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Reproductive success: Which meaning?
Abstract
The theory of kin selection (the part played by behavior in the changes of mean inclusive fitness) induced many human sociobiologists to think that since behavior was involved in the increase in fitness, this last entity could apply to the individual. Approximated by the individual's lifetime reproductive success, this measure became the keyword of studies linking social and cultural behavior to biological adaptive processes. To be commonly applicable to human populations, it had to be simplified to represent the number of offspring reaching sexual maturity and most existing studies are based on this definition. The current trend, however, seems to consider that, like inbreeding, reproductive success takes its signification in the depth of successive generations. These diverse measures were tested in two traditional populations, Berber and Aymara, and show that finding a satisfactory evaluation of reproductive success is a problem that is still far from a solution.
... Pragmatically, LRS data are difficult to obtain because they require that individuals be marked and tracked for their entire lifetimes in longitudinal studies. Conceptually, the type (e.g., sex), life stage (e.g., neonate), or even generation at which descendants should be counted to quantify LRS has long been a source of discussion, with no consensus on a single best metric (Grafen 1988;Newton 1989a;Rhine 1997;Brommer et al. 2002;Crognier 2003;Strassmann and Gillespie 2003;Brommer et al. 2004). Consequently, reported metrics of reproductive success across many taxa range from proximal single-generation measures, such as the number of eggs laid or neonates born (e.g., Madsen and Shine 1994;Setchell et al. 2005), to increasingly distal metrics, such as number of offspring that attain some measure of independence such as fledging or weaning (e.g., Oring et al. 1991;Dugdale et al. 2010), number of offspring that achieve a specific predefined age (e.g., King et al. 1991;Wilson et al. 2005), number of offspring that reach reproductive maturity (e.g., Käär and Jokela 1998;Jensen et al. 2004), number of mature offspring that become reproductively active (Fitzpatrick and Woolfenden 1989;Brommer et al. 2004), and, occasionally, multi-generational metrics such as counts of grandoffspring or later descendants (e.g., Kelly 2001;Brommer et al. 2004;Dugdale et al. 2010). ...
... Lifetime studies of wild vertebrate populations with sample sizes on the level of hundreds of individuals are rare, but they provide important empirical demonstrations of magnitude and variability in individual fitness (e.g., Clutton-Brock 1988;Newton 1989a;Oli and Armitage 2003;Wilson et al. 2005;McAdam et al. 2007;Dugdale et al. 2010). Outside of human studies where existing census records can be used (e.g., Crognier 2003;Gillespie et al. 2008), grandoffspring studies have been conducted for only a few species of free-living mammals (Fedigan and Pavelka 2001;Kelly 2001;Dugdale et al. 2010;Nuckolls 2010) and birds (Brommer et al. 2004;MacColl and Hatchwell 2004), although the importance of this distal metric for representing individual fitness has been noted (Brommer et al. 2002). ...
Lifetime reproductive success (LRS) in the form of number of descendants is a commonly used measure of individual fitness, but the life stage at which descendants are counted varies among studies. Conceptual and logistical trade-offs exist along the gradient of proximal-to-distal LRS metrics. Although proximal metrics, such as number of offspring weaned, are logistically easier to collect than distal metrics, such as number of reproductively mature offspring or grandoffspring, they may be of little evolutionary significance if stochastic events heavily influence the realized number of descendants. We use a 25-year demographic data set based on 954 adult female Richardson’s ground squirrels (Urocitellus richardsonii) from 22 annual cohorts to characterize and compare 6 metrics for LRS: lifetime production of litters, numbers of weanlings, weanling daughters, adult daughters, weanling matrilineal granddaughters, and adult matrilineal granddaughters. Most adult females weaned only 0 (21%), 1 (47%), or 2 (22%) lifetime litters. All LRS distributions were right skewed, with 53% and 77% of adult female Richardson’s ground squirrels having no adult female matrilineal descendants after 1 and 2 generations, respectively. Survival of daughters and mothers covaried with calendar year, and LRS was strongly influenced by the calendar year in which females recruited into the breeding population as yearlings. Catastrophic flooding in 2005 killed nearly all descendants from the 2004 and 2005 cohorts. Daughter survival to adulthood explained more variance in lifetime production of adult daughters and granddaughters than number, size, or sex ratio of litters. Overall, to have a ≥ 50% chance of 1 adult granddaughter, a female needed to produce 2 litters, 7 weanling daughters, or 2 adult daughters. All distal (response) versus proximal (predictor) LRS metrics had significantly positive relationships, but variability increased with each distal step in the response variable. Our long-term study highlights sampling issues with LRS studies, variability within and among cohorts, and the role of stochastic events in uncoupling a mammal’s reproductive effort from realized number of descendants.
... En una revisión sistemática en poblaciones de diferentes grupos étnicos, hecha desde 1980 hasta julio de 2013, se observaron factores no genéticos que contribuyeron al adelanto de la edad de la menarquia, destacando el peso corporal, la alta ingesta de proteínas animales y el estrés en la relación entre padre e hija, entre otros (Yermachenko y Dvornyk, 2014). La edad de la menarquia presenta nítidas variaciones según múltiples parámetros, como el ambiente y la pertenencia social (Hernández et al., 2007;Méndez, Valencia y Meléndez, 2006;Wangermez, 1984), el ambiente climático y la altitud (Crognier, 2003;Dieu-Cambrezy y Froment, 1993;González y Ortiz, 1994;González y Villena, 1996;González, Villena y Ubilluz;, la alimentación y las condiciones de vida (Hernández et al., 2007), e incluso la práctica deportiva (Konovalova, 2013) o la pertenencia étnica (Ossa, Bustos, Muñoz y Amigo, 2012). Además, se afirmó que la educación mixta posibilita la precocidad en las primeras menstruaciones, así como la situación genética caracterizada por la edad de la menarquia en madres y hermanas (Torrealva, 2017). ...
relación entre la edad del primer embarazo y el índice de desarrollo humano en diez países de América Latina. Enseñanza e Investigación en Psicología, 2(2), 246-254. Artículo recibido el 23 de octubre de 2019 y aceptado el 7 de enero de 2020. RESUMEN Dentro de las características biológicas que describen la adolescencia se encuentra la edad de la menarquia, que es un factor determinante de la capacidad reproductiva ligada a la propor-ción de los ciclos de ovulación, que en las últimas décadas se ha adelantado sobre todo en los países de América Latina. La aceleración del desarrollo puberal es un importante problema médico y social, ya que puede resultar en un aumento de morbilidad y mortalidad en la edad adulta, así como en el adelanto de la vida sexual y la maternidad temprana. Como uno de los factores descritos para este adelanto se encuentra la pobreza, cuyo índice es medido con el factor de desarrollo humano, por lo que en el presente trabajo se hizo un análisis cronológico del adelanto puberal y de los cambios en el índice de desarrollo humano, eso en diez países de América Latina, encontrándose una correlación entre la disminución de la menarquia en las últimas dos décadas y un aumento en el porcentaje de embarazos adolescentes. ABSTRACT Among the biological characteristics that describe adolescence, the age of menarche is a determining factor of the reproductive capacity, linked to the proportion of ovulation cycles that in recent decades has advanced, especially in Latin American countries. Acceleration of pubertal development is a major medical and social problem because it may result in an increased morbidity and mortality in adulthood, as well as the advancement of sexual life and
... From an evolutionary perspective, reproductive success represents an individuals number of offspring reaching sexual maturity [1,2]. This implies that beyond the transmission of heritable characteristics to successive generations, offspring quality in terms of survival and fecundity is shaped by a multitude of environmental factors. ...
Animal studies have shown that maternal resource allocation can be sex-biased in order to maximize reproductive success, yet this basic concept has not been investigated in humans. In this study, we explored relationships between maternal factors, offspring sex and prenatal and postnatal weight gain. Sex-specific regression models not only indicated that maternal ethnicity impacted male ( n = 2456) and female ( n = 1871) childrens postnatal weight gain differently but also that parity and mode of feeding influenced weight velocity of female ( β ± s.e. = −0.31 ± 0.11 kg, p = 0.005; β ± s.e. = −0.37 ± 0.11 kg, p < 0.001) but not male offspring. Collectively, our findings imply that maternal resource allocation to consecutive offspring increases after a male firstborn. The absence of this finding in formula fed children suggests that this observation could be mediated by breast milk. Our results warrant further mechanistic and epidemiological studies to elucidate the role of breastfeeding on the programming of infant growth as well as of metabolic and cardiovascular diseases, with potential implications for tailoring infant formulae according to sex and birth order.
... This contrasts (but is not mutually exclusive) with the "forward method," in which a researcher infers the features of a relevant evolutionary environment and the fitness of different phenotypic variants in that environment before asking whether observed phenotypes are those hypothesized to have the highest fitness (Sherman and Reeve 1997). The concept of RS has been controversial in the scientific literature, often because of the different definitions that have been used (Crognier 2003). Here we define RS in biological terms as the number of offspring produced over the life course (fertility) and the number of offspring surviving past the age of five (number of surviving offspring). ...
Identifying the determinants of reproductive success in small-scale societies is critical for understanding how natural selection has shaped human evolution and behavior. The available evidence suggests that status-accruing behaviors such as hunting and prosociality are pathways to reproductive success, but social egalitarianism may diminish this pathway. Here we introduce a mixed longitudinal/cross-sectional dataset based on 45 years of research with the Batek, a population of egalitarian rain forest hunter-gatherers in Peninsular Malaysia, and use it to test the effects of four predictors of lifetime reproductive success: (i) foraging return rate, (ii) sharing proclivity, (iii) cooperative foraging tendency, and (iv) kin presence. We found that none of these factors can explain variation in lifetime reproduction among males or females. We suggest that social egalitarianism, combined with strikingly low infant and juvenile mortality rates, can mediate the pathway between foraging, status-accruing behavior, and reproductive success. Our approach advocates for greater theoretical and empirical attention to quantitative social network measures, female foraging, and fitness outcomes.
... This contrasts (but is not mutually exclusive) with the "forward method," in which a researcher infers the features of a relevant evolutionary environment and the fitness of different phenotypic variants in that environment before asking whether observed phenotypes are those hypothesized to have the highest fitness (Sherman and Reeve 1997). The concept of RS has been controversial in the scientific literature, often because of the different definitions that have been used (Crognier 2003). Here we define RS in biological terms as the number of offspring produced over the life course (fertility) and the number of offspring surviving past the age of five (number of surviving offspring). ...
Sharing and generalized reciprocity have long been thought to have played an important role in human evolution. As such, patterns of sharing interactions in hunter-gatherers have received a great deal of interest from anthropologists seeking to understand how and why sharing occurs in small-scale societies that engage in the widespread exchange of food resources, child care, and information. In this study we apply social network analysis to a historical economic dataset collected in the 1970’s on Batek hunter-gatherers in Peninsular Malaysia. To understand both the endogenous and exogenous factors underlying the exchange of food resources, we analyzed the structure of Batek sharing networks through time using pooled temporal exponential family random graph models. Our analysis reveals a strong effect of delayed reciprocity, in addition to relatedness, marriage ties, and transitivity, on the structure of Batek food sharing networks. Further, we found that the cumulative degree distribution of sharing networks was “scale-free,” suggesting that these networks are structurally similar to many other real world networks that demonstrate the small-world property. Our results suggest an important role for delayed reciprocity in structuring Batek sharing networks and demonstrate the utility of placing hunter-gatherer sharing networks in the context of real-world networks more generally.
... An example is the concept of reproductive success. Its meanings can range from the number of offspring reaching sexual maturity to the degree of one's genes contributing to the gene pool of successive generations, resulting in a situation where " it cannot be compared cross-culturally " (Crognier 2003, p. 358). Cross-cultural comparison based on such constructs is impossible as its meaning is not cross-culturally comparable. ...
This chapter provides a comprehensive overview of the theoretical and methodological issues confronting cross-cultural and intercultural research. Informative cross-cultural research is based on sound theoretical analysis, and four major theoretical issues have to be tackled: specifying the relevant cultural constructs, theoretical frameworks for explaining the influence of culture, theoretical justifications for the selection of cultures, and construct equivalence. With regard to methodological challenges, a number of issues are important, including comparability of samples, similarity in data collection procedures across cultures, measurement equivalence across cultures, proper statistical analysis, and strategies to strengthen causal claims. In contrast to cross-cultural research, intercultural research is concerned with the interaction of people with different cultural backgrounds. Knowledge about cultural similarities and differences helps us understand how people interact across cultural boundaries, but additional theoretical frameworks are needed for a full analysis. This type of interaction is influenced by individual differences and intergroup dynamics, including in-group favoritism. With regard to methodological concerns, identity issues are important and have to be taken into account, and intercultural interaction may be face-to-face or mediated by media technologies.
A variety of research designs, including experiments and longitudinal designs, are feasible for intercultural research.
... However, it is of crucial importance for determining the likelihood of survival of populations (Calder 1983, Tremblay, 2000 and should be part of any population viability analysis (PVA). Moreover evaluation of the mean (or medium) life and variance (or quartile) in lifespan is a variable of vital importance when considered for analyses of the likelihood of natural selection and genetic drift (Barrowclough & Rockwell 1993, Merila & Sheldon 2000, Crognier 2003). Many parameters are correlated with lifespan such as effective population size and lifetime reproductive success. ...
Tremblay (2000) published a three years survey of
lifespan in four species of Lepanthes including L.
caritensis, however, after the three years survey most
juveniles and adults were still alive (64 and 91%,
respectively), while most seedlings died during the survey
period with only 25% still alive at the end of the
survey. As a result, the survivorship curve only represented
a partial life history of the species with a maximum
possible lifespan as function of the survey period.
Limited survey’s can easily bias the results, which can
suggest smaller mean and variance in the projected
lifespan. A second preoccupation with the previously
published paper is that the number of seedling surveyed
was very small and consequently the estimates
could have been affected from random mortality and
survivorship and not be a good representation of the
typical lifespan of this life stage. Thus in this survey
we present a continuation of the survey of one of the
species, Lepanthes caritensis Tremblay & Ackerman
and include a third population to the previously two
studied with a total survey period of approximately
eleven years for the first two populations.
... As described extensively elsewhere (Crognier 2003, Crognier et al. 2001, Tymicki 2004, in order to account for the positive relation between kin oriented help and reproductive success of the recipient, both components of reproductive success have to be considered; the number of produced offspring and the number of surviving offspring. The hypothesis concerning the kin effect on the reproduction assumes that this effect operates through both components. ...
The present study aims to investigate the parity specific effect of kin help on the transition between births among natural
and controlled fertility birth cohorts of the Bejsce parish. The hypothesis states that kin help should be of particular importance
in the case of higher order births. Thus, kin effects understood as reduction in the costs of childbearing (direct childcare,
provision of the resources) or nutritional effects should be of particular importance at higher parities. The analyses are
based on the multilevel hazard models of parity transition with kin effects represented by time-constant and time-varying
covariates. The data used for the estimation of the models come from the reconstitution of the registers from Bejsce parish
located in south-central Poland. The reconstitution covers the period between 1730 and 1968. The results suggest that there
was a strong kin effect especially at higher parities. These effects were mostly associated with the presence of nongenerative
relatives (grandparents). The analyses reveal only weak differences in the kin effect between natural and controlled fertility
regimes.
... A woman's fitness could be affected by factors such as her own longevity, the time and place where she lived, and her own genetic makeup. Since it has been shown that mitochondrial polymorphisms are associated with differential longevity (Alexe et al., 2007), these polymorphisms could result in increased lifetime reproductive success (LRS—the total number of children produced ), (Crognier, 2003). Castrıèt al. (2009) determined that subjects who carry the 150T mutation have extended longevity whereas subjects who carry the 5178A marker in haplogroup D (HGD) have shortened longevity. ...
To determine if individuals who carry mitochondrial markers which have been previously shown to affect longevity also have differential lifetime reproductive success (LRS).
We extracted the mtDNA from living subjects residing in Atenas, Costa Rica. Since mtDNA does not recombine, and its probability of mutation is low, we assume that all maternal ancestors of the living subjects have the same mtDNA. We reconstructed the maternal genealogy of the living subjects, so that we have information on the LRS and longevity of the maternal ancestors of the living subjects. We compared the LRS of women who carried the 5178A marker in haplogroup D (associated with decreased longevity) and who carried the 150T polymorphism (associated with increased longevity) with the LRS of controls born in the same half century time period from 1750 to 1939.
We found that the LRS of neither group of women with a longevity-associated polymorphism (LAP) differed from the LRS of controls, even if these women differed significantly from the controls in their longevity.
Although LAPS significantly affect longevity, such differential longevity does not result in differential lifetime reproductive success. From an evolutionary perspective, these longevity-associated polymorphisms do not affect the carriers' Darwinian fitness.
This paper studies the influence of biological and behavioural determinants on the reproductive period and fertility of older, rural Portuguese women (Municipality of Oleiros) and their mothers and describes the secular trends 1880-1940.
The reproductive histories of 337 women who were 44 years and older were obtained through personal interviews. Participants were also asked about the reproductive events of their mothers.
The women's mean values of reproductive variables were: marriage age, 22.80 ± 3.89 years, age at first birth, 24.46 ± 4.11 years, age at last birth, 32.26 ± 6.56 years, childbearing periods, 7.76 ± 6.20 years, and effective number of children, 3.25 ± 2.06 children. The childbearing periods of participants' mothers were 8.59 ± 6.98 years, with an age at first birth of 25.11 ± 4.31 years, age at last birth of 35.33 ± 6.14 years and effective number of children of 4.45 ± 2.19 children. The women's age at marriage and the age at last birth were related to childbearing period and effective number of children. Participants were younger at first birth and at last birth and had a lower effective number of children compared to their mothers.
Over time mean values of the reproductive events and the effective number of children decreased slowly and were likely influenced through cultural, behavioural and healthcare changes occurring in the region.
The phenomenon of kin-oriented help, according to inclusive fitness theory, should be of crucial importance with respect to the process of reproduction. This is due to the fact that the devoted time and resources might indirectly contribute to the reproductive performance of a donor. This study aimed at analyzing the kin effects on fertility in order to check whether help received from kinsmen enhance a recipient's reproduction in terms of parity transition risk, completed fertility, and the number of survivors. The data came from reconstitution of church registers from Bejsce parish, Poland. To estimate the kin effect, regression models for count outcomes and techniques of multilevel event history analysis were applied. The analyses have shown that completed fertility and parity-specific transition risks are strongly influenced by various kin groups. Moreover, a multilevel hazard model revealed differences in the patterns of the kin influence among controlled fertility than among natural fertility birth cohorts. Female reproductive outcome is influenced mainly by the presence of siblings and postreproductive helpers (grandparents). However, there is a negative impact of so-called helpers-at-the-nest (older children in the household) on parity transition risks.
In most social species, position in the male social hierarchy and reproductive success are positively correlated; in humans, however, this relationship is less clear, with studies of traditional societies yielding mixed results. In the most economically advanced human populations, the adaptiveness of status vanishes altogether; social status and fertility are uncorrelated. These findings have been interpreted to suggest that evolutionary principles may not be appropriate for the explanation of human behavior, especially in modern environments. The present study tests the adaptiveness of social status with actual mating and reproductive data in a representative sample of males from an industrial society. Reproductive success, even when assessed by a more reliable measure of actual male fertility than the one commonly used, fails to correlate with social status. In striking contrast, however, status is found to be highly correlated with potential fertility, as estimated from copulation frequency. Status thus accounts for as much as 62% of the variance in this proximate component of fitness. This pattern is remarkably similar to what is found in many traditional societies and would result in a substantial positive relationship between cultural and reproductive success in industrial populations were it not for the novel conditions imposed by contraception and monogamy. Various underlying mechanisms are suggested for these findings, illustrating the value of current behavioral and reproductive data in the study of adaptation. It is concluded that evolutionary explanations of human behavior remain entirely relevant in modern societies.
A comparison of demographic data from a sample of traditional, natural-fertility societies demonstrates that the mean total fertility of populations which practise intensive agriculture is significantly higher than that of foragers and horticulturalists. These findings support the association that demographers and economists have long maintained between the intensification of subsistence technology and increases in human fertility. This higher fertility probably results from changes in nutritional status, marriage patterns, and breastfeeding practices that frequently accompany subsistence intensification. A fuller explanation of these fertility differentials, however, will require the collection of further high-quality microdemographic data from a variety of traditional societies.
n Abstract Life history theory offers evolutionary explanations for the timing of life events, with a particular focus on age-schedules of fertility and mortality and growth. Traditional models examine trade-offs between current and future reproduction and quality versus quantity of offspring. These models can be used to understand questions concerning time of gestation, age of wean-ing, juvenile mortality profiles, age at maturation, adult body size, fertility rates, senescence, menopause, and the length of the life span. The trajectory of energy acquisition and its allocations is also an important part of life history theory. Modifications of these models have been developed to examine the period of learning, postweaning parental investment, and patterns of development. In this article, we combine energetic and demographic approaches in order to examine the human life course from an optimality perspective. The evolved life history solves related problems across two generations. The first set of decisions con-cerns how to maximize own lifetime net energy production that can be used for reproduction. The second set of decisions concerns how to maximize total off-spring energy production (summed over all offspring).
The evolutionary biological hypothesis that culturally defined values and goals are proximate means of enhancing reproductive success is tested on data from the Mukogodo, a small group of Maa-speaking pastoralists in north-central Kenya who value the accumulation of livestock. The results support the prediction that, at least among males, livestock wealth should correlate with reproductive success. This correlation appears to be due mainly to greater polygyny among wealthier men. Lower age at first marriage among wealthier men may also contribute to the correlation between livestock wealth and reproductive success. The association between livestock wealth and reproductive success does not appear to be due to the productivity of wives and children, to bridewealths obtained when daughters marry, or to the effects of wealth on the reproductive success of men's wives.
Fertility appears to be reduced in at least some high altitude populations relative to their counterparts at lower elevations. Inferring from the difficulties with reproduction of newcomers to high altitude and from animal experiments, it has been hypothesized that this apparent reduction is the result of hypoxia acting to reduce fecundity and/or increase fetal loss. In humans, however, several behavioral as well as biological factors may affect fertility levels. These many factors have been organized by demographers into a framework of seven proximate determinants that includes fecundability (the monthly probability of conception) of which successful ovulation is one component. To test whether ovarian function is impaired in women indigenous to high altitude, we measured salivary progesterone (P) in a sample (n = 20) of Quechua women (aged 19-42 years) residing at 3,100 m. It was found that mean luteal P = 179 pmol/L and mean midluteal P = 243 pmol/L, levels that fall about midway in the range of known values for several populations and are higher than some lower altitude populations. These findings suggest that hypoxia does not appear to significantly impair ovarian function in those with lifelong residence at high altitude. There are, however, several factors common to many high altitude populations that may act to reduce fecundability and fertility including intercourse patterns (affected by marriage and migration practices), prolonged lactation, dietary insufficiency, and hard labor.
This volume surveys the state of knowledge and research on the determinants of human reproduction. It adopts an inter-disciplinary approach and integrates information from demographic, epidemiological and biological studies of fertility. The chapters provide a comprehensive overview of reproductive processes, including puberty and menopause, conception and fetal loss, and the effects of sexually transmitted diseases and lactation. The volume also considers the effects on fertility of nutrition and stress, environmental and occupational hazards, and social behaviour, and includes clinical papers on fertility following contraceptive use and treatment of infertility. Findings from original research on the determinants of human reproduction are also presented.
This volume surveys the state of knowledge and research on the determinants of human reproduction. It adopts an inter-disciplinary approach and integrates information from demographic, epidemiological and biological studies of fertility. The chapters provide a comprehensive overview of reproductive processes, including puberty and menopause, conception and fetal loss, and the effects of sexually transmitted diseases and lactation. The volume also considers the effects on fertility of nutrition and stress, environmental and occupational hazards, and social behaviour, and includes clinical papers on fertility following contraceptive use and treatment of infertility. Findings from original research on the determinants of human reproduction are also presented.
This volume surveys the state of knowledge and research on the determinants of human reproduction. It adopts an inter-disciplinary approach and integrates information from demographic, epidemiological and biological studies of fertility. The chapters provide a comprehensive overview of reproductive processes, including puberty and menopause, conception and fetal loss, and the effects of sexually transmitted diseases and lactation. The volume also considers the effects on fertility of nutrition and stress, environmental and occupational hazards, and social behaviour, and includes clinical papers on fertility following contraceptive use and treatment of infertility. Findings from original research on the determinants of human reproduction are also presented.
This chapter is a preliminary attempt to characterise reproductive patterns in traditional, pre-industrial societies, including huntergatherers, tribal horticulturalists and pastoralists and settled peasant agriculturalists. Assertions about the level of fertility in such societies have played a key role in the development of theoretical models in demography and anthropology and, more recently, in reproductive biology. In classic demographic transition theory, for example, it was assumed that pre-transitional societies were characterised by uniformly high fertility rates, which provided the starting point for the recent secular decline in fertility (Knodel, 1977). Most ecological anthropologists, in contrast, have come to believe that many traditional societies, especially unacculturated hunter-gatherers, have regulated their reproductive output at relatively low levels (Dumond, 1975; Peacock, 1986). It has even been suggested that there occurred an earlier, stone-age demographic transition toward higher. birth and death rates associated with the emergence of settled village life during the Neolithic (Handwerker, 1983; Roth, 1985).1
It has been suggested by some that the acquisition of symbolic capital in terms of honor, prestige, and power translates into an accumulation of material capital in terms of tangible belongings, and that on the basis of these goods high reproductive success may be achieved. However, data on completed fertility rates over more than one generation in so-called traditional societies have been rare. Ethnographic and demographic data presented here on the pastoral Bakkarwal of northern India largely corroborate the hypothesis concerning the interdependence between the attainment of various cultural goals and differential reproduction rates and indicate that the numbers of (especially male) surviving offspring and siblings are crucial to a man's position in society.
A study of female reproductive histories from nineteenth-century Utah shows that although women who married polygynously had fewer children, their number of grandchildren was equal to that of women who married monogamously. Women who chose to marry high-status men polygynously traded decreased fertility for enhanced reproductive performance of offspring. High status can be associated with low fertility and yet still be consistent with fitness optimization. These results suggest how female reproductive decisions influence social structure and challenge previous assumptions concerning proximate measures of fitness.
The Ache, whose life history the authors recounts, are a small indigenous population of hunters and gatherers living in the neotropical rainforest of eastern Paraguay. This is part exemplary ethnography of the Ache and in larger part uses this population to make a signal contribution to human evolutionary ecology.
A framework for analyzing the relationship between intermediate fertility variables and fertility levels is presented. An intermediate fertility variable is defined as any behavioral or biological factor through which socioeconomic, cultural, or environmental variables affect fertility. A complete set of eight intermediate fertility variables is proposed, but it is shown that only four are important determinants of fertility differentials among populations: proportions married, contraception, induced abortion, and lactational infecundability. A simple model that allows quantitative estimation of the fertility effects of each of these four variables is outlined, and its application is illustrated.
This paper has two interrelated goals. The first is to offer a general theory of fertility and parental investment across a broad spectrum of human societies. The second is to provide a perspective that unifies traditionally separate domains of anthropology. The basic foundation for the analysis is life history theory and evolutionary biological models of optimal fertility regulation. This tradition is combined with human capital theory in economics to produce a more general theory of investments in embodied capital within and between generations. This synthesis results in a series of optimality models to examine the decision processes underlying fertility and parental investment upon which natural selection is expected to act. Those models are then applied to the hunting and gathering lifeway. This analysis focuses both on problems that all hunting and gathering peoples face and on the production of variable responses in relation to variable ecologies. Next, this consideration of optimal parental investment and fertility behavior in hunter-gatherers is united with existing models of the proximate determinants of human fertility. The analysis of proximate mechanisms is based on the idea that natural selection acts on the final phenotypic outcome of a coordinated system of physiological, psychological and cultural processes. The important conditions affecting parental investment and fertility in modern socioeconomic contexts are then discussed. An explanation of modern fertility and parental investment behavior in terms of the interaction of those conditions with the physiological and psychological mechanisms that evolved during our hunting and gathering history is proposed. The proposal is that skills-based competitive labor markets increase the value of parental investment in children and motivate better-educated, higher income parents to invest more per child than their less-educated, lower-earning counterparts. It is also suggested that the deviation from fitness maximization associated with low modern fertility is due to excess expenditures on both parental and offspring consumption, indicating that our evolved psychology is responding to cues in the modern environment that are not directly related to the fitness impacts of consumption. © 1996 Wiley-Liss, Inc.
The fundamental postulate of sociobiology is that individuals exploit favorable environments to increase their genetic representation in the next generation. The data on fertility differentials among contemporary humans are not cotvietent with this postulate. Given the importance of Homo sapiens as an animal species in the natural world today, these data constitute particularly challenging and interesting problem for both human sociobiology and sociobiology as a whole.The first part of this paper reviews the evidence showing an inverse relationship between reproductive fitness and “endowment” (i.e. wealth, success, and measured aptitudes) in contemporary, urbanized societies. It is shown that a positive relationship is observed only for those cohorts who bore their children during a unique period of rising fertility, 1935–1960, and that these cohorts are most often cited by sociobiologists as supporting the central postulate of sociobiology. Cohorts preceding and following these show the characteristic inverse relationship between endowment and fertility. The second section reviews the existing so-ciobiological models of this inverse relationship, namely, those of Barkow, Burley, and Irons, as well as more informal responses among sociobiologists to the persistent violation of sociobiology's central postulate, such as those of Alexander and Dawkins. The third section asks whether the goals of sociobiology, given the violation of its fundamental postulate by contemporary human societies, might not be better thought of as applied rather than descriptive, with respect to these societies. A proper answer to this question begins with the measurement of the pace and direction of natural selection within modern human populations, as compared to other sources of change. The vast preponderance of the shifts in human trait distributions, including the IQ distribution, appears to be due to environmental rather than genetic change. However, there remains the question of just how elastic these distributions are in the absence of reinforcing genetic change.
Failure of the behavioral sciences to develop an adequate general theory is seen as a result of the difficulty in deriving from evolutionary theory a subtheory, or set of subtheories, with satisfying applicability to the study of behavior. Efforts at general theories based on reflex concepts, or simple movements such as in orientation, have been unsuccessful in dealing with complex behaviors. Recent arguments that selection is focused at the level of the individual organism suggest the additional inadequacy that such theories fail to emphasize the selective compromises that exist at suborganismic levels. Evolutionary theories about behavior have tended to concentrate chiefly on patterns of historical change (phylogenies) without stressing adaptive (= reproductive) strategies, or have generally viewed adaptiveness erroneously as focused at group, population, or species levels. Human society is discussed briefly, in a context of selection focused at the individual level, considering six principal aspects: group-living, sexual competition, incest avoidance, nepotism, reciprocity, and parenthood. An effort is made to combine the approaches and data of biologists and social scientists in analyzing reciprocity in social interactions.
Sociobiology provides a perspective from which much of human behavior seems to make sense, and we expect that its importance will grow in years to come. We believe, however, that its current development is flawed by several widespread misunderstandings.
Like other students of nature, sociobiologists are greatly interested in adaptation, particularly as embodied in W.D. Hamilton's theory of inclusive fitness. However, natural selection produces adaptation only when traits' fitnesses do not depend on their frequencies. Yet the fitness effects of social behaviors are generally frequency dependent, and there is, therefore, no reason to expect social behaviors to be adaptive. Neither is there any reason to expect evolution to maximize inclusive fitness. Hamilton's theory has been too widely applied.
For similar reasons, evolution can produce mechanisms of cultural transmission that lead to maladaptive behaviors. The circumstances under which adaptive learning mechanisms will evolve are poorly understood.
The ethnographic literature is a poor source of data for testing hypotheses because it often fails to distinguish what people do from what they say they do. If, as seems likely, language is used more for manipulation than communication, the ethnographic record will be practically useless.
In spite of all this, sociobiological arguments seem to account for a great deal of observed variability in human behavior.
Infant and child mortality in 18th and 19th century Krummhörn exhibits a remarkable feature: significantly more daughters than sons of comparably prosperous high status farmers achieve adulthood. We interpret this difference as being the outcome of differential parental care reflecting varying reproductive perspectives and social role expectations, to which sons and daughters from farmer families were exposed. This is verified by sex differences in the children's probability of marrying and their differing chances of social persistence. Against the background of severely restricted reproductive opportunities and recognizably higher upbringing costs for a son, parental underinvestment in the survival of sons can be best understood as an economically motivated measure in the course of a farmer's efforts to concentrate his property and to maintain his family's social status. At the same time, such a scenario also had a biological adaptive value, because it contributed to the intergenerational perpetuation of above average chances of life and reproduction, and hence, to the genetic persistence of the farmer families.
The “Leslie matrix” of demography is extended to deal with categories of wealth, rather than age, and is used to build an evolutionary model of the effect of heritable wealth on reproductive decisions. Optimal reproductive strategies are assumed to be those that maximize the long-term rate of growth in the numbers of one's descendents. In poor environments, the optimal strategy is to maximize the wealth inherited by each offspring, which requires limiting their numbers. In rich environments, on the other hand, it pays to maximize the number of offspring. Strong positive correlations between wealth and the number of offspring are predicted only in rich environments. Therefore, evidence that the rich reproduce more slowly than the poor is not inconsistent with the hypothesis that reproductive strategies have been shaped by evolution.
Summary Using relatively simple mathematical techniques, an analysis is made of a comprehensive reproductive model that describes the relationships between a set of intermediate fertility variables and the marital fertility rate. Two types of intermediate fertility variables are distinguished: (1) biological parameters and (2) control variables. A homogeneous model is outlined first. Next, this version is extended to include heterogeneity with respect to fecundability and coital rates. Tests of the model with data from two historical populations (i.e. Crulai, 1674-1742, and Tourouvre au Perche, 1665-1765) demonstrate that the model is, indeed, consistent with observed reproductive behaviour in actual populations.
Demographic analyses from 3 cohorts of Kenyan Kipsigis women married between 1940 and 1973 demonstrate that early maturing women have higher reproductive success than do late maturing women, due to longer reproductive lifespans and higher fertility. This result is independent of confounding effects of husband's wealth, but not of the wealth of a woman's parents which affects both menarcheal age and subsequent reproductive success. Data on bridewealth payments at 194 marriages occurring after 1959 show that men make higher marriage payments for early maturing women than for late maturing women. Together these results suggest that Kipsigis men vary their marriage payments in accordance with the reproductive value of their brides. The question of why men use age at menarche rather than bride's parents' wealth as a cue to their bride's subsequent reproductive performance is discussed in the light of changing social and economic conditions experienced by Kipsigis since the late 1920s. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/46888/1/265_2004_Article_BF00292097.pdf
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.
The estimation of fecundability from survey data is plagued by methodological problems such as misreporting of dates of birth and marriage and the occurrence of premarital exposure to the risk of conception. Nevertheless, estimates of fecundability from World Fertility Survey data for women married in recent years appear to be plausible for most of the surveys analyzed here and are quite consistent with estimates reported in earlier studies. The estimates presented in this article are all derived from the first interval, the interval between marriage or consensual union and the first live birth conception.
A genetical mathematical model is described which allows for interactions between relatives on one another's fitness. Making use of Wright's Coefficient of Relationship as the measure of the proportion of replica genes in a relative, a quantity is found which incorporates the maximizing property of Darwinian fitness. This quantity is named “inclusive fitness”. Species following the model should tend to evolve behaviour such that each organism appears to be attempting to maximize its inclusive fitness. This implies a limited restraint on selfish competitive behaviour and possibility of limited self-sacrifices.
Special cases of the model are used to show (a) that selection in the social situations newly covered tends to be slower than classical selection, (b) how in populations of rather non-dispersive organisms the model may apply to genes affecting dispersion, and (c) how it may apply approximately to competition between relatives, for example, within sibships. Some artificialities of the model are discussed.
Grounds for thinking that the model described in the previous paper can be used to support general biological principles of social evolution are briefly discussed.Two principles are presented, the first concerning the evolution of social behaviour in general and the second the evolution of social discrimination. Some tentative evidence is given.More general application of the theory in biology is then discussed, particular attention being given to cases where the indicated interpretation differs from previous views and to cases which appear anomalous. A hypothesis is outlined concerning social evolution in the Hymenoptera; but the evidence that at present exists is found somewhat contrary on certain points. Other subjects considered include warning behaviour, the evolution of distasteful properties in insects, clones of cells and clones of zooids as contrasted with other types of colonies, the confinement of parental care to true offspring in birds and insects, fights, the behaviour of parasitoid insect larvae within a host, parental care in connection with monogyny and monandry and multi-ovulate ovaries in plants in connection with wind and insect pollination.
Reproductive patterns were studied from data collected in 1,450 Berber households in the province of Marrakesh, Morocco in 1984. Women aged 45-49 years had a mean of 8.9 pregnancies to achieve 5.7 living children. Social influences on fertility rates show the importance of tradition, particularly through time-dependent variables such as age at marriage, waiting time to first birth, interbirth intervals, and duration of breastfeeding. Birth control does not appear to affect the tempo of fertility; rather, its main use is to bring the reproductive period to a close. The comparison of two subsamples of women separated by a 25-year interval indicates an actual acceleration of the tempo of fertility by the reduction of waiting time to first birth and of interbirth intervals. The supposed ongoing process of demographic transition is not clearly observed in this population.
The relationship between female age and infertility is examined using a single-island Micronesian population case. Demographic data, derived primarily from reproductive history interviews, show that a significant age-associated decline in marital reproductive performance is absent before women reach their late thirties in this population but a substantial decline is present once women reach their forties. Ethnographic data support the demographic inference that couples are maintaining relatively high levels of conjugal coital activity with both advancing female age and increasing marital duration. Thus coital activity levels appear to be an important factor in the maintenance of fertility in this group before the mid-thirties but decreases in fecundability after this age are due primarily to reductions in fecundity, not to declines in coital activity. The description of the Butaritari case lends support to Underwood's (1990) suggestion that a "Micronesian pattern" of reproductive performance may exist for the region's atoll-based populations and underscores the promise of further investigations of these special cases in the fields of demography and reproductive ecology.
A sample of 842 rural women from Morocco (Amizmiz, Marrakech) was used to examine the relationship between a number of biosocial variables and fertility patterns. For women still in their reproductive years there were significant correlations between family size and woman's age, age at marriage and years of marriage. Among women with completed families, those with early age at marriage ceased childbearing about 10 years before reaching menopause, while women who married later continued to bear children until the end of their fertile life.
The reproductive histories of women aged 45-70 years from a homogeneous Berber population of South Morocco were sampled from three contrasting environments: a small town (n = 75), villages in the lowlands (n = 217), and villages in the highlands (n = 128). The main reproductive variables oppose the relatively better conditions of fertile life in the lowlands to the more hostile ones in the highlands. Path analysis confirms this difference through reproductive behaviors and suggests the existence of mechanisms for controlling family size in town and in the rural lowlands, but not in the highlands. The estimates of survival function show significant differences among the three groups, the conditions for survival in the highlands being clearly less favorable. Rank tests of the association of survival data with several covariates indicate the association of survival data with vaccinations and with conditions of delivery. In spite of the lower rate of offspring survival, the highlander group would demonstrate a higher overall number of children reaching reproductive maturity, thanks to an extended reproductive span.
Birth interval lengths are analysed from reproductive life histories of 517 Berber peasant women of the region of Marrakesh (Southern Morocco), whose fertility developed in a full traditional context. The high mortality rates associated with short birth intervals indicate that a rapid succession of births is detrimental to the progeny. The reproductive efficiency of the traditional propensity to a large family size is therefore examined by means of two different evaluations of reproductive success: the 'absolute' reproductive success (the absolute number of offspring surviving to maturity) and the 'relative' reproductive success (the proportion of live born surviving to maturity). The first shows that close pregnancies increase the fertility rate to such an extent that the associate higher number of deaths is more than compensated for, so that the women practising short birth intervals produce more surviving offspring than the others by the end of their reproductive life. The second shows that the probability of survival is directly associated with birth interval length, the efficiency of the reproductive process being therefore greater as birth intervals grow. It is suggested that these two behaviours are not contradictory, and that they represent two successive steps of the same reproductive adjustment to evolving environmental conditions.
"Helpers at the nest," usually offspring of a preceding litter who contribute by feeding the young to increase the reproductive success of a breeding pair, are known in many species of birds and mammals. Although similar behaviors were described by ethnological observations in several human societies, there is a lack of data on their existence and role. This study of 794 reproductive life histories of post-menopausal Berber women of Southern Morocco aims to provide such information. Results show that the presence of "probable helpers" in the household is related to higher fertility scores and is associated with improved survival of offspring to sexual maturity. In contrast to sparse observations from other human societies, there is no indication that child caretaking would be specific to eldest daughters. Although the association between offspring survival and helping patterns seems highly probable, there is no confirmation that child caretaking per se is the relevant variable. Contrary to nonhuman helpers at the nest, workloads of children range from housekeeping to light agricultural tasks, and are not focused on assisting younger siblings. The improvement of reproductive success is probably the result of multiple interactions, among which the network of kinship would play a role at both the levels of economy and reciprocal assistance.
"Helpers at the nest," young adults remaining in their parents home to take care of younger siblings, are known in many species of birds and mammals. Similar behaviors are occasionally observed in human societies but their frequency and significance for parental reproductive success are still not fully appraised. This study was designed to document this issue in a traditional Aymara peasant society of the Bolivian Altiplano, It is based on 359 reproductive life histories of women 45 years of age or older and on a survey of children's workload in 1998 and 1999. The presence of "potential helpers" in the household is significantly associated with higher fertility and with improved survival of siblings to sexual maturity. Caretaking is not particularly assigned to older daughters. The positive relationship between the availability of offspring help and reproductive success does not demonstrate a causal role for child caretaking because, in contrast with nonhuman helpers, workloads of children range from housekeeping to agricultural tasks, instead of being focused on feeding or protecting younger siblings. Correlation and multiple regression analyses, however, suggest that the total amount of care given by the older offspring and the amount of care received by each recipient are, along with offspring contribution to household economy, among the determinants of parental reproductive success.
"In this review I draw upon statistical demography and, to a lesser extent, reproductive endocrinology to formulate a coherent strategy for investigating fertility and reproduction in anthropological populations. The object, it must be emphasized, is not to reduce anthropology to demography or endocrinology, but rather to acquaint anthropologists with a powerful set of tools with which they can address issues of anthropological interest." The author first discusses the concept of natural fertility. Next, he summarizes the most significant generalizations concerning variations in fertility among preindustrial societies using the concept of proximate determinants developed by John Bongaarts. Finally, he outlines an alternative approach that might be more suited to the analysis of such fertility variations.
Reproductive characteristics at high altitude are described based on the reproductive histories of 720 Aymara women, collected in 1998 and 1999 in a group of twelve peasant communities at a mean altitude of 4000 m in the Bolivian Altiplano. The reproductive pattern is shaped by a late onset of childbearing, associated with a rather short reproductive span and large birth intervals. Environmental conditions could explain the particularly late age at menarche of rural girls compared with their urban counterparts, whereas the age at first birth is likely to be under cultural control. The short reproductive span appears to result from a large mean interval between last birth and menopause, which is essentially determined by cultural decisions. The birth intervals, which are longer than in many traditional societies, could be the result of a slower restoration of postpartum fecundability induced by the hard way of life inherent in the Altiplano (including poor sanitary and nutritional conditions and high workload), perhaps aggravated by hypoxia. However, a secular trend in fertility is perceptible, towards earlier menarche, earlier age at first birth, increasing reproductive span and a slight increase in live births and surviving offspring, which is probably the result of a slow improvement in living conditions. The existence of birth control on the one hand, and a total fertility rate averaging six live births among the couples who do not practise contraception on the other, are other arguments against the hypothesis of a low natural fecundity in these Aymara groups.
Data on the natural fertility (complete absence of birth control) of 13 populations are examined with the study restricted to legitimate birthrates and those of unmarried women in a stable union. Very different fertility levels were found among these populations despite a similar pattern of fertility as it varies from 1 age group to another. Where the fertility rates for European populations are greater than for the non-European differences can be attributed to variation in birth spacing. It is hypothesized that differences in fertility level are either the result of variations in behavior related to resumption of sexual relations and the duration of lactation or to the differences of a physiological nature related to frequency and duration of anovulation during lactation.