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.     

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.     

Estimating parity of parents: application to the history of infertility among the !Kung of Southern Africa

We derive a method for interpreting information about the reproductive performance of mothers of a sample of informants and apply it to the history of low fertility among the !Kung of the Kalahari desert of southern Africa. The method formalizes the commonsense procedure of weighting mothers with 1 birth by 1, with 2 births by 1/2, with 3 births by 1/3, etc. An expression for the confidence interval of the estimate of mean completed fertility of fertile women in the mothers' cohort is given. Several independent studies show that !Kung women have low fertility, with a population average completed family size of slightly over 4. Our method, applied to the cohort of old informants, shows that fertility is comparable among !Kung women, mothers of old informants, who were born in the late nineteenth and early twentieth centuries. If the cause is an endemic infectious disease of long standing in this region, there are important implications for the history of southern and central Africa.     

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.     

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.     

The effect of paternal investment on female fertility intention in South Korea

Life history theory views reproduction as an outcome of resource allocation. The allocation of resources such as parental investments of time, energy and material resources involves trade-offs between number of offspring and timing of reproduction. Within the framework of mammalian parental investment, the outstanding feature of human reproduction is the high level of paternal care. Although empirical evidence suggests that human paternal investment may have evolved as a reproductive strategy to reduce infant and child mortality rates, the effects of actual paternal investment, including allocating time to child care, on female reproductive decisions have received relatively little attention. We examined the trade-off from two perspectives using a representative sample of married South Korean women aged 20–44 in 2005 (n=977). First, paternal investment in domestic labor, including child care and housework, was expected to be associated with women's preference regarding future reproduction. Second, relative paternal investment was expected to increase women's preference for future reproduction, especially among employed women. We found that increased paternal investment in child care and housework remarkably enhanced women's intention to have a second child, especially among employed women. In addition, although family members provide a low percentage of child care in South Korea, such help is likely to be a useful resource for second childbirth among employed women. Somewhat expectedly, older age and longer time since first birth had negative effects on women's second-child intention. There is growing evidence that, in the lowest fertility societies, paternal investment may be an essential resource for promoting future reproductive behavior of women, especially employed women.     

Why pigs are important in Papua? Wealth,height and reproductive success among the Yali tribe of West Papua

Many studies have investigated how different variables influence the reproductive success (RS) in the populations of natural birth control. Here, we tested hypotheses about positive relationship between wealth, height and several measures of RS in an indigenous, traditional society from West Papua. The study was conducted among the Yali tribe in a few small, isolated mountain villages. In this tribe, a man's wealth is measured by the number of pigs he possesses. We found that wealth was related to fertility and number of living children, but not to child mortality in both men and women. Additionally, child mortality increased with the number of children in a family. Finally, we did not observe any relationship between height and reproductive success measures or wealth. We provide several possible explanations of our results and also put forward hypothetical background for further studies of indigenous populations.     

The Anthropology of Simple Societies: The Kalahari !Kung

Demography of the Dobe !Kung . Nancy Howell
The Bushmen A. P. Wannenburgh, Peter Johnson, and Anthony Bannister.
The !Kung San: Men, Women, and Work in a Foraging Society Richard B. Lee     

Economic and social correlates of infant mortality: A cross‐sectional and longitudinal analysis of 63 selected countries

Abstract

Genealogical records containing birth and death dates for completed families have been analyzed to compare the longevity of twins, sibs and parents. The data are restricted to twins and sibs who survived to adulthood and married. The findings, similar to those found with respect to fertility (Wyshak and White, 1969), show that twins, especially male twins, are disadvantaged in comparison with their male sibs. Sib‐sib and parent‐offspring correlation analysis confirmed that there is a genetic component in the determination of life span, but environmental factors contribute more to the total variation. No evidence of a stronger maternal than paternal effect was found. Twin bearers also lived longer than nontwin bearers. Even among persons who survived to age SO or more, parents of twins had more children and lived slightly longer than their twin and nontwin offspring. Regression analysis for persons who survived to SO or longer indicated that, in addition to life span of parents, secularity (year of death) and fertility (number of children borne) were the best predictors of longevity, though only a small proportion of the variation could be accounted for by these and other demographic variables. Life span has shown a consistent increase over time from the seventeenth century through the nineteenth century, while fertility has tended to decline. However, among persons surviving to age SO, when the relation between secularity and fertility and secularity and longevity is controlled, a significant correlation between fertility and longevity remains. This relation, observed in populations that did not practice voluntary family size limitation, would not be found in contemporary data. Maternal mortality accounted for the shorter life span of women than for men; eliminating its effect gives women a slight advantage. The fertility and longevity experience of migrants who survived to age SO is more favorable than that of persons who did not migrate.     

Selection intensities in menopausal and permanent contraceptor women of a rural population in India

Changing fertility and mortality patterns due to socioeconomic forces have a profound effect on natural selection in human populations. The opportunity for selection was estimated in the rural population of Visakhapatnam District, Andra Pradesh, India, in 1979. A total of 1570 women were interviewed, 1544 among them had ever been pregnant. Data were analyzed to compute fertility and mortality components of the population. The index of total selection was found to be low. Selection in relation to birth control reveals that opportunity for selection is lower among women who completed their fertility by family planning methods than in women who completed their fertility by attaining menopause. Further, the results showed that differential fertility and mortality make equal contributions to the total measure of selection in both groups, whereas in developed countries like the US the mortality component contributes only a small fraction to the total index, due to improved health conditions.     

Child fostering as a reproductive strategy among Southern African pastoralists

Information on child fosterage was obtained on 1902 individuals in the course of collecting 611 reproductive histories from Herero and Mbanderu pastoralists in northwestern Botswana in southern Africa. The hypothesis that women foster out their children as a strategy for increasing the number of children they produce is tested. Analysis of these data indicate that sex and birth year of children and parents' marital status significantly influence a child's risk of being fostered, but the relationship of fostering to fertility is unclear. More recently born individuals, females, and children born to parents who were not married to each other are more likely to be fostered than older individuals, males, and children born to parents who were married to each other. A child's risk of being fostered does not appear to be influenced by mother's age if the parents were married to each other, while children born to parents who were not married to each other are more likely to be fostered if the mother is very young, but this pattern is not clear. No association between fostering and mortality was found.     

A model of fertility motivation,desires, and expectations early in women's reproductive careers

Abstract

We propose a general model of the relationships connecting fertility motivations, desires, and expectations in which motivations are antecedent to desires, which in turn are antecedent to expectations. We describe differences in the model as it applies to women early in their reproductive careers, specifically, after marriage but before their first birth and after their first birth but before their second. We test the model with data from 311 just‐married and 318 just‐mothered women, using LISREL to estimate structural equations for both groups. The general model fits the data well. There are three primary differences in the model with respect to the two groups of married women. How soon a woman expects her next child affects how many children she expects only among the just‐married woman. The number of children expected by the woman is the only antecedent to how soon she expects her next child among the just‐mothered women. Both positive and negative maternal motivation has effects among the just‐married women, but only negative maternal motivation has effects among the just‐mothered women. These and other differences are discussed in terms of psychological and situational developments occurring early in women's reproductive careers.     

A model of fertility motivation, desires, and expectations early in women's reproductive careers

We propose a general model of the relationships connecting fertility motivations, desires, and expectations in which motivations are antecedent to desires, which in turn are antecedent to expectations. We describe differences in the model as it applies to women early in their reproductive careers, specifically, after marriage but before their 1st birth and after their 1st birth but before their 2nd. We test the model with data from 311 just-married and 318 just-mothered US women enrolled in a prospective psychology of reproduction study, using LISREL (analysis of linear structural relationships by the method of maximum likelihood) software to estimate structural equations for both groups. The general model fits the data well. There are 3 primary differences in the model with respect to the 2 groups of married women. How soon a woman expects her next child affects how many children she expects only among the just-married woman. The number of children expected by the woman is the only antecedent to how soon she expects her next child among the just-mothered women. Both positive and negative maternal motivation has effects among the just-married women, but only negative maternal motivation has effects among the just-mothered women. These and other differences are discussed in terms of psychological and situational developments occurring early in women's reproductive careers.     

A dynamic framework for the study of optimal birth intervals reveals the importance of sibling competition and mortality risks

Human reproductive patterns have been well studied, but the mechanisms by which physiology, ecology and existing kin interact to affect the life history need quantification. Here, we create a model to investigate how age‐specific interbirth intervals adapt to environmental and intrinsic mortality, and how birth patterns can be shaped by competition and help between siblings. The model provides a flexible framework for studying the processes underlying human reproductive scheduling. We developed a state‐based optimality model to determine age‐dependent and family‐dependent sets of reproductive strategies, including the state of the mother and her offspring. We parameterized the model with realistic mortality curves derived from five human populations. Overall, optimal birth intervals increase until the age of 30 after which they remain relatively constant until the end of the reproductive lifespan. Offspring helping each other does not have much effect on birth intervals. Increasing infant and senescent mortality in different populations decreases interbirth intervals. We show that sibling competition and infant mortality interact to lengthen interbirth intervals. In lower‐mortality populations, intense sibling competition pushes births further apart. Varying the adult risk of mortality alone has no effect on birth intervals between populations; competition between offspring drives the differences in birth intervals only when infant mortality is low. These results are relevant to understanding the demographic transition, because our model predicts that sibling competition becomes an important determinant of optimal interbirth intervals only when mortality is low, as in post‐transition societies. We do not predict that these effects alone can select for menopause.     

How contemporary human reproductive behaviors influence the role of fertility-related genes: the example of the p53 gene

Studies on human fertility genes have identified numerous risk/protective alleles involved in the occurrence of reproductive system diseases causing infertility or subfertility. Investigations we carried out in populations at natural fertility seem to suggest that the clinical relevance that some fertility genes are now acquiring depends on their interaction with contemporary reproductive behaviors (birth control, delayed childbearing, and spacing birth order, among others). In recent years, a new physiological role in human fertility regulation has emerged for the tumor- suppressor p53 gene (P53), and the P53 Arg72Pro polymorphism has been associated with recurrent implantation failure in humans. To lend support to our previous observations, we examined the impact of Arg72Pro polymorphism on fertility in two samples of Italian women not selected for impaired fertility but collected from populations with different (premodern and modern) reproductive behaviors. Among the women at near-natural fertility (n = 98), the P53 genotypes were not associated with different reproductive efficiency, whereas among those with modern reproductive behaviors (n = 68), the P53 genotypes were associated with different mean numbers of children [Pro/Pro = 0.75相似文献   

Teenage childbearing: structural determinants in developing countries

Data for a sample of 50 developing countries are analysed to investigate the social correlates of the teenage birth rate. Of five major factors considered as predictors of national birth rates (socioeconomic development, family planning programmes, women's status, the sex ratio, and marriage patterns), regression analyses reveal that only the average age at marriage for women has a significant effect on the teenage birth rate. In contrast, all variables except the sex ratio and the average age at marriage for women have a significant effect on the total fertility rate.     

Testing evolutionary hypotheses about human biological adaptation using cross-cultural comparison

Physiological data from a range of human populations living in different environments can provide valuable information for testing evolutionary hypotheses about human adaptation. By taking into account the effects of population history, phylogenetic comparative methods can help us determine whether variation results from selection due to particular environmental variables. These selective forces could even be due to cultural traits-which means that gene-culture co-evolution may be occurring. In this paper, we outline two examples of the use of these approaches to test adaptive hypotheses that explain global variation in two physiological traits: the first is lactose digestion capacity in adults, and the second is population sex-ratio at birth. We show that lower than average sex ratio at birth is associated with high fertility, and argue that global variation in sex ratio at birth has evolved as a response to the high physiological costs of producing boys in high fertility populations.     

The delineation of fertility strategies in a tribal population of India: the Koyas of Koraput District,Orissa

Demographic data collected for a tribal population of India, the Koyas of Koraput District, Orissa, were examined in light of 2 models of reproductive behavior associated with the economic value of children: the replacement effect and son survivorship motivation. Both models are united in the concept that infant/child mortality affects subsequent fertility. The database consists of retrospective fertility histories of Koya women who had completed their reproductive period. The total number was 260, with the total offspring numbering 1407. 2 distinct cohorts of women were formed for the purpose of analysis, separated only by the criterion of offspring survival: women who had experienced infant child mortality (129 women with 739 children); and women who completed their reproductive period without suffering offspring loss of this nature (132 women with 668 children). The cohort without child loss had a mean parity of 5.10, lower than the average parity of 5.73 recorded for the cohort whose reproductive histories included at least 1 infant/child death. Age specific marital fertility and birth interval analyses indicated that this differential was because of biological, not behavioral, factors. The age pattern of fertility of females suffering offspring mortality failed to demonstrate a high rate of childbearing in the later age intervals of the reproductive period, a characteristic pattern of couples attempting to "replace" lost offspring. Birth interval analysis pointed to biological "interval effect," whereby infant/child mortality caused a cessation of lactation and hence a shortening of postpartum amenorrhea. Computer simulation further indicated that the higher fertility differential of the cohort experiencing offspring loss still did not result in high son survivorship values. The findings agree with earlier studies indicating that for predemographic transitional populations, economically motivated fertility strategies are ineffectual.     

Caloric requirements of human populations: A model

Currently available models used for predicting human caloric requirements do not reflect the great variability in activity patterns observed among populations, and are insensitive to important anthropometric, demographic, and environmental variables. They are thus inadequate for application to many populations and problems of anthropological interest. We present a model for determining caloric requirements which more accurately accommodates the effects of variation in activity and in anthropometries on individual needs, and which predicts population requirements based on individual needs and demographic parameters. The model is tested on four populations (the Andean community of Nuñoa, Peru, the Dobe !Kung of Botswana, and two New Guinean villages) and is found to provide consistently better estimates of caloric requirements than are generated by the Food and Agriculture/World Health Organization's model. This model should be useful to anthropologists and human ecologists concerned with problems involving human energy consumption, such as the efficiency of subsistence strategies, optimum family composition, or certain consequences of increased labor migration or technological change.This research was supported by a grant from the Research Foundation of the State University of New York. Funds for computer time and materials were provided by the State University of New York at Binghamton and The Pennsylvania State University.