An evolutionary perspective on the patterning of maternal investment in pregnancy

A study of the effect of number of children on depression among rural older women: empirical evidence from China

The association between early reproductive events and health status in later life has always been of interest across disciplines. The purpose of this study was to investigate whether there was an association between the number of children born in the early years of elderly women and their depression in later life based on a sample of older women aged 65 years and above with at least one child in rural China. Data from the Chinese Longitudinal Healthy Longevity Survey in 2018, this study used the ordinary least square method to conduct empirical research. This study has found a significant correlation between an increase in the number of children and depression in older rural women. When considering the sex of the child, the number of daughters had a greater and more significant impact on depression. Number of children may exacerbate depression of older women through declining self-rated health and reduced social activity, while increased inter-generational support alleviated depression. The association between number of children born and depression also existed in urban older women, though not significant. Therefore, it is suggested to accelerate the improvement of supporting policies related to childbirth, developing a healthy and scientific fertility culture, and improving rural maternal and child health services. Women should be assisted in balancing their roles in the family and in society, and in particular in sharing the burden of caring for children. Targeted efforts to increase old-age protection for older people.

Recognizing normal reproductive biology: A comparative analysis of variability in menstrual cycle biomarkers in German and Bolivian women

The idealized "normal" menstrual cycle typically comprises a coordinated ebb and flow of hormones over a 28-day span with ovulation invariably shown at the midpoint. It's a pretty picture-but rare. Systematic studies have debunked the myth that cycles occur regularly about every 28 days. However, assumptions persist regarding the extent and normalcy of variation in other cycle biomarkers. The processes of judging which phenotypic variants are "normal" is context dependent. In everyday life, normal is that which is most commonly seen. In biomedicine normal is often defined as an arbitrarily bounded portion of the phenotype's distribution about its statistical mean. Standards thus defined in one population are problematic when applied to other populations; population specific standards may also be suspect. Rather, recognizing normal female reproductive biology in diverse human populations requires specific knowledge of proximate mechanisms and functional context. Such efforts should be grounded in an empirical assessment of phenotypic variability. We tested hypotheses regarding cycle biomarker variability in women from a wealthy industrialized population (Germany) and a resource-limited rural agropastoral population (Bolivia). Ovulatory cycles in both samples displayed marked but nonetheless comparable variability in all cycle biomarkers and similar means/medians for cycle and phase lengths. Notably, cycle and phase lengths are poor predictors of mid-luteal progesterone concentrations. These patterns suggest that global and local statistical criteria for "normal" cycles would be difficult to define. A more productive approach involves elucidating the causes of natural variation in ovarian cycling and its consequences for reproductive success and women's health.

Body composition helps: Differences in energy expenditure between pregnant and nonpregnant females

OBJECTIVES

Human pregnancy is associated with important physiological changes that usually increase energetic requirements. However, great variability exists in the costs and mechanisms required to bear pregnancy. Since body mass (BM) and composition are modified during gestation, it is of great interest to compare the influence of BM on energy expenditure (EE) in pregnant and nonpregnant females.

METHODS

BM, body composition, and EE of 77 volunteers (35 pregnant and 42 nonpregnant females) were measured. The pregnant volunteers completed two measurement rounds at 28 and 32 gestation weeks. Differences on the measured parameters were sought, and comparison of regression lines was computed to test how BM affected the EE of the volunteers.

RESULTS

BM and body composition parameters are significantly higher in pregnant females, but EE is not statistically different. Pregnant females have a larger percentage of fat mass, but lower percentage of fat-free mass (FFM). The EE per kg of FFM is similar in both groups. Comparison of regression lines shows that pregnancy does not change the relationship between BM and EE, but for similar BM pregnant females expend less energy than nonpregnant females.

CONCLUSIONS

We propose that their larger percentage of passive body tissues is the reason why pregnant females expend less energy than nonpregnant females of similar BM, without changing the scaling of EE on BM. Thus, pregnancy could not be as energetically constraining as usually assumed, with important consequences for human reproductive ecology.

Field methods and strategies for assessing female reproductive functioning

A detailed understanding of female reproductive functioning is important to many disciplines including anthropology, evolutionary theory, demography, psychology, and biomedicine. In this article, I describe strategies and methods that have been used successfully in community-based studies of human reproduction, many in remote locales, to produce high quality biomarker data. These techniques are applicable to a wide range of research questions and populations, and to persons from adolescence through senescence. I give particular attention to the inherent challenges imposed by the cyclical and somewhat unpredictable nature of the hypothalamic-pituitary-ovarian axis including the necessity and difficulty of ascertaining the timing and occurrence of ovulation, the limits of different sampling regimes for capturing fluctuations in reproductive hormones, and the critical importance of recognizing and, when possible, reducing selection bias. I discuss the relative advantages and disadvantages of collecting saliva, urine, and dried blood spots, and describe some of the subtleties involved in collecting contamination-free samples. Once samples are collected, they must be stored in a manner that minimizes degradation; I describe techniques to keep samples cold even without access to electricity or dry ice. I also discuss various issues that should be considered during initial discussions with a laboratory and when samples are assayed by the laboratory. I include examples of techniques that have worked well in actual field studies, and examples of flawed analytical approaches that should be avoided. With these and other tools, even under technology-sparse conditions, researchers can investigate variability in human physiology across the breadth of human habitats.

When and Where Birth Spacing Matters for Child Survival: An International Comparison Using the DHS

A large body of research has found an association between short birth intervals and the risk of infant mortality in developing countries, but recent work on other perinatal outcomes from highly developed countries has called these claims into question, arguing that previous studies have failed to adequately control for unobserved heterogeneity. Our study addresses this issue by estimating within-family models on a sample of 4.5 million births from 77 countries at various levels of development. We show that after unobserved maternal heterogeneity is controlled for, intervals shorter than 36 months substantially increase the probability of infant death. However, the importance of birth intervals as a determinant of infant mortality varies inversely with maternal education and the strength of the relationship varies regionally. Finally, we demonstrate that the mortality-reducing effects of longer birth intervals are strong at low levels of development but decline steadily toward zero at higher levels of development. These findings offer a clear way to reconcile previous research showing that birth intervals are important for perinatal outcomes in low-income countries but are much less consequential in high-income settings.

A life history approach to prenatal supplementation: Building a bridge from biological anthropology to public health and nutrition

While prenatal supplementation with protein, lipids, carbohydrates, and micronutrients has been used to improve infant outcomes in undernourished populations since the 1960s with inconsistent results, a flourishing body of literature within biological anthropology has used life history theory to explain why supplemental resources are often allocated to maternal survival and future reproduction and not to the current offspring. To date, however, public health and nutrition researchers have not adopted evolutionary perspectives in designing or analyzing prenatal supplementation studies. The result is a long series of supplementation trials with unpredictable and often disappointing outcomes for women and children, as well as serious lacunae in the understanding of long-term consequences of supplementation for women. The goal of this article is to open a tactical conversation about how to build a bridge between the evolutionary logic of biological anthropology and the evidentiary standards and methods of public health and nutrition with the aim of advancing knowledge about reproductive and metabolic physiology and improving women's health over the life course. The article reviews recent prenatal supplementation studies and proposes programmatic strategies by which biological anthropologists and public health and nutrition workers may collaborate to define different conditions of prenatal supplement resource allocation and to target more effective interventions.

Low birth weight, maternal birth-spacing decisions, and future reproduction : A cost-benefit analysis

The aim of this study is an analysis of the possible adaptive consequences of delivery of low birth weight infants. We attempt to reveal the cost and benefit components of bearing small children, estimate the chance of the infants' survival, and calculate the mothers' reproductive success. According to life-history theory, under certain circumstances mothers can enhance their lifetime fitness by lowering the rate of investment in an infant and/or enhancing the rate of subsequent births. We assume that living in a risky environment and giving birth to a small infant may involve a shift from qualitative to quantitative production of offspring. Given high infant mortality rates, parents will have a reproductive interest in producing a relatively large number of children with a smaller amount of prenatal investment. This hypothesis was tested among 650 Gypsy and 717 non-Gypsy Hungarian mothers. Our study has revealed that 23.8% of the Gypsy mothers had low birth weight (<2,500 g) children, whose mortality rate is very high. These mothers also had more spontaneous abortions and stillbirths than those with normal weight children. As a possible response to these reproductive failures, they shortened birth spacing, gaining 2-4 years across their reproductive lifespan for having additional children. Because of the relatively short interbirth intervals, by the end of their fertility period, Gypsy mothers with one or two low birth weight infants have significantly more children than their ethnic Hungarian counterparts. They appear to compensate for handicaps associated with low birth weights by having a larger number of closely spaced children following the birth of one or more infants with a reduced probability of survival. The possible alternative explanations are discussed, and the long-term reproductive benefits are estimated for both ethnic groups.

Unconscious dynamics in twin pregnancy emerging from the Lüscher color test

OBJECTIVE

To investigate unconscious dynamics in women pregnant with twins, using Lüscher's color test to objectively measure subjective color preferences, and compare them with women with singleton pregnancies.

METHODS

The color test was administered to 50 Italian women with twin pregnancies and 100 women with singleton pregnancies.

RESULTS

Both the twin and singleton pregnancy groups said that violet was their favorite color (50 versus 49%) and brown was their least favorite color (52 versus 44%), indicating that they idealized being pregnant, but also found it physically stressful. The twin pregnancy group chose yellow as their second favorite (28 versus 17%) and were most likely to combine it with their first choice of violet (44 versus 19%, p=0.0006), indicating that they were worried about their relationships with their new babies and wanted to give birth soon. In addition, both groups preferred form 6, the sine curve on a dark background, but rejected the associated brown color 6, revealing that the women felt the need to look after themselves.

CONCLUSIONS

Both groups idealized being pregnant, but also found it physically stressful. However, the twin pregnancy group was afraid of building relationships with their babies and wanted to give birth soon.

The evolutionary ecology of attachment organization

Life history theory's principle of allocation suggests that because immature organisms cannot expend reproductive effort, the major trade-off facing juveniles will be the one between survival, on one hand, and growth and development, on the other. As a consequence, infants and children might be expected to possess psychobiological mechanisms for optimizing this trade-off. The main argument of this paper is that the attachment process serves this function and that individual differences in attachment organization (secure, insecure, and possibly others) may represent facultative adaptations to conditions of risk and uncertainty that were probably recurrent in the environment of human evolutionary adaptedness.

Appetite sensations in pregnancy among agropastoral women in rural Tanzania

Women all over the globe report physical and appetite sensations in early pregnancy, and this study contributes to this growing literature by reporting on the appetite sensations experienced by pregnant women from rural Tanzania. Appetite changes associated with 545 pregnancies were compiled from surveys conducted to report on the prevalence of appetite loss, nausea, vomiting, dizziness, joint pain, cravings, aversions, and pica experienced by agropastoral women from rural north-central Tanzania. In addition to these symptoms, specific craved and aversive food groups are described. Statistical associations among appetite sensations, NVP, and birthweight are tested. The only symptom associated with a lower average birth weight for newborns was vomiting. In addition to investigating micronutrient content and chemical properties of specific food and non-food items, future research should include assessing relationships among various appetite sensations and short- and long-term health outcomes for both the mother and child.

The roles of the immune system in women's reproduction: evolutionary constraints and life history trade-offs

Life history theory posits that, as long as survival is assured, finite resources are available for reproduction, maintenance, and growth/storage. To maximize lifetime reproductive success, resources are subject to trade-offs both within individuals and between current and future investment. For women, reproducing is costly and time-consuming; the bulk of available resources must be allocated to reproduction at the expense of more flexible systems like immune function. When reproducing women contract infectious diseases, the resources required for immune activation can fundamentally shift the patterns of resource allocation. Adding to the complexity of the reproductive-immune trade-offs in women are the pleiotropic effects of many immune factors, which were modified to serve key roles in mammalian reproduction. In this review, we explore the complex intersections between immune function and female reproduction to situate proximate immunological processes within a life history framework. After a brief overview of the immune system, we discuss some important physiological roles of immune factors in women's reproduction and the conflicts that may arise when these factors must play dual roles. We then discuss the influence of reproductive-immune trade-offs on the patterning of lifetime reproductive success: (1) the effect of immune activation/infectious disease on the timing of life history events; (2) the role of the immune system, immune activation, and infectious disease on resource allocation within individual reproductive events, particularly pregnancy; and (3) the role of the immune system in shaping the offspring's patterns of future life history trade-offs. We close with a discussion of future directions in reproductive immunology for anthropologists.

Malaria during pregnancy in endemic areas: a lens for examining maternal-fetal conflict

Most of our knowledge about maternal-fetal conflict derives from the battle over scarce nutritional resources. How do other stressors like infectious diseases alter the maternal-fetal relationship? In this article, we use the example of malaria infection during pregnancy to explore the altered maternal-fetal relationship in the presence of an infectious disease. While adults living in regions endemic to Plasmodium falciparum malaria are generally immune, pregnant women experience significantly more frequent and severe infections. These infections generally resolve within a few days of birth and rarely cross the placenta, but the infants often experience poor birth outcomes, particularly low birth weight. This article summarizes what is known about the proximate, or physiological, mechanisms by which malaria causes more severe or frequent infections for pregnant versus nonpregnant women in endemic regions and then utilizes an evolutionary approach to focus on the altered maternal-fetal relationship during malaria-infected pregnancy.

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.

Maternal stress and fetal responses: evolutionary perspectives on preterm delivery

New epidemiological and neurohormonal evidence provides insights into the persistent public health issue of preterm delivery and its long-term health consequences for the newborn. Mechanisms linked to preterm delivery may originate early in gestation as a result of maternal cues signaling a stressful intrauterine environment. When these signals are present, the fetus responds with a series of facultative responses, including accelerated organ maturation. If these responses are unsuccessful and the environment remains insufficient, a series of feed-forward mechanisms initiate the hormonal cascade that leads to parturition, and thus, early expulsion from a stressful environment. The internal environmental cues are delivered via glucocorticoids (stress hormones) in the circulatory system, but fetal responses and the initiation of the final terminal pathway to parturition are regulated by placentally derived corticotropin-releasing hormone (CRH). The potential costs of early expulsion from a stressful intrauterine environment are high and include an increased likelihood of perinatal and infant mortality. Permanent alterations in organ and metabolic functioning may occur, suggesting considerable fitness trade-offs. There is some evidence that preterm parturition is a maternal adaptation to limit the energetic costs of individual pregnancies in the face of poor condition at the time of conception. Moreover, nutritional stress is not the only indicator that signals a stressful environment: maternal psychosocial stress, and thus her response to an assessment of the social environment, also signal an insufficient internal environment to the fetus. The epidemiological and neurohormonal evidence for these relationships and mechanisms responsible for regulating such delicate negotiations are explored. In turn, the implications of such findings are examined from life history and public health perspectives.

Risk factors and mechanisms of preterm delivery in Malawi

PROBLEM

We examined risk factors and mechanisms of preterm delivery (PTD) in malaria-exposed pregnant women in Blantyre, Malawi.

METHOD OF STUDY

The human immunodeficiency virus (HIV), malaria, syphilis, and anemia were assessed in a cross-sectional study of 572 pregnant women. In a nested case-control study, chorioamnionitis (CAM) was examined; tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-8, macrophage inflammatory protein (MIP)-1alpha, monocyte chemotactic protein (MCP)-1, transforming growth factor (TGF)-beta, cortisol, and corticotropin-releasing hormone were measured in placental, maternal and/or cord blood.

RESULTS

HIV, infrequent antenatal clinic attendance, low-maternal weight, no intermittent preventive malaria therapy (IPT), and CAM were associated with PTD, while malaria was not. Of the 18 compartmental cytokine measurements, elevations in placental and/or cord IL-6 and IL-8 were associated with both CAM and PTD. In contrast, there was no overlap between the cytokines affected by malaria and those associated with PTD.

CONCLUSIONS

The HIV and CAM were the major infections associated with PTD in this study. CAM, but not malaria, causes PTD via its effect on proinflammatory cytokines.

Energetics and reproductive effort

Natural selection favors the optimal allocation of energy and other limiting resources to reproduction. Human reproductive physiology displays characteristic patterns that can be viewed as mechanisms that help optimize reproductive effort in the face of environmental energetic constraints. Female ovarian function is particularly sensitive to energy balance and energy flux, resulting in a synchronization of conception with favorable energetic conditions. Reproductive effort during gestation is highly buffered from environmental energetic constraints, but the duration of gestation and final birthweight are both very sensitive to maternal energy availability. Milk production during lactation is relatively buffered from maternal energetic constraints as well, but the duration of lactational amenorrhea is sensitive to the relative metabolic load of lactation. Male gamete production is very insensitive to energetic constraints, but variation in testosterone production in response to both age and longer-lasting energetic conditions contributes to the modulation of somatic and behavioral aspects of male reproductive effort, aspects that are more energetically costly for a male. There is also new evidence that testosterone may also help to modulate the trade-off between male parenting and mating effort.

Maternal trade-off in treating high-risk children

Low birthweight and the infant's health status are expected to strongly influence the child's reproductive value and, thus, the maternal decisions on the amount and timing of investment. A total of 590 Hungarian primiparous mothers giving birth in the late 1980s were recruited for the longitudinal study. Mothers of high-risk infants shortened the duration of breast-feeding and interbirth intervals, compared to those with an infant of higher survival prospects. The most powerful predictor of the length of the lactation period was the infant's weight at birth, whereas birth spacing was significantly influenced by the health status of the older child. Socioeconomic status had a positive effect on maternal care as well, but it did not change the basic pattern of diminishing maternal care as a function of the infants' low reproductive value. The combination of the above factors resulted in a cumulative effect on maternal investment of mothers with handicapped children of various degrees of risk. An attempt has been made to exclude alternative explanations and to discuss the proximate mechanisms of discriminative parental solicitude.

Age, reproductive history, seasonality, and maternal body composition during pregnancy for nomadic Turkana of Kenya

To evaluate the potential differences in maternal nutritional investment in pregnancy, data collected from nomadic Ngisonyoka Turkana women during a July 1993-July 1994 field season were utilized. The roles maternal age, parity, duration of the previous nonpregnant interval, overlap between pregnancy and lactation on trimester changes in weight and summed skinfolds during pregnancy were examined. Because seasonality is an important aspect of the Turkana environment, the effects of seasonality were also assessed. First trimester weight gain is positively associated with overlap in pregnancy and lactation. Second trimester maternal weight gain is negatively influenced by higher parity and by overlap between lactation and early pregnancy. Third trimester weight gain is influenced only by seasonally induced morbidity. First trimester changes in maternal skinfolds are negatively influenced by older maternal age and parity, and positively influenced by a longer nonpregnant interval, and overlap between pregnancy and lactation. Second and third trimester skinfolds are significantly associated only with overlap between lactation and pregnancy (negatively in the second, positively in the third). Seasonality does not influence maternal skinfolds. Differences in age- and parity-related patterns of maternal nutritional investment in pregnancy are not supported by the data. The possibility that Turkana cultural beliefs may influence nutritional status during pregnancy is discussed. Am. J. Hum. Biol. 11:658-672, 1999. Copyright 1999 Wiley-Liss, Inc.

EVOLUTIONARY ECOLOGY OF HUMAN REPRODUCTION

▪ Abstract  Evolutionary ecology of human reproduction is defined as the application of natural selection theory to the study of human reproductive strategies and decision-making in an ecological context. The basic Darwinian assumption is that humans—like all other organisms—are designed to maximize their inclusive fitness within the ecological constraints to which they are exposed. Life history theory, which identifies trade-off problems in reproductive investment, and evolutionary physiology and psychology, which analyzes the adaptive mechanisms regulating reproduction, are two crucial tools of evolutionary reproductive ecology. Advanced empirical insights have been obtained mainly with respect to the ecology of fecundity, fertility, child-care strategies, and differential parental investment. Much less is known about the ecology of nepotism and the postgenerative life span. The following three theoretical aspects, which are not well understood, belong to the desiderata of future improvement in evolutionary human reproductive ecology: (a) the significance of and the interactions between different levels of adaptability (genetic, ontogenetic, and contextual) for the adaptive solution of reproductive problems; (b) the dialectics of constraints and adaptive choices in reproductive decisions; and (c) the dynamics of demographic change.

Developmental influences on adult ovarian hormonal function

The hypothesis is advanced that levels of ovarian hormonal function in adult women are associated with the tempo of growth and maturation in childhood and adolescence. Empirical data are presented documenting a correlation between menarcheal age and adult ovarian hormonal function both within and between populations. It is noted that this relationship is compatible with current understanding of the mechanisms underlying pubertal maturation of ovarian hormonal function. Functionally, such a relationship could serve the purpose of modulating adult fecundity to chronic environmental conditions. Alternative hypotheses include the possibility that the relationship is not causal but rather reflects either confounded effects of some common cause or the persistence of acute environmental effects through time. Proper testing of such alternative hypotheses will require longitudinal data on migrant populations, changing environments, or secular trends within populations. The importance of establishing the relationship between development and adult ovarian hormonal function is not limited to issues of fecundity and fertility, but includes other areas of female general and reproductive health. © 1996 Wiley-Liss, Inc.

Ecological demography: a synthetic focus in evolutionary anthropology

"Ecological demography arises from two facts: that the reproductive behavior of humans, like that of other species, is influenced by natural selection and that current fitness is the product of both genotype and environment. Here I review classic atheoretical and recent theoretical studies that contribute to this emerging field, exploring the extent to which human reproductive responses to ecological conditions, considered in their broadest context, follow the same selective rules as other species."