Comparative study of breastfeeding structure and its relation to human reproductive ecology

Beyond Milk and Nurture: Breastfeeding's Powerful Impact on Breast Cancer

Breast cancer (BC) stands as a global concern, given its high incidence and impact on women's mortality. This complex disease has roots in various risk factors, some modifiable and others not. Understanding and identifying these factors can be instrumental in both preventing BC and improving survival rates. Remarkably, women's reproductive behaviors have emerged as critical determinants of BC susceptibility. Numerous studies have shed light on how aspects including age of menarche, first pregnancy and menopause along with number of pregnancies, hormone replacement therapies, can influence one's risk of developing BC. Furthermore, the act of breastfeeding and its duration have shown an inverse relationship with BC risk. This review delves into the biological and molecular mechanisms associated with breastfeeding that contribute to BC protection. It highlights the role of endocrine processes triggered by suckling stimulation, the gradual onset of lactational amenorrhea, delayed weaning, reduced lifetime menstrual cycles, chromosomal repair mechanisms, and immunological events throughout the lactation cycle. These insights provide a potential explanation for the protective effects conferred by breastfeeding against breast carcinomas.

Cesarean section and breastfeeding outcomes in an Indigenous Qom community with high breastfeeding support

BACKGROUND AND OBJECTIVES

Cesarean section may lead to suboptimal breastfeeding outcomes, though evidence has been mixed. Factors, such as premature birth, birth weight and maternal age may independently increase risk of cesarean and hinder breastfeeding initiation, while maternal preferences, support and sociostructural barriers may influence breastfeeding practices beyond the immediate postpartum period.

METHODOLOGY

We assessed impacts of cesarean section and gestational factors on breastfeeding duration among Indigenous Qom mothers in Argentina who have strong traditional breastfeeding support. We modeled transitions from exclusive breastfeeding to complementary feeding and from complementary feeding to full weaning in a Bayesian time-to-event framework with birth mode and gestational covariates (n = 89 infants).

RESULTS

Estimated median time to full weaning was 30 months. Cesarean-delivered babies were weaned an average of 5 months later adjusting for gestational age, maternal parity and infant sex. No factors were associated with time-to-complementary feeding, and time-to-complementary feeding was not associated with time-to-full weaning.

CONCLUSIONS AND IMPLICATIONS

Among Indigenous Qom mothers in Argentina, cesarean section was not associated with suboptimal breastfeeding outcomes. Although some Qom mothers do experience early breastfeeding problems, particularly following first birth, problems are not more frequent following cesarean delivery. Traditional postpartum kin and community support during prolonged postpartum periods may be instrumental in helping mothers to overcome early breastfeeding problems due to cesarean or other risk factors.

Breastfeeding Beyond 12 Months: Is There Evidence for Health Impacts?

Because breastfeeding provides optimal nutrition and other benefits for infants (e.g., lower risk of infectious disease) and benefits for mothers (e.g., less postpartum bleeding), health organizations recommend that healthy infants be exclusively breastfed for 4 to 6 months in the United States and 6 months internationally. Recommendations related to how long breastfeeding should continue, however, are inconsistent. The objective of this article is to review the literature related to evidence for benefits of breastfeeding beyond 1 year for mothers and infants. In summary, human milk represents a good source of nutrients and immune components beyond 1 year. Some studies point toward lower infant mortality in undernourished children breastfed for >1 year, and prolonged breastfeeding increases interbirth intervals. Data on other outcomes (e.g., growth, diarrhea, obesity, and maternal weight loss) are inconsistent, often lacking sufficient control for confounding variables. There is a substantial need for rigorous, prospective, mixed-methods, cross-cultural research on this topic. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Reconstructing prehistoric demography: What role for extant hunter-gatherers?

Demography is central to biological, behavioral, and cultural evolution. Knowledge of the demography of prehistoric populations of both Homo sapiens and earlier members of the genus Homo is, therefore, key to the study of human evolution. Unfortunately, demographic processes (fertility, mortality, migration) leave little mark on the archeological and paleoanthropological records. One common solution to this issue is the application of demographic data from extant hunter-gatherers to prehistory. With the aim of strengthening this line of enquiry, here we outline some pitfalls and their interpretative implications. In doing so, we provide recommendations about the application of hunter-gatherer data to the study of demographic trends throughout human evolution. We use published demographic data from extant hunter-gatherers to show that it is the diversity seen among extant hunter-gatherers-both intra- and inter-population variability-that is most relevant and useful for understanding past hunter-gatherer demography.

A Close and Supportive Interparental Bond During Pregnancy Predicts Greater Decline in Sexual Activity From Pregnancy to Postpartum: Applying an Evolutionary Perspective

A common topic for advice given to parents after childbirth – both from relationship experts and popular media – is how to “bounce back” to one’s pre-pregnancy sexuality, with warnings that postpartum declines in sexual frequency will take a serious toll on one’s relationship. However, these admonishments may not accurately reflect the ways in which the unique reproductive context of pregnancy and the postpartum transition alter associations between sexual frequency and relationship quality. Evolutionary perspectives on reproductive strategies would suggest that in the postpartum context, decreased sexual activity would help target parental investment in the current offspring (rather than creating new offspring); however, if the parental relationship is lacking in intimacy and support, continued sexual activity may help seal the cracks in the bond. We tested this theory in a longitudinal dyadic study of changes in relationship quality and sexual frequency from pregnancy to 6 months postpartum among 159 heterosexual couples. We found that across three different measures of relationship quality taken from interviews and behavioral observation of couple interactions, higher relationship quality (i.e., greater support, intimacy, and responsiveness) predicted greater decline in sexual frequency whereas sexual frequency remained relatively stable in lower quality relationships. These findings suggest that, during the postpartum transition, decreased sexual frequency may not be a reliable signal of poor relationship quality.

Impacts of nocturnal breastfeeding, photoperiod, and access to electricity on maternal sleep behaviors in a non-industrial rural Bolivian population

OBJECTIVES

We tested 4 main predictions, derived from life history theory and self-evident human diurnality, regarding maternal sleep behaviors in a non-industrialized population in which mother-nursling co-sleeping is universal and prolonged: (1) Night breastfeeding incurs a sleep cost to co-sleeping mothers; (2) Night breastfeeding increases with infant age, causing mothers to sleep less; (3) Sleep duration co-varies with darkness duration; (4) Access to electricity reduces sleep duration.

DESIGN

Mothers self-recorded and reported nursing and sleep behaviors for a 48-hour period once per month (median = 5 months).

SETTING

Rural Bolivian altiplano homesteads, primarily reliant on agropastoralism, scattered throughout the countryside surrounding a main town (altitude 3800 m; 17°14'S, 65°55'W; darkness duration 10-12 hours over the year).

PARTICIPANTS

One hundred eighty-four co-sleeping mother-infant pairs (infant age 22-730 days).

MEASUREMENTS

Breastfeeding frequency, and retiring and rising times for 885 48-hour observation periods.

RESULTS

Maternal sleep duration covaried with darkness duration. Sleep duration was shorter in those with access to electricity (ie, living nearer to town) than those without access (more distant homesteads). Night breastfeeding rate was fairly steady until it began to decline after the first year postpartum. At a given infant age, higher night breastfeeding rates were associated with less maternal sleep. As their infants aged, mothers without electricity slept more, whereas mothers with access slept less.

CONCLUSIONS

During the first year postpartum, more frequent night nursing shortens maternal sleep more than any other predictor variable. For older infants, the effect of night nursing diminishes, and even modest "modernization" (eg, access to electricity) is associated with shorter maternal sleep.

Milk ejection patterns remain consistent during the first and second lactations

OBJECTIVE

Milk ejection is a critical physiological process for successful lactation in humans and without it little milk can be removed. Individual milk ejection patterns have been shown to remain consistent between breasts at different lactation stages and using different vacuum patterns with an electric breast pump. Little is known about the milk ejection characteristics during the second lactation period in the same mother. The objective of this study was to examine milk ejection characteristics in the same woman over two lactations.

METHODS

One mother took part in two pumping studies during consecutive lactations. One pumping study examined milk ejection characteristics during simultaneous breast expression during the first lactation. The second pumping study (second lactation) used two different pumping patterns. Three distinct milk ejections were measured during each pumping session.

RESULTS

Measurements of milk flow were used to compare the duration and time taken to reach the peak of each milk ejection for two pumping sessions from each of the lactations. There were no significant differences in milk ejection characteristics between breasts, using different pumping patterns, or between lactations in this mother (P > .05).

CONCLUSION

Milk ejection appears to be a physiological response that is consistent across consecutive lactations within the same mother. This suggests that milk ejection characteristics are established during or prior to the first lactation. The infant appears to have little influence on the milk ejection characteristics of the mother.

Girl helpers and time allocation of nursing women among the Toba of Argentina

In this paper we outline the activities of young girls in a Toba community of northern Argentina and examine the effect of girl helpers on time allocation of nursing women. Activity budgets were obtained for 41 girls aged 3 to 15 using spot observations. Girls spent substantial portions of observations engaged in helping behaviors. Individual values varied with age, anthropometric characteristics, and birth order. Activity budgets of 21 nursing women were obtained through focal observation sessions. Women living in households with girls aged 7 to 15 allocated 17% less time to domestic work and 9% more time to socializing during afternoon observation sessions. For nursing women in this community, direct childcare (provided by the infant's own mother) seemed to be a priority. Living with a girl helper did not have any measurable effect on the frequency or duration of nursing, or on the time that women spent caring for their infants. Based on these findings, hypotheses are outlined for future work on the effect of girl helpers on women's fertility.

Parent-offspring conflict and the cultural ecology of breast-feeding

Lactation constitutes a major focus for research in international health because of its dramatic impact on child survival; evolutionary biology has investigated lactation as an important aspect of parenting strategy, with implications for understanding parent-offspring conflict. These perspectives are brought together in an attempt to develop integrated models for an issue of key international health concern: the duration of exclusive breast-feeding and the timing of weaning. This analysis highlights the relevance of evolutionary theory for practical problems in public health, and it suggests the utility of public health outcomes for addressing evolutionary questions.

Breastfeeding over two years is associated with longer birth intervals, but not measures of growth or health, among children in Kilimanjaro, TZ

OBJECTIVES

Breastfeeding has been associated with numerous health and well-being benefits for both children and their mothers, including prolonging the birth interval to the subsequent sibling. The clearest associations between breastfeeding and health outcomes, per se, reflect exclusive breastfeeding in the first months of postnatal life and are most evident during infancy. Fewer studies explore the consequences of breastfeeding for multiple years. In this article, we ask whether breastfeeding for more than 2 years is associated with discernible health and well-being benefits to children.

METHODS

Data were collected from 315 children, aged 2 to 7, and their caretakers residing in Kilimanjaro, Tanzania. Basic demographic and health information was solicited, and anthropometric and blood markers of health were evaluated.

RESULTS

Our results indicate a strong positive relationship between breastfeeding for 2 or more years and interbirth interval, but little evidence for a relationship between prolonged breastfeeding and several indicators of child growth and health.

CONCLUSIONS

We suggest that these relationships may support the recently rekindled birth spacing hypothesis, positing selection for longer interbirth intervals, rather than, or in addition to, more direct health benefits associated with breastfeeding for 2 or more years. Our results may indicate attenuating health benefits associated with longer breastfeeding.

Does observed fertility maximize fitness among New Mexican men? : A test of an optimality model and a new theory of parental investment in the embodied capital of offspring

Our objective is to test an optimality model of human fertility that specifies the behavioral requirements for fitness maximization in order (a) to determine whether current behavior does maximize fitness and, if not, (b) to use the specific nature of the behavioral deviations from fitness maximization towards the development of models of evolved proximate mechanisms that may have maximized fitness in the past but lead to deviations under present conditions. To test the model we use data from a representative sample of 7,107 men living in Albuquerque, New Mexico, between 1990 and 1993. The model we test proposes that low fertility in modern settings maximizes number of grandchildren as a result of a trade-off between parental fertility and next generation fertility. Results do not show the optimization, although the data do reveal a trade-off between parental fertility and offspring education and income.We propose that two characteristics of modern economies have led to a period of sustained fertility reduction and to a corresponding lack of association between income and fertility. The first is the direct link between costs of investment and wage rates due to the forces of supply and demand for labor in competitive economies. The second is the increasing emphasis on cumulative knowledge, skills, and technologies in the production of resources. Together they produce historically novel conditions. These two features of modern economies may interact with evolved psychological and physiological mechanisms governing fertility and parental investment to produce behavior that maximizes the economic productivity of lineages at the expense of fitness. If cognitive processes evolved to track diminishing returns to parental investment and if physiological processes evolved to regulate fertility in response to nutritional state and patterns of breast feeding, we might expect non-adaptive responses when returns from parental investment do not diminish until extremely high levels are reached. With high economic payoffs from parental investment, people have begun to exercise cognitive regulation of fertility through contraception and family planning practices. Those cognitive processes maynot have evolved to handle fitness trade-offs between fertility and parental investment.

The evolutionary ecology of early weaning in Kilimanjaro, Tanzania

Public health recommendations promote prolonged breastfeeding of all children; however, parental investment (PI) theory predicts that breastfeeding will be allocated among a mothers' offspring to maximize her reproductive success. We evaluated PI in terms of risk for weaning before age two among 283 children in Kilimanjaro, Tanzania. Results demonstrate: (i) a Trivers-Willard effect--high socioeconomic status (SES) females and low SES males were more likely to be weaned early; (ii) later-born children were less likely to be weaned early; (iii) higher birthweight children were less likely to be weaned early, and (iv) no effect of cattle (a source of supplementary milk) ownership. These associations were largely independent and remained significant in models controlling for potential confounders; however, the inverse association between early weaning and birth order lost significance in the model containing birthweight. These patterns were observed despite public health recommendations encouraging breastfeeding for at least two years.

Fifty fertile years: anthropologists' studies of reproduction in high altitude natives

Early European colonists of the Andes had difficulties in reproducing, a fact that underpins the hypothesis that reproduction is impaired amongst all humans at high altitudes. Yet a 16th century missionary wrote, "… the Indians are healthiest and where they multiply the most prolifically is in these same cold air-tempers, … [yet most children of the Spaniards] when born in such regions do not survive." These observations suggest that humans at high altitudes are subjected to strong natural selection from hypoxia, cold and limited food sources and, furthermore, that human populations can and have adapted, and continue to adapt, to these conditions. Informed by multiple approaches and theoretical frameworks, anthropologists have investigated to what extent and precisely how high altitude environments impact human reproductive functioning and fertility. Analyses of the proximate determinants of natural fertility suggest that behaviors (breast/infant feeding practices in the Andes, and marriage practices and religious celibacy in the Himalaya) are major determinants of fertility in high altitude populations. Furthermore, data from Project REPA (Reproduction and Ecology in Provincía Aroma), a longitudinal study in rural Bolivia, demonstrate that fecundity is not impaired in this indigenous altiplano population, and that the risk for early pregnancy loss (EPL) is not elevated by environmental hypoxia but does vary seasonally with the agricultural cycle (contra to the assumption that EPLs are due almost entirely to genetically flawed concepti). This review discusses these and other findings that reveal the complex and dynamic adaptations of human reproductive functioning in high altitude environments.

Darwin's legacy: an evolutionary view of women's reproductive and sexual functioning

On the Origin of species, published just over 150 years ago, has deeply influenced thinking in both scientific and wider communities. Darwin's legacy includes recognition of the fact that all organisms evolve; that variation within and between species is natural and normal; and that an evolutionary approach to understanding the sources and consequences of this variation comprises theoretical frameworks, testable hypotheses, and rigorously collected evidence. With an eye toward facilitating communication and productive collaboration among researchers from different intellectual traditions who nonetheless share a common interest in women's reproductive and sexual functioning, we discuss evolutionary concepts and models, summarize the known variability in ovarian functioning and consider the implications of this variability for conducting sex research, and evaluate the relative merits of various biomarkers that serve as proxy measurements of a woman's reproductive and hormonal status. With these perspectives and methods from reproductive ecology at hand, we examine several contentious issues: the links between hormones and sexuality in premenopausal and perimenopausal women, the causes of premenstrual syndrome, and the existence (or not) of menstrual synchrony. In none of these cases is as much known as is often claimed. In each, there are abundant opportunities for innovative, albeit challenging, research.

Menopause, A Biocultural Perspective

Each menopausal body is the product of decades of physiological responses to an environment composed of cultural and biological factors. Anthropologists have documented population differences in reproductive endocrinology and developmental trajectories, and ethnic differences in hormones and symptoms at menopause demonstrate that this stage of life history is not exempt from this pattern. Antagonistic pleiotropy, in the form of constraints on the reproductive system, may explain the phenomenon of menopause in humans, optimizing the hormonal environment for reproduction earlier in the life course. Some menopausal symptoms may be side effects of modernizing lifestyle changes, representing discordance between our current lifestyles and genetic heritage. Further exploration of women's experience of menopause, as opposed to researcher-imposed definitions; macro- and microenvironmental factors, including diet and intestinal ecology; and folk etiologies involving the autonomic nervous system may lead to a deeper understanding of the complex biocultural mechanisms of menopause.

Evolutionary perspectives on mother-infant sleep proximity and breastfeeding in a laboratory setting

Human maternal and infant biology likely coevolved in a context of close physical contact and some approximation of frequent, "infant-initiated" breastfeeding. Still, mothers and infants commonly sleep apart from one another in many western societies, indicating a possible "mismatch" between cultural norms and infant biology. Here we present data from a 3-night laboratory-based study that examines differences in mother-infant sleep physiology and behavior when mothers and infants sleep together on the same surface (bedsharing) and apart in separate rooms (solitary). We analyze breastfeeding frequency and interval data from the first laboratory night (FN) for 52 complementary breastfeeding mothers and infants (26 total mother-infant pairs), of which 12 pairs were routine bedsharers (RB) and 14 were routine solitary sleepers (RS). RB infants were 12.0 ± 2.7 (SD) weeks old; RS infants were 13.0 ± 2.4 weeks old. On the FN, RB mother-infant pairs (while bedsharing) engaged in a greater number of feeds per night compared to RS (while sleeping alone) (P < 0.001). RB also showed lower intervals (min) between feeds relative to RS (P < 0.05). When we evaluated data from all three laboratory nights (n = 36), post hoc, RB breastfed significantly more often (P < 0.01) and showed a trend towards lower intervals between feeds (P < 0.10). Given the widely known risks associated with little or no breastfeeding, the demonstrated mutually regulatory relationship between bedsharing and breastfeeding should be considered in future studies evaluating determinants of breastfeeding outcomes.

Paradoxical malnutrition in mother-child pairs: untangling the phenomenon of over- and under-nutrition in underdeveloped economies

As economic development and urbanization proceed globally, the coexistence of under- and over-nutrition within the same household, sometimes termed 'paradoxical' or 'dual burden' malnutrition is increasingly being reported. We used Demographic and Health Survey data sets from 18 lower and middle income countries to explore paradoxical forms of malnutrition (maternal overweight with child underweight or stunting) in mother-child pairs. Multinomial logistic regression was used to estimate the odds of discordant pairs after adjusting for a number of important covariates. Several factors were significantly associated with an increased relative odds of discordant mother-child pairs, including working in subsistence agriculture, low levels of maternal education, more siblings in the household, and relative household poverty. However, many of these factors also predicted other combinations of poor nutritional status in mother-child pairs. We conclude that it is difficult to identify any specific factors that elevate risk above and beyond those that predict risk of maternal over-nutrition or child under-nutrition. Based on these analyses, it appears that paradoxical weight status between mothers and children can be best understood as a consequence of rapid secular increases in maternal weight, rather than a distinct nutritional condition with a discrete etiology.

Evolutionary Models of Women's Reproductive Functioning

Life history theory posits that natural selection leads to the evolution of mechanisms that tend to allocate resources to the competing demands of growth, reproduction, and survival such that fitness is locally maximized. (That is, among alternative allocation patterns exhibited in a population, those having the highest inclusive fitness will become more common over generational time.) Strategic modulation of reproductive effort is potentially adaptive because investment in a new conception may risk one's own survival, future reproductive opportunities, and/or current offspring survival. Several physiological and behavioral mechanisms modulate reproductive effort in human females. This review focuses on the hormonal changes that vary the probability of ovulation, conception, and/or continuing pregnancy and discusses evolutionary models that predict how and why these hormonal changes occur. Anthropological field studies have yielded important insights into the environmental correlates of variation in ovarian steroids, but much remains to be learned about the evolutionary determinants, proximate mechanisms, and demographic significance of variation in women's reproductive functioning.

Life history theory and the immune system: steps toward a human ecological immunology

Within anthropology and human biology, there is growing interest in immune function and its importance to the ecology of human health and development. Biomedical research currently dominates our understanding of immunology, and this paper seeks to highlight the potential contribution of a population-based, ecological approach to the study of human immune function. Concepts from life-history theory are applied to highlight the major challenges and demands that are likely to shape immune function in a range of ecological contexts. Immune function is a major component of maintenance effort, and since resources are limited, trade-offs are expected between investment in maintenance and other critical life-history functions involving growth and reproduction. An adaptationist, life-history perspective helps make sense of the unusual developmental trajectory of immune tissues, and emphasizes that this complex system is designed to incorporate information from the surrounding ecology to guide its development. As a result, there is substantial population variation in immune development and function that is not considered by current biomedical approaches. In an attempt to construct a framework for understanding this variation, immune development is considered in relation to the competing life-history demands that define gestation, infancy, childhood, adolescence, and adulthood. Each life stage poses a unique set of adaptive challenges, and a series of hypotheses is proposed regarding their implications for immune development and function. Research in human ecological immunology is in its earliest stages, but this is a promising area of exploration, and one in which anthropology is well-positioned to make important contributions.

Explaining the Neolithic Demographic Transition

Three main questions are raised in this chapter.

1. The part of the signal of the NDT which is demographically identifiable, based on the proportion of the immature skeletons in cemeteries, shows that a baby-boom occurred and, beyond that, with the onset of the change in the economic system, a fertility transition towards high values also occurred. What was the biodemographic cause of this fertility explosion, beyond the proxy variable represented by sedentarism? The cause is a major shift in the maternal energetics of farming communities relative to mobile foragers. In the energy balance there was (i) on the intake side, an underlying trend towards a reduction in low-calorie food from hunting and fishing, and a correlative increase in high-calorie food from agriculture, (ii) on the expenditure side, a reduction in the physical energy devoted to mobility and the maternal stress of child transportation.

2. The NDT is detectable from a signal representing a fertility transition, but the transition relating to mortality is missing and must be inferred. If, during the fertility transition, mortality had remained the same as in the preceding forager period, then the population would have grown infinitely. The assumption of unchanged mortality during the entire fertility transition is therefore not realistic. Mortality, in its turn, must have begun to rise well before the end of the fertility transition. But when? Why? One of the answers consists of a model where, except at the start of the process, birth and mortality rates rise more or less simultaneously, bringing about a typical rate of increase for pre-industrial populations of slightly above zero.

3. According to the level and speed of the population growth, what should we expect in terms of population structure? What are the expected effects of this growth, not only on the population in numbers but also on age distribution, the distribution and structure of families, the distribution of households and on family systems?

Weaning age among foragers at Matjes river rock shelter, South Africa, from stable nitrogen and carbon isotope analyses

Matjes River Rock Shelter is a large shell midden on the southern coast of South Africa. Stable nitrogen (delta(15)N) and carbon (delta(13)C) isotope ratios were measured in bone collagen and dentine from human skeletons excavated from this site in order to establish a weaning curve in mid-Holocene hunter-gatherers. delta(15)N results show a progressive increase in individuals from birth to 1.5 years old. delta(13)C results are more tightly clustered and mirror the steady progressive change seen for delta(15)N. We deduce that children at Matjes River Rock Shelter were breastfed for at least the first 1.5 years after birth, and were weaned sometime between 2-4 years of age. A similar pattern was documented for historic-era Kalahari foraging people, where the interbirth spacing was approximately 3 years. This study provides the first direct evidence for an extended period of breastfeeding, and thus long interbirth intervals, among prehistoric foragers, even when those foragers lived in an environment with abundant food resources.

Weaning practices of the Makushi of Guyana and their relationship to infant and child mortality: A preliminary assessment of international recommendations

The World Health Organization (WHO) recommends exclusive breastfeeding (EBF) for the first 6 months of life, primarily because of potential immunological benefits which are deemed to outweigh nutritive costs for infants. This recommendation is controversial, as studies of the relationship between the term of EBF and infant and child health have produced conflicting results. The purpose of this paper is to evaluate the relationship between the term of EBF and infant and child mortality among a group of swidden-horticulturalists in lowland South America. Consistent with the WHO, we hypothesized that EBF <6 months will compromise the survival of the infant or child. This relationship was assessed via recall data generated in 2001 in structured interviews with 60 Makushi Amerindian women in Guyana's North Rupununi region. The data were analyzed with t-tests, Fisher's exact test, and logistic regression. The results do not support our hypothesis; the term of EBF is not found to be related to infant or child mortality. This is surprising given the potential for contamination in nonbreast-milk foods in this environment. Notably, this is occurring among mothers who are not energetically stressed. We propose that the apparent lack of benefit of EBF >or=6 months is due to insufficient energy supply from breast milk alone, which may predispose the child to morbidity when subsequently stressed. This study concurs with others which revealed no significant benefits to the infant of EBF >6 months, and the recognition that universal recommendations must be situated within local ecological contexts.

THE ECOLOGIES OF HUMAN IMMUNE FUNCTION

Immune function is notoriously complex, and current biomedical research elaborates this complexity by focusing on the cellular and molecular mechanisms that characterize immune defenses. However, the human immune system is a product of natural selection that develops and functions in whole organisms that are integral parts of their surrounding environments. A population-level, cross-cultural, adaptationist perspective is therefore a necessary complement to the micro levels of analysis currently favored by biomedical immunology. Prior field-based research on human immunity is reviewed to demonstrate the relevance of cultural ecological factors, with an emphasis on the ecologies of nutrition, infectious disease, reproduction, and psychosocial stress. Common themes and anthropological contributions are identified in an attempt to promote future research in human ecological immunology that integrates theory and method for a more contextualized understanding of this important physiological system.

Culture and symptom reporting at menopause

The purpose of the present paper is to review recent research on the relationship of culture and menopausal symptoms and propose a biocultural framework that makes use of both biological and cultural parameters in future research. Medline was searched for English-language articles published from 2000 to 2004 using the keyword 'menopause' in the journals--Menopause, Maturitas, Climacteric, Social Science and Medicine, Medical Anthropology Quarterly, Journal of Women's Health, Journal of the American Medical Association, American Journal of Epidemiology, Lancet and British Medical Journal, excluding articles concerning small clinical samples, surgical menopause or HRT. Additionally, references of retrieved articles and reviews were hand-searched. Although a large number of studies and publications exist, methodological differences limit attempts at comparison or systematic review. We outline a theoretical framework in which relevant biological and cultural variables can be operationalized and measured, making it possible for rigorous comparisons in the future. Several studies carried out in Japan, North America and Australia, using similar methodology but different culture/ethnic groups, indicate that differences in symptom reporting are real and highlight the importance of biocultural research. We suggest that both biological variation and cultural differences contribute to the menopausal transition, and that more rigorous data collection is required to elucidate how biology and culture interact in female ageing.

The role of changing childhood diets in the prehistoric evolution of food production: An isotopic assessment

Earlier weaning has often been suggested as a cause for population growth after the evolution of food production. However, evidence for weaning-time reduction is largely circumstantial. Collagen stable nitrogen- and carbon-isotope ratios from juvenile and adult burials from four sites in eastern North America were measured to estimate weaning onsets and durations before and after the appearance of intensive food production. Two preagricultural Late Archaic sites (Indian Knoll and Carlston Annis) are compared with two highly agricultural Middle Mississippian sites (Angel and Tinsley Hill). Isotopic data and paleodemographic measures of birth rates provide no evidence for changes in weaning behavior or fertility with the development of food production in the prehistoric Lower Ohio Valley. Birth rates and weaning behavior appear to have been roughly the same at all four sites. These results indicate that models attributing population growth after the appearance of food production to earlier weaning are not universally applicable.

The proximate determinants of fertility in populations exposed to chronic hypoxia

It has been hypothesized that hypoxia reduces fertility, but comparative studies of high and low altitude populations have been unable to verify or refute this proposal because it is difficult to control for the behavioral and sociocultural factors that may also either underlie fertility differentials or compensate for physiological changes caused by varying partial pressure of oxygen. Taken collectively, estimates of fertility in populations exposed to chronic hypoxia range widely and do not suggest any reproductive patterns specific to high altitude. Here we review the available data from the Andes and the Himalaya on the proximate determinants of fertility, that set of factors through which any and all environmental, behavioral, and sociocultural factors must act to influence fertility levels. Although hypoxia could potentially affect some of these factors, there is no unequivocal evidence that this occurs in human populations indigenous to high altitude. At this time, it appears that local variations in infant feeding beliefs and practices, often coupled with prolonged breast-feeding, play a major role in determining fertility variation in Andean populations. In the Himalaya, large numbers of adults are not in sanctioned sexual unions as a consequence of polyandrous marriage practices and religious celibacy. The absence of a clear negative effect of hypoxia on fertility in populations indigenous to high altitude, even though migrants report reproductive difficulties, argues that these populations have adapted to the conditions at high altitude. The experimental and clinical evidence presented in this issue suggests proximate mechanisms by which such adaptation, shaped by natural selection and developmental processes, is possible.

Impact of breastfeeding on anthropometric changes in peri-urban Toba women (Argentina)

We present an analysis of the effect of lactation on average maternal anthropometric and body composition measures in a population of Toba women in Formosa, Argentina. This indigenous population is undergoing a continuing transition from a seminomadic hunter-gatherer lifestyle to a sedentary, peri-urban one. Using a mixed-longitudinal design, we measured monthly maternal body mass index (BMI), body fat percentage, and triceps and subscapular skinfold thickness between birth and the 18th month postpartum in 113 breastfeeding women. The pattern of change in postpartum body composition varied with maternal age. Adult women (20 years old and older) did not show significant changes in any of the anthropometric measures during the entire study. Older adult women (30 years old and older) consistently had the highest values in measures of BMI and percentage fat, and tended to retain weight postpartum. Adolescent subjects (19 years old and younger) tended to lose weight during the first 6 months postpartum but regain their prepregnancy weight by 12 months postpartum. The same patterns were observed for changes in body fat percentage and in skinfold thickness. We conclude that in this population the energetic stress of lactation does not pose a serious challenge to the maintenance of long-term maternal energy balance or to short-term energy balance in women over 20 years of age. From a public health perspective, postpartum weight retention in older women may represent a more serious health threat. The low level of energetic stress associated with lactation may also contribute to the relatively short duration of lactational amenorrhea in this population despite a cultural pattern of intensive breastfeeding. Am. J. Hum. Biol. 15:717-724, 2003.

The home team advantage: reproduction in women indigenous to high altitude

Although there is substantial evidence that environmental conditions disrupt reproductive function among newcomers to hypoxic settings, it is not certain that low oxygen pressure reduces fertility among those indigenous to high altitude. Even when fertility does appear to be relatively lower, numerous behavioral and sociocultural factors may be responsible. These are best examined within demographic frameworks that delineate a finite list of the proximate determinants of fertility. The findings presented here are based on several studies of indigenous Andean populations (Peruvian Quechua at 4000m, Bolivian Quechua at 3100m, Bolivian Aymara at 4000m). Data on ovarian function suggest that neither progesterone levels nor menstrual cycle length or regularity are significantly different from those of women at lower altitudes. Data on two behavioral factors that determine fertility levels, coital frequency and infant feeding practices, suggest that the former is not likely to be of significance in co-habitating couples, but that variation in breastfeeding patterns has probably made a substantial contribution to differences in fertility among at least some populations at high altitude.

Weaning, complementary feeding, and maternal decision making in a rural east African pastoral population

A small-scale, prospective study of breastfeeding and weaning practices was conducted in a seminomadic pastoral population in northern Tanzania (Datoga of Mbulu District, Arusha Region). Focus group participants suggested that weaning patterns are strongly influenced by seasonal factors, and individual mothers often stated an intention to introduce adult staple foods and terminate breastfeeding at the end of the long rains. However, a combination of maternal self-perception, assessment of infant well-being, and indicators of household food supply influenced the actual progression of weaning for individual children. The results suggest that interventions to promote exclusive breastfeeding and improve complementary feeding practices in rural East African pastoral populations could build on maternal attention to infant-centered cues and address household-level constraints on caregivers making decisions about young child feeding.

Hydration as a limiting factor in lactation

A relatively untapped area of research concerning lactation and hydration exists for bioanthropologists interested in the ecology of breastfeeding. This review details current limited knowledge on the topic with recommendations about relevant research questions for human biologists. On average, lactating mothers produce >750 ml/day milk for their infants. Breastfeeding thus promotes a powerful thirst stimulus in the lactating mother, resulting in a 12-16% increase in fluid intake among Western women. Thirst during lactation may be mediated by oxytocin release, since this hormone is structurally similar to the antidiuretic hormone, vasopressin. Prolactin also may be involved. A few studies among Western women concluded that moderate dehydration does not affect milk production, but it is not known how lactating women in hot and/or dry climates respond to chronically limited water supplies. Lactating women in such environments may use both physiological and behavioral adaptations to conserve fluid requirements, such as reducing activities during breastfeeding and carrying extra fluid supplies while travelling. Given the limited state of knowledge on lactation and hydration, future field studies should incorporate research to determine the importance of adequate fluids in maintaining milk production and the health of both mothers and infants. Am. J. Hum. Biol. 10:151-161, 1998. © 1998 Wiley-Liss, Inc.