, particularly among the urban sample, and indeed seems to be on the decline. The data presented here also suggest that breastfeeding pratices are changing in this area of Africa where little economic development has occurred in the past 20 years. This research suggests that if current trends continue, fertility levels are likely to increase significantly. The findings also indicate that Bas Zairian mothers want to space their births and for this reason may be receptive to family planning programs that use appropriate strategies.     

Outcomes of planned home births with certified professional midwives: large prospective study in North America

Objective To evaluate the safety of home births in North America involving direct entry midwives, in jurisdictions where the practice is not well integrated into the healthcare system.Design Prospective cohort study.Setting All home births involving certified professional midwives across the United States (98% of cohort) and Canada, 2000.Participants All 5418 women expecting to deliver in 2000 supported by midwives with a common certification and who planned to deliver at home when labour began.Main outcome measures Intrapartum and neonatal mortality, perinatal transfer to hospital care, medical intervention during labour, breast feeding, and maternal satisfaction.Results 655 (12.1%) women who intended to deliver at home when labour began were transferred to hospital. Medical intervention rates included epidural (4.7%), episiotomy (2.1%), forceps (1.0%), vacuum extraction (0.6%), and caesarean section (3.7%); these rates were substantially lower than for low risk US women having hospital births. The intrapartum and neonatal mortality among women considered at low risk at start of labour, excluding deaths concerning life threatening congenital anomalies, was 1.7 deaths per 1000 planned home births, similar to risks in other studies of low risk home and hospital births in North America. No mothers died. No discrepancies were found for perinatal outcomes independently validated.Conclusions Planned home birth for low risk women in North America using certified professional midwives was associated with lower rates of medical intervention but similar intrapartum and neonatal mortality to that of low risk hospital births in the United States.     

An adaptive model for predicting !Kung reproductive performance: A stochastic dynamic programming approach

A stochastic dynamic programming model is presented that supports and extends work on the reproductive performance of the !Kung Bushmen (Lee 1972; Blurton Jones and Sibly 1978; Blurton Jones 1986), proposing that !Kung women and their reproductive systems may be maximizing reproductive success. The stochastic dynamic programming approach allows the construction of a whole-life model where the physical/environmental constraints along with the uncertainty about future events !Kung women face when making reproductive choices can be explicitly built in. The model makes quantitative predictions for the optimal reproductive strategy assuming !Kung women are maximizing expected lifetime reproduction (ELR) given the physical parameters of !Kung life.The model relies on data gathered from the works cited above and some considerations from simple probability theory. The model predictions for optimal birth spacing match the !Kung reproductive data very well and support earlier findings (Blurton Jones and Sibly; Blurton Jones 1986). The utility of the dynamic modeling approach is illustrated when the effects of varying certain model parameters are investigated.By including the effect of the mother's mortality, which was not included in the Blurton Jones and Sibly (1978) analysis, the model allows for further exploration of the application of an adaptive approach to human reproductive performance. By adding some considerations about the risks of childbirth for the mother the model not only predicts optimal birth spacing, which is site specific, but also predicts the optimal time for a woman to begin and cease having children. These predictions coincide with menarche and menopause and shed light on their possible adaptive value.     

Sex hormone level and body shape of the !Kung San. A contribution to explaining pedomorphic stature of San populations

In order to test the hypothesis that oestrogens as well as androgens modify the body shape resp. the paedomorphous habitus of !Kung San circulating levels of sex hormones (testosterone, 5 alpha-dihydrotestosterone, and oestradiol in the serum as well as free testosterone in the saliva) of 114 healthy !Kung San men from the Northern Kalahari of Namibia were quantitatively determined by means of the radioimmunoassay method. The probands were described by 50 body measurements and morphological characters like gynaecomastia. The results show that sex hormone concentrations and body shape are significantly correlated in !Kung San men as it has been previously demonstrated for non-African samples. This does not only refer to the expression of sexual dimorphism but also to the special aspect of paedomorphism. The hypothesis of Marrett, Davies and Tobias, however, that the paedomorphous type of the San is due to "over-oestrogenization" could not be confirmed. Rather, the comparatively low testosterone level of the !Kung San men seems to be of significance.     

Changes in maternal age distribution and their possible impact on demand for prenatal diagnostic services

Between 1977 and 1985 there was a 65% increase in births to women aged 35 or more in England and Wales, but only a 15% increase in all births. Two factors of roughly equal importance were responsible for this differential increase. Firstly, the proportion of older women (35-44) among all women of reproductive age (15-44) increased from 28% in 1977 to 31% in 1985; and, secondly, in the same period the fertility rate for women aged 35-39 increased from 18·2 to 24·1 per 1000 and for women aged 40-44 from 4·1 to 4·6 per 1000.The increased fertility rate among older women is not due to an extension of the reproductive period but to a delay in childbearing. This delay was seen in women married only once and also in those who had remarried.As prenatal diagnosis for the exclusion of chromosome abnormalities is customarily offered to older mothers the increased numbers of women aged 35 or more and their increased fertility rate have important implications for the provision of obstetric and laboratory services. There were 51 859 live births to women aged 35 and over in 1985; the projected figure for 2001 is 85 000. If the use of prenatal diagnosis continues to increase facilities for about 70 000 prenatal cytogenetic analyses will be needed in 2001.     

Demography of the Hadza, an increasing and high density population of Savanna foragers.

This is a report on the demography of the Hadza, a population of East African hunter-gatherers. In it, we describe the results of a census, and our estimation of age structure, survivorship, mean age of women at childbearing, number of live children, total population size and density, and rate of change since 1967. We show that relevant measures fit closely the stable population model North 6 chosen by Dyson to represent Hadza demography in the 1960s. We compare aspects of Hadza demography with surrounding non-Hadza and with the !Kung. Among other things, we find that the Hadza have a higher population density, higher fertility, and a faster population growth rate than do the !Kung. These demographic differences are consistent with our expectations, which were based on differences in the costs and benefits of foraging in the two regions. We also show that Hadza demographic parameters display remarkable consistency over the past 20 years. Since neighboring populations have been encroaching on the area used by the Hadza, and Hadza foragers have been subject to interludes of externally imposed settlement, this consistency is surprising. We discuss some of the implications.     

Low forager fertility: demographic characteristic or methodological artifact?

Anthropological literature has long held that traditional foraging populations have low fertility levels. This research examines the number of live births per woman for 9 non-western forager groups who have been investigated in the last 20 years. Data are derived from 1) birth registration systems, 2) surveys conducted during short stays with the group, and 3) surveys conducted as part of longer ethnographic studies. Fertility rates for the groups are 1) 3.5 for the Kiunga area of Papula, New Guinea, 2) 4.2 for Northern Territory Australian aborigines, 3) 5.0 for Cayapo groups in Brazil, 4) 5.3 for Hiowe people of New Guinea, 5) 5.7 for 3 Xavante groups in Brazil, 6) 6.0 for West Alaskan Eskimos, 7) 6.9 for Nunamiut Eskimos of Alaska, 8) 7.6 for the Bisman-Asmat group of Indonesian New Guinea, and 9) 8.4 for the Winikina Warao of Venezuela. Since fertility rates are highest when ethnographic studies, which allow for question clarification, memory recall, and cross-checking, are used, the author believes that high fertility rates most accurately represent forager societies. Research on the Dobe ]Kung (fertility rate - 4.7), may contradict these findings, but the author believes that the ]Kung fertility rates are higher than reported because of infanticide practices, sexual abstinence during lactation, and disease related fertility problems. In summary, the study finds high fertility (7-9 births) in traditional foraging societies. Although the study examines small populations, correlation strength and overall consistency help verify the results.     

The Changing Timing of Births in Iran: An Explanation of the Rise and Fall in Fertility After the 1979 Islamic Revolution

Studies exploring the course of period fertility in Iran after the 1979 Islamic Revolution have not examined systematically the role played by changes in the timing of births. Using retrospective data from the 2000 Iran Demographic and Health Survey and frailty hazard models, this study finds that the rise in fertility in the early 1980s was due to faster transitions to the first birth among all social groups of women and to the fourth birth largely among illiterate and less educated women. In contrast, the rapid fertility decline after 1985 is attributed to slower transition to successive births, especially to the second, third, and fourth births. These findings point to the importance of education and contraceptive use (measured by length of previous birth interval) as key determinants of birth timing in Iran. Interaction between age at marriage and education positively influenced the timing of births, with stronger effects among highly educated women, suggesting that the onset of rapid fertility decline was likely driven by these highly educated women. Another interaction between the gender of prior children and education shows that birth timing, even among highly educated women, appears to have been influenced by son preference in Iran.     

The disadvantaged child: A product of the culture of poverty,his education,and his life chances

Abstract

This study concerns the fertility of Sherpa and Tibetan women living at altitudes over 3,400 meters in Nepal. The average completed fertility (4.77 live births) and estimated crude birth rate (31 to 33 per 1,000) are low relative to low altitude peasant populations as well as to high altitude Andean peasants. Environmental phenomena (hypoxia, iodine deficiency) may be associated with retarded menarchial age and high infant mortality; but the major factors causing the low fertility appear to be cultural rather than environmental. Traditional ceremonial requirements delay the age at marriage until the mid or late twenties. Religious practices promote male and female celibacy. Migrant females and women married to migrant males report reduced fertility, probably because of poor nutrition and health care. Nonmigrant women living in villages that participate extensively in the cash economy have greater access to the growing market economy, health care, and education and report higher numbers of live births and fewer child deaths.