Evolutionary and empirical perspectives on 'demand' breastfeeding: The baby in the driver's seat or the back seat?

Background/Objectives: The concept of 'demand' breastfeeding is central in public health. A key feature of the concept is that the infant is the locus of control in the breastfeeding process; when the breast is demanded by the infant, it is given the opportunity to feed. This study questions this notion of the infant as the locus of control in demand breastfeeding for empirical and theoretical reasons. From an evolutionary perspective, infants are expected to seek maximal investment and, against this backdrop of maximal investment-seeking, parents decide how much investment to put into offspring. Methodology: Focal follows were conducted among 113 mother-infant dyads in Papua New Guinea. During these follows, response times and types of responses, including breastfeeding to offspring fussing and crying, were recorded. Results: Infants were breastfed an average of 3.6 times/hour for just over 2 min/feed. Fussing and crying were responded to quickly, with most response times under 1 min. When the mother responded, she breastfed the child approximately 52% of the time. The other 48% of the time, mothers responded to infants with other forms of pacification. Mothers were significantly less likely to respond to infants by breastfeeding if the child had been breastfed within the past 59-76 min. Conclusion/Implications: As predicted by evolutionary parental investment theory, infants make frequent demands on their parents for investment, but mothers are ultimately the locus of control in the investment process. The mother decides whether and how frequently to breastfeed her offspring against this backdrop of near-continuous investment demands.

Determinants of infant carrying behavior in rural Papua New Guinea.

OBJECTIVES: The determinants of variability in infant carrying within and across societies is an understudied area within parental investment research. Carrying has positive and negative consequences as it may protect the infant from predators and pathogens but is energetically costly for caregivers. Moreover, carrying may delay independent locomotor development and exposure to antigens necessary for immunological competence. The purpose of this study is to compare infants' ages and developmental milestone attainment as predictors of carrying behavior and to identify other determinants of carrying behavior among traditional forager-horticulturalists in Papua New Guinea. METHODS: We analyze quantitative data collected among 107 infant-caregiver dyads during 354 hours of focal follows on infant carrying. Random effects logistic regression was used to model carrying behavior in these dyads. RESULTS: Infants' chronological age and milestone achievement are equally reliable predictors of abatement of carrying and increased time on the ground. Further, the presence or absence of the mother and the location of the infant-caregiver pair are significantly associated with carrying behavior. Indices of mother's and infants' condition such as anthropometric measures were not predictive of carrying behaviors. CONCLUSIONS: Mothers vary the amount of carrying based on assessment of infant and local environmental conditions. Age and milestone attainment are equally predictive of time on the ground and therefore increased exposure to antigens, pathogens and other dangers. High levels of infant carrying function as offspring protection by increasing exposure to ground level pathogens gradually, thus allowing for the naïve immune system to develop immunocompetence incrementally.

Prenatal stress exposure and early childhood BMI: Exploring associations in a New Zealand context.

OBJECTIVES: The primary purpose of this study was to (i) examine associations between prenatal objective vulnerability and subjective stress, and (ii) investigate the relationships between prenatal vulnerability and subjective stress and early childhood BMI at 24- and 54- months of age after controlling for covariates. METHODS: The Growing Up in New Zealand (GUiNZ) longitudinal study provided information on 5839 pregnant women and their children to assess the study objectives. Vulnerability, operationalized by nine objective-risk factors, and subjective stress, operationalized by the Perceived Stress Scale, were independently investigated. Hierarchical linear regression models were conducted to analyze the associations between both prenatal measures and childhood BMI at 24- and 54- months of age. RESULTS: Correlations between subjective stress and objective vulnerability were low but significant (r = .28, P < .01). Exposure to one additional risk factor during pregnancy was significantly associated with a .11 increase in BMI z-score at 24-months (P < .01) and a .15 increase in BMI z-score at 54-months (P < .01), after controlling for covariates including maternal prepregnancy BMI. Subjective prenatal stress was not significantly associated with either child BMI outcome. CONCLUSIONS: Vulnerability and subjective stress were minimally correlated in this sample. Vulnerability, but not subjective stress, was associated with childhood BMI at 24- and 54- months of age. This study informs our understanding of how risk exposures and stress responses early in life impacts offspring obesity risk, and it may help identify strategies that decrease early life predisposition to adult disease.

Social image concerns promote cooperation more than altruistic punishment.

Human cooperation is enigmatic, as organisms are expected, by evolutionary and economic theory, to act principally in their own interests. However, cooperation requires individuals to sacrifice resources for each other's benefit. We conducted a series of novel experiments in a foraging society where social institutions make the study of social image and punishment particularly salient. Participants played simple cooperation games where they could punish non-cooperators, promote a positive social image or do so in combination with one another. We show that although all these mechanisms raise cooperation above baseline levels, only when social image alone is at stake do average economic gains rise significantly above baseline. Punishment, either alone or combined with social image building, yields lower gains. Individuals' desire to establish a positive social image thus emerges as a more decisive factor than punishment in promoting human cooperation.

Breastfeeding structure as a test of parental investment theory in Papua New Guinea.

Evolutionary parental investment theory predicts that parents invest preferentially in offspring best able to translate investments into fitness payoffs. It has also been proposed that where the reproductive prospects of offspring are directly correlated with parental investment and variance in fertility is higher for males than females, parents in better condition should bias investment toward males while those in poorer condition should bias investment toward females. Lactation is arguably among the costliest forms of investment expended by mothers and is thus expected to be allocated in ways consistent with fitness payoffs. Quantitative data collected among 110 Papua New Guinean mother-infant pairs during 470 h of focal follows on nursing frequency and duration and responses to infant demands by maternal and offspring characteristics are presented to provide empirically-based descriptions of infant care and tests of evolutionary parental investment theory. Results indicate that mothers show very high levels of investment in offspring. However, although breastfeeding in developing countries is often characterized as on-demand, fussing and crying by infants were only attended to with breastfeeding about 30% of the time. Contrary to expectations of parental investment theory that parents should invest less in poorer quality offspring, mothers increased investment in offspring in poorer condition. The expectation that mothers in better condition would bias investment toward male offspring was also not supported; better nourished mothers biased investment toward female offspring. This study illustrates how infant feeding data may be used for testing larger evolutionary questions such as those derived from parental investment theory.

Diagnosis of rickets and reassessment of prevalence among rural children in northern China.

BACKGROUND: Rates of rickets from 15.9 to 26.7% have been reported in China. METHODS: Combining the methods of epidemiology and the behavioral sciences, this study investigated the prevalence of rickets in children in rural Shanxi Province, China. A total of 250 children age 12-24 months were examined physically for the presence of rickets, blood was drawn for laboratory analysis, and X-rays were taken of each child's wrists. RESULTS: Vitamin D deficiency in the spring was found among 65.3% of children. Rickets diagnosis relying on clinical signs alone determined a rickets prevalence of 41.6%, declining to 17.0% in the fall after a summer of sun exposure (chi(2) = 8.356, P = 0.004). But an integrated diagnostic method exploiting clinical signs, X-ray and alkaline phosphatase levels found the prevalence of active rickets to be 3.7%. Furthermore, it was demonstrated that only five clinical signs reflect active rickets--wide wrists, frontal bossing, rachitic rosary, Harrison's sulcus, and bowed legs. CONCLUSIONS: The prevalence of active rickets in young children in northern China is lower than previously reported. Even in poor countries, simple tests such as X-rays and alkaline phosphatase can be added to physical examination to more accurately diagnose active rickets.

Somatic versus reproductive energy allocation in Papua New Guinea: life history theory and public health policy.

A fundamental evolutionary problem faced by organisms is how to allocate energy to somatic and reproductive functions in ways that optimize fitness. Given that energy is limited in all environments, energy allocation necessarily involves physiological tradeoffs between such factors as growth and reproduction, reproduction and condition, and current reproduction and future survival. Ultimately, the "decisions" that are made about energy allocation among growth, survival, and reproduction determine life history patterns and trajectories of organisms. For humans, knowing how energy allocation to reproduction will likely impact other aspects of the somatic well-being of individuals may also have practical implications for public health policy. This article reviews the evidence for energy tradeoffs between somatic and reproductive functioning in a range of human societies. It also seeks to corroborate the results of earlier work in Papua New Guinea on lactation-related maternal energy depletion using an independent measure of maternal energy reserves, tetrapolar bioelectrical impedance analysis. The current analysis shows that maternal energy reserves decline over the course of lactation and that a cumulative parity-specific decline in maternal energy reserves also exists. A longitudinal follow-up of five women over 11 years shows the decline to amount to about 3 mm of subcutaneous fat per round of pregnancy and lactation. The results corroborate predictions from life history theory and have applied public health implications. In particular, It is suggested that policies such as lactation advocacy that encourage enhanced energy allocation to reproduction in order to promote child health may have the unintended result of compromising maternal well-being, particularly in nations of the developing world. Consequently, it is recommended that nutritional support of mothers be implemented in concert with lactation promotion.

Two decade trends in birth weight and early childhood growth in Papua New Guinea.

At present, little is known about patterns of early growth and development in foraging and small-scale horticultural populations. Moreover, still less is known about secular changes in growth in these populations. Data collected in 1967, 1976, and 1989 are presented for birth weight and subsequent growth to 5 years among Au forager-horticulturalists of Papua New Guinea. Despite the launching of health campaigns over the last two decades aimed at bettering the nutritional status of the Au, the data show that average birth weight has remained stable and low at just over 2600 g. Weight- and length-for-age also show no significant changes among traditional Au over the last 20 years, and remain on average, at just below the U.S. 5th percentile. Evidence for the start of a positive secular trend in birth weight and subsequent growth is seen, however, among a small group of Au children residing in households that have a source of wage income. These individuals show an increase of 150 g in birth weight, and over the subsequent 5 years of growth average 93 g heavier and 2.6 cm taller than Au children in traditional households. In addition to providing baseline data on child growth in a forager-horticulturalist society, the findings provide evidence for a secular trend newly underway, and suggest that health promotion campaigns alone without socioeconomic development may be insufficient in effecting change in growth status in rural communities. Am. J. Hum. Biol. 10:483-493, 1998. © 1998 Wiley-Liss, Inc.