Human Lactation, Pair-bonds, and Alloparents: A Cross-Cultural Analysis

Humanity's evolved nest and its relation to cardiac vagal regulation in the first years of life

BACKGROUND

The Evolved Developmental Niche (EDN) is a millions-year-old developmental system that matches the maturational schedule of the offspring, optimizing health. Every animal has a developmental niche.

AIMS

Humanity has fallen away from providing its EDN. Does it matter?

STUDY DESIGN

Several components of humanity's EDN were reviewed (breastfeeding, positive touch, allomothers, responsive care, free play) in relation to cardiac vagal nerve regulation, a signal of healthy development. Focal subjects were young children.

OUTCOME MEASURES

A review of research on the selected EDN components in relation to vagal nerve function was performed. Data were available for all but the allomother component, which is typically not measured by western researchers, although allomothers provide EDN components alongside parents.

RESULTS

Apart from the lack of research on allomother effects, all these EDN components have been shown to influence cardiac vagal regulation in young children.

CONCLUSIONS

Converging evidence suggests that providing the EDN in early life may not only support aspects of a child's primal health system, but bolster capacities for social health and wellness, the cornerstone of a positive life trajectory.

Effects of household composition on infant feeding and mother-infant health in northern Kenya

BACKGROUND

Households with alloparents, individuals other than the mother who care for an infant, can shift members' roles and affect mother-infant health.

AIMS

To investigate how household composition relates to infant feeding and infectious disease risk in mother-infant dyads, the team utilized data from breastfeeding dyads (n 208) surveyed during a prolonged drought and food scarcity in northern Kenya.

METHODS

Households were classified by the presence/absence of potential alloparents, distinguishing non-siblings and siblings of the infant. Regression models for breastfeeding frequency, complementary feeding status, and recent infections (n 83) evaluated these outcomes' associations with household type while accounting for food insecurity, adjusted for infant age, infant sex, and maternal age.

RESULTS

Household type was unassociated with breastfeeding frequency, but the presence of non-sibling alloparents interacted with food insecurity, predicting increasing breastfeeding frequency as food insecurity intensified among dyads living with non-sibling alloparents. Households with non-sibling alloparents were also inversely associated with complementary feeding but had no association with infection. Households with siblings were inversely associated with (protective against) infant and maternal infection.

CONCLUSION

Further research is needed to understand the interactive influence of household social and food ecologies on mother-infant diet and health under diverse cultural rules and norms for alloparenting.

The evolved nest, oxytocin functioning, and prosocial development

Prosociality, orientation to attuned, empathic relationships, is built from the ground up, through supportive care in early life that fosters healthy neurobiological structures that shape behavior. Numerous social and environmental factors within early life have been identified as critical variables influencing child physiological and psychological outcomes indicating a growing need to synthesize which factors are the most influential. To address this gap, we examined the influence of early life experiences according to the evolved developmental niche or evolved nest and its influence on child neurobiological and sociomoral outcomes, specifically, the oxytocinergic system and prosociality, respectively. To-date, this is the first review to utilize the evolved nest framework as an investigatory lens to probe connections between early life experience and child neurobiological and sociomoral outcomes. The evolved nest is comprised of characteristics over 30 million years old and is organized to meet a child's basic needs as they mature. Converging evidence indicates that humanity's evolved nest meets the needs of a rapidly developing brain, optimizing normal development. The evolved nest for young children includes soothing perinatal experiences, breastfeeding, positive touch, responsive care, multiple allomothers, self-directed play, social embeddedness, and nature immersion. We examined what is known about the effects of each evolved nest component on oxytocinergic functioning, a critical neurobiological building block for pro-sociomorality. We also examined the effects of the evolved nest on prosociality generally. We reviewed empirical studies from human and animal research, meta-analyses and theoretical articles. The review suggests that evolved nest components influence oxytocinergic functioning in parents and children and help form the foundations for prosociality. Future research and policy should consider the importance of the first years of life in programming the neuroendocrine system that undergirds wellbeing and prosociality. Complex, interaction effects among evolved nest components as well as among physiological and sociomoral processes need to be studied. The most sensible framework for examining what builds and enhances prosociality may be the millions-year-old evolved nest.

Female cooperation: evolutionary, cross-cultural and ethnographic evidence

Women and girls cooperate with each other across many domains and at many scales. However, much of this information is buried in the ethnographic record and has been overlooked in theoretic constructions of the evolution of human sociality and cooperation. The assumed primacy of male bonding, hunting, patrilocality and philopatry has dominated the discussion of cooperation without balanced consideration. A closer look at the ethnographic record reveals that in addition to cooperative childcare and food production, women and girls collectively form coalitions, have their own cooperative political, ceremonial, economic and social institutions, and develop female-based exchange and support networks. The numerous ethnographic examples of female cooperation urge reconsideration of gender stereotypes and the limits of female cooperation. This review brings together theoretic, cross-cultural and cross-lifespan research on female cooperation to present a more even and empirically supported view of female sociality. Following the lead from trends in evolutionary biology and sexual selection theory, the hope going forward is that the focus shifts from rote characterizations of sex differences to highlighting sources of variation and conditions that enhance or constrain female cooperative engagement. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

What is a pair bond?

Pair bonding is a psychological construct that we attempt to operationalize via behavioral and physiological measurements. Yet, pair bonding has been both defined differently in various taxonomic groups as well as used loosely to describe not just a psychological and affective phenomenon, but also a social structure or mating system (either social monogamy or just pair living). In this review, we ask the questions: What has been the historical definition of a pair bond? Has this definition differed across taxonomic groups? What behavioral evidence do we see of pair bonding in these groups? Does this observed evidence alter the definition of pair bonding? Does the observed neurobiology underlying these behaviors affect this definition as well? And finally, what are the upcoming directions in which the study of pair bonding needs to head?

The expression of care: Alloparental care frequency predicts neural control of facial muscles in primates

The adaptive value of facial expressions has been debated in evolutionary biology ever since Darwin's seminal work. Among mammals, primates, including humans, exhibit the most intricate facial displays. Although previous work has focused on the role of sociality in the evolution of primate facial expressions, this relationship has not been verified in a wide sample of species. Here, we examine the relationship between allomaternal care (paternal or alloparental) and the morphology of three orofacial brainstem nuclei (facial; trigeminal motor; hypoglossal) across primates to test the hypothesis that allomaternal care explains variation in the complexity of facial expressions, proxied by relative facial nucleus size and neuropil fraction. The latter represents the proportion of synaptically dense tissue and may, therefore, correlate with dexterity. We find that alloparental care frequency predicts relative neuropil fraction of the facial nucleus, even after controlling for social system organization, whereas allomaternal care is not associated with the trigeminal motor or hypoglossal nuclei. Overall, this work suggests that alloparenting requires increased facial dexterity to facilitate nonverbal communication between infants and their nonparent caregivers and/or between caregivers. Accordingly, alloparenting and complex facial expressions are likely to have coevolved in primates.

The Human Family—Its Evolutionary Context and Diversity

The family defines many aspects of our daily lives, and expresses a wide array of forms across individuals, cultures, ecologies and time. While the nuclear family is the norm today in developed economies, it is the exception in most other historic and cultural contexts. Yet, many aspects of how humans form the economic and reproductive groups that we recognize as families are distinct to our species. This review pursues three goals: to overview the evolutionary context in which the human family developed, to expand the conventional view of the nuclear family as the ‘traditional family’, and to provide an alternative to patrifocal explanations for family formation. To do so, first those traits that distinguish the human family are reviewed with an emphasis on the key contributions that behavioral ecology has made toward understanding dynamics within and between families, including life history, kin selection, reciprocity and conflict theoretical frameworks. An overview is then given of several seminal debates about how the family took shape, with an eye toward a more nuanced view of male parental care as the basis for family formation, and what cooperative breeding has to offer as an alternative perspective.

Crucial Contributions : A Biocultural Study of Grandmothering During the Perinatal Period

Maternal grandmothers play a key role in allomaternal care, directly caring for and provisioning their grandchildren as well as helping their daughters with household chores and productive labor. Previous studies have investigated these contributions across a broad time period, from infancy through toddlerhood. Here, we extend and refine the grandmothering literature to investigate the perinatal period as a critical window for grandmaternal contributions. We propose that mother-daughter co-residence during this period affords targeted grandmaternal effort during a period of heightened vulnerability and appreciable impact. We conducted two focus groups and 37 semi-structured interviews with Himba women. Interviews focused on experiences from their first and, if applicable, their most recent birth and included information on social support, domains of teaching and learning, and infant feeding practices. Our qualitative findings reveal three domains in which grandmothers contribute: learning to mother, breastfeeding support, and postnatal health and well-being. We show that informational, emotional, and instrumental support provided to new mothers and their neonates during the perinatal period can aid in the establishment of the mother-infant bond, buffer maternal energy balance, and improve nutritional outcomes for infants. These findings demonstrate that the role of grandmother can be crucial, even when alloparenting is common and breastfeeding is frequent and highly visible. Situated within the broader anthropological and clinical literature, these findings substantiate the claim that humans have evolved in an adaptive sociocultural perinatal complex in which grandmothers provide significant contributions to the health and well-being of their reproductive-age daughters and grandchildren.

Typologies of postnatal support and breastfeeding at two months in the UK: Research participant commentary

Emmott et al. (2020) presented in this journal an analysis of different types of postnatal support that mothers in the UK experience and their relationship to breastfeeding status at two months. This paper is a commentary by two participants in the study. It draws on the research literature alongside recent lived experiences of breastfeeding and formula feeding in the UK to question and expand on Emmott et al.'s analysis. It highlights how a decision to stop breastfeeding in the current UK context may lead to experiencing a lack of support from healthcare professionals. We argue that a family-centred approach to infant feeding should acknowledge the benefits and costs of all infant feeding methods and enable all families to be fully informed and supported in feeding their babies.

Hired helpers at the nest: The association between life-cycle servants and net fertility in North Orkney, 1851-1911

OBJECTIVES

The presence of kin is often, but not always, associated with higher fertility in historical populations. However, the effect of other household members on fertility is less frequently studied. While not genetically related, life-cycle servants lived and worked alongside household members and may have provided assistance to reproducing families. Female servants in particular may have helped mothers with small children through direct help with childcare activities or by replacing the economic effort of mothers whose work was not compatible with childcare. This study examines the presence of servants in the households of married women of reproductive age to assess whether households with young children are more likely to also have servants.

MATERIALS AND METHODS

This study uses individual-level census data from North Orkney, Scotland (1851-1911) to investigate the relationship between the presence of servants in households and a measure of recent net marital fertility, the number of women's own-children under age 5, using logistic regression models.

RESULTS

Households with young children were more likely to have a female, but not male, servant in the household after controlling for the effects of other possible helpers, including older children, mothers, and mothers-in-law.

DISCUSSION

These findings are consistent with prior research that indicates the importance of female labor to smallholder agricultural households and suggests that female servants may have provided support to reproducing families. Life-cycle servants should be considered one component of biocultural reproduction in historical Northwest Europe. The use of hired help is not restricted to contemporary or elite groups.

Typologies of postnatal support and breastfeeding at two months in the UK

There is extensive evidence to suggest that social support improves breastfeeding outcomes. Building on this evidence-base, public health services and interventions aiming to improve breastfeeding rates have primarily targeted informational and emotional support to mothers, reflecting an individual behaviour-change approach. However, mothers exist within a wider social network, and the characteristics of their broader support networks may be an important predictor of breastfeeding outcomes. Here we explore the typologies of postnatal support for mothers in the UK; a population with one of the lowest breastfeeding rates in Europe. Using retrospective data from an online survey (data collection period December 2017 - February 2018), we carry out a latent class regression (n = 432) to identify "clusters" of postnatal support in our data. Mothers in our sample were most likely to report receiving practical and emotional support from partners and maternal grandmothers, and breastfeeding information from health professionals. We identify three distinct typologies of postnatal support: 1) Extensive support, where mothers received support from a wide range of supporters including partners, maternal grandmothers, friends and health professionals, but mothers were the only ones to feed the infant; 2) Family support, where mothers received support from partners and maternal grandmothers, including with infant feeding, but less likely to receive support from health professionals; and 3) Low support, where mothers primarily received support from partners. 94% of women with extensive support were predicted to be breastfeeding at two months, followed by 48% of mothers in the low support group, and 13% in the family support group. Our findings highlight the complexities of family support and its potential impact on breastfeeding, as well as the significance of professional support. Overall, our results hint at the potential value for health professionals to engage with wider family in order to achieve extensive support for mothers.

Women's experiences of ceasing to breastfeed: Australian qualitative study

OBJECTIVE

To investigate mothers' infant feeding experiences (breastfeeding/formula milk feeding) with the aim of understanding how women experience cessation of exclusive breastfeeding.

DESIGN

Multimethod, qualitative study; questionnaire, focus groups and interviews.

SETTING

Northern and Southern Tasmania, Australia.

PARTICIPANTS

127 mothers of childbearing age from a broad sociodemographic context completed a questionnaire and participated in 22 focus groups or 19 interviews across Tasmania, 2011-2013.

RESULTS

Mothers view breastfeeding as 'natural' and 'best' and formula milk as 'wrong' and 'unnatural'. In an effort to avoid formula and prolong exclusive breastfeeding, mothers will endure multiple issues (eg, pain, low milk supply, mastitis, public shaming) and make use of various forms of social and physical capital; resources such as father/partner support, expressing breast milk, bottles and dummies. The cessation of exclusive breastfeeding was frequently experienced as unexpected and 'devastating', leaving mothers with 'breastfeeding grief' (a prolonged sense of loss and failure).

CONCLUSIONS AND IMPLICATIONS

For many mothers, the cessation of exclusive breastfeeding results in lingering feelings of grief and failure making it harmful to women's emotional well-being. Reframing breastfeeding as a family practice where fathers/partners are incorporated as breastfeeding partners has the potential to help women negotiate and prolong breastfeeding. Proactive counselling and debriefing are needed to assist women who are managing feelings of 'breastfeeding grief'.

Oxytocin- and arginine vasopressin-containing fibers in the cortex of humans, chimpanzees, and rhesus macaques

Oxytocin (OT) and arginine-vasopressin (AVP) are involved in the regulation of complex social behaviors across a wide range of taxa. Despite this, little is known about the neuroanatomy of the OT and AVP systems in most non-human primates, and less in humans. The effects of OT and AVP on social behavior, including aggression, mating, and parental behavior, may be mediated primarily by the extensive connections of OT- and AVP-producing neurons located in the hypothalamus with the basal forebrain and amygdala, as well as with the hypothalamus itself. However, OT and AVP also influence social cognition, including effects on social recognition, cooperation, communication, and in-group altruism, which suggests connectivity with cortical structures. While OT and AVP V1a receptors have been demonstrated in the cortex of rodents and primates, and intranasal administration of OT and AVP has been shown to modulate cortical activity, there is to date little evidence that OT-and AVP-containing neurons project into the cortex. Here, we demonstrate the existence of OT- and AVP-containing fibers in cortical regions relevant to social cognition using immunohistochemistry in humans, chimpanzees, and rhesus macaques. OT-immunoreactive fibers were found in the straight gyrus of the orbitofrontal cortex as well as the anterior cingulate gyrus in human and chimpanzee brains, while no OT-immunoreactive fibers were found in macaque cortex. AVP-immunoreactive fibers were observed in the anterior cingulate gyrus in all species, as well as in the insular cortex in humans, and in a more restricted distribution in chimpanzees. This is the first report of OT and AVP fibers in the cortex in human and non-human primates. Our findings provide a potential mechanism by which OT and AVP might exert effects on brain regions far from their production site in the hypothalamus, as well as potential species differences in the behavioral functions of these target regions.

Shodagor Family Strategies : Balancing Work and Family on the Water

The Shodagor of Matlab, Bangladesh, are a seminomadic community of people who live and work on small wooden boats, within the extensive system of rivers and canals that traverse the country. This unique ecology places particular constraints on family and economic life and leads to Shodagor parents employing one of four distinct strategies to balance childcare and provisioning needs. The purpose of this paper is to understand the conditions that lead a family to choose one strategy over another by testing predictions about socioecological factors that impact the sexual division of labor, including a family’s stage in the domestic cycle, aspects of the local ecology, and the availability of alloparents. Results show that although each factor has an impact on the division of labor individually, a confluence of these factors best explains within-group, between-family differences in how mothers and fathers divide subsistence and childcare labor. These factors also interact in particular ways for Shodagor families, and it appears that families choose their economic strategies based on the constellation of constraints that they face. The results of these analyses have implications for theory regarding the sexual division of labor across cultures and inform how Shodagor family economic and parenting strategies should be contextualized in future studies.

Mothers and Fathers Perform More Mate Retention Behaviors than Individuals without Children

Human life history is unique among primates, most notably the extraordinary length of infant dependency and the formation of long-term pair-bonds. Men and women are motivated to remain pair-bonded to maintain the distribution of male-provisioned resources to a woman and her offspring, or to protect offspring from infanticide. Men and women can employ several strategies to retain their mate and prevent their partner from defecting from the relationship, including individual mate retention (behaviors performed alone) and coalitional mate retention (behaviors performed by a close ally). The current research investigates whether men and women with children perform more frequent mate retention behaviors than men and women without children. Participants (n = 1003) currently in a heterosexual romantic relationship completed a survey, reporting whether they had genetic children with their current romantic partner and how frequently they performed various mate retention behaviors. The results indicate that men (n = 262) and women (n = 234) who share genetic children with their current partner performed more frequent individual mate retention behaviors and requested more frequent coalitional mate retention behaviors than men (n = 280) and women (n = 227) who do not share genetic children with their current partner. The results are interpreted as they relate to hypotheses concerning the evolution of pair-bonding in humans, and mate retention behaviors more generally. Limitations of the current research are discussed, and profitable avenues for future research in this domain are suggested.

The neurobiological causes and effects of alloparenting

Alloparenting, defined as care provided by individuals other than parents, is a universal behavior among humans that has shaped our evolutionary history and remains important in contemporary society. Dysfunctions in alloparenting can have serious and sometimes fatal consequences for vulnerable infants and children. In spite of the importance of alloparenting, they still have much to learn regarding the underlying neurobiological systems governing its expression. Here, they review how a lack of alloparental behavior among traditional laboratory species has led to a blind spot in our understanding of this critical facet of human social behavior and the relevant neurobiology. Based on what is known, they draw from model systems ranging from voles to meerkats to primates to describe a conserved set of neuroendocrine mechanisms supporting the expression of alloparental care. In this review we describe the neurobiological and behavioral prerequisites, ontogeny, and consequences of alloparental care. Lastly, they identified several outstanding topics in the area of alloparental care that deserve further research efforts to better advance human health and wellbeing. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 214-232, 2017.

What can cross-cultural correlations teach us about human nature?

Many recent evolutionary psychology and human behavioral ecology studies have tested hypotheses by examining correlations between variables measured at a group level (e.g., state, country, continent). In such analyses, variables collected for each aggregation are often taken to be representative of the individuals present within them, and relationships between such variables are presumed to reflect individual-level processes. There are multiple reasons to exercise caution when doing so, including: (1) the ecological fallacy, whereby relationships observed at the aggregate level do not accurately represent individual-level processes; (2) non-independence of data points, which violates assumptions of the inferential techniques used in null hypothesis testing; and (3) cross-cultural non-equivalence of measurement (differences in construct validity between groups). We provide examples of how each of these gives rise to problems in the context of testing evolutionary hypotheses about human behavior, and we offer some suggestions for future research.

The evolution of paternal care can lead to population growth in artificial societies

Evolutionary models of paternal care predict that when female reproductive effort is higher than male reproductive effort, selection might favour the emergence of unconditional male cooperation towards females, even when the latter group does not reciprocate. However, previous models have assumed constant population sizes, so the ecology of interacting individuals and its effects on population dynamics have been neglected. This paper reports an agent-based model that incorporates ecological dynamics into evolutionary game dynamics by allowing populations to vary. As previous models demonstrate, paternal care only evolves when female reproductive effort is higher than that of males, and the optimal strategy for females is to exploit male unconditional cooperation. The model also shows that evolution of this behaviour drives some simulations towards regimes of population growth. Thanks to the evolution of paternal care, females׳ inter-birth intervals are shortened and causing them to reproduce faster. Thus, it is suggested that the evolution of paternal care in species with differential reproductive effort between sexes could be associated to population growth. Nevertheless, the modelled evolutionary dynamics are stochastic, so differences in reproductive effort are necessary but not sufficient conditions for the evolution of paternal care.

Practical Support from Fathers and Grandmothers Is Associated with Lower Levels of Breastfeeding in the UK Millennium Cohort Study

Mothers face trade-offs between infant care and subsistence/economic activities. In traditional populations, allomothers such as fathers and grandmothers support mothers with young infants, allowing them to reduce labour activities and focus on breastfeeding. Similarly, the positive impact of social support on breastfeeding has been highlighted in developed populations. However, these studies have generally focused on emotional support from fathers, peers and healthcare professionals. Given the availability of formula milk in developed populations, an evolutionary anthropological perspective highlights that practical support, unlike emotional support, may have negative associations with breastfeeding by enabling substitution of maternal care. Other kin, mainly grandmothers, may also be important allomothers influencing maternal breastfeeding levels. Here we explore the associations between different types of social support mothers receive from fathers/grandmothers and breastfeeding in the UK Millennium Cohort Study. We find frequent grandmother contact and father’s parenting involvement are both associated with lower levels of breastfeeding, suggesting a negative relationship between practical support and breastfeeding. In contrast, father presence, potentially capturing emotional support, is associated with greater breastfeeding initiation. Our findings suggest that practical support and emotional support functions differently, and practical support may not encourage breastfeeding in developed populations.

The insectan apes

I present evidence that humans have evolved convergently to social insects with regard to a large suite of social, ecological, and reproductive phenotypes. Convergences between humans and social insects include: (1) groups with genetically and environmentally defined structures; (2) extensive divisions of labor; (3) specialization of a relatively restricted set of females for reproduction, with enhanced fertility; (4) extensive extramaternal care; (5) within-group food sharing; (6) generalized diets composed of high-nutrient-density food; (7) solicitous juveniles, but high rates of infanticide; (8) ecological dominance; (9) enhanced colonizing abilities; and (10) collective, cooperative decision-making. Most of these convergent phenotypic adaptations stem from reorganization of key life-history trade-offs due to behavioral, physiological, and life-historical specializations. Despite their extensive socioreproductive overlap with social insects, humans differ with regard to the central aspect of eusociality: reproductive division of labor. This difference may be underpinned by the high energetic costs of producing offspring with large brains.

The evolutionary psychology of human mating: a response to Buller's critique

In this paper, I critique arguments made by philosopher David Buller against central evolutionary-psychological explanations of human mating. Specifically, I aim to rebut his criticisms of Evolutionary Psychology regarding (1) women's long-term mating preferences for high-status men; (2) the evolutionary rationale behind men's provisioning of women; (3) men's mating preferences for young women; (4) women's adaptation for extra-pair sex; (5) the sex-differentiated evolutionary theory of human jealousy; and (6) the notion of mate value. In sum, I aim to demonstrate that Buller's arguments contra Evolutionary Psychologists are left wanting.

Dating and sexual behavior among single parents of young children in the United States

Theory and research on partnered parents suggests trade-offs between parenting and sexuality, with those trade-offs most pronounced among mothers of young children. However, little research has focused on how a growing demographic of single parents negotiates dating and sexual activity. The current study drew upon a 2012 nationally representative sample of 5,481 single Americans 21 years of age and older, of whom 4.3% were parents of a child age five or younger. Dependent variables were sexual thoughts, frequency of sexual activity, number of sexual partners in the past year, dates during the previous three months, and whether one was actively seeking a relationship partner. Covariates included parental age, sex/gender, sexual orientation, education, and income. Using the entire sample of singles, we found no main effects of number (0, 1, 2+) of children aged five years and younger or number of children aged two years and younger on dating and sexual behavior variables. Next, using analyses restricted to single parents (n = 2,121), we found that single parents with a child aged five years or younger, adjusting for covariates, reported greater frequency of sexual activity and first dates but no differences in other outcomes compared with single parents of older children.

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.

Autonomic substrates of the response to pups in male prairie voles

Caregiving by nonparents (alloparenting) and fathers is a defining aspect of human social behavior, yet this phenomenon is rare among mammals. Male prairie voles (Microtus ochrogaster) spontaneously exhibit high levels of alloparental care, even in the absence of reproductive experience. In previous studies, exposure to a pup was selectively associated with increased activity in oxytocin and vasopressin neurons along with decreased plasma corticosterone. In the present study, physiological, pharmacological and neuroanatomical methods were used to explore the autonomic and behavioral consequences of exposing male prairie voles to a pup. Reproductively naïve, adult male prairie voles were implanted with radiotransmitters used for recording ECG, temperature and activity. Males responded with a sustained increase in heart-rate during pup exposure. This prolonged increase in heart rate was not explained by novelty, locomotion or thermoregulation. Although heart rate was elevated during pup exposure, respiratory sinus arrhythmia (RSA) did not differ between these males and males exposed to control stimuli indicating that vagal inhibition of the heart was maintained. Blockade of beta-adrenergic receptors with atenolol abolished the pup-induced heart rate increase, implicating sympathetic activity in the pup-induced increase in heart rate. Blockade of vagal input to the heart delayed the males’ approach to the pup. Increased activity in brainstem autonomic regulatory nuclei was also observed in males exposed to pups. Together, these findings suggest that exposure to a pup activates both vagal and sympathetic systems. This unique physiological state (i.e. increased sympathetic excitation of the heart, while maintaining some vagal cardiac tone) associated with male caregiving behavior may allow males to both nurture and protect infants.

Profiles of Maternal Care Observed in a Group of Brazilian Mothers: an Exploratory Study

The aim of this study is to characterize profiles of maternal care in a sample of Brazilian mothers, using a culturalbiological perspective. Participants were 315 women who had at least one child younger than six years-old. They were selected from six Brazilian states. In each state, two groups of mothers were studied, one from the capital and one from a small city. They were interviewed and answered scales on allocentrism, social support, adult attachment, and practices of childcare. Mothers' characteristics, the type of context (capital vs. small city), and reported childcare practices were used in a Tree analysis. A Correspondence analysis was performed using the four clusters obtained and mothers' answers regarding their youngest child. Univariate GLM analyses were performed to compare mothers in the four clusters in terms of their scores on the different scales. Four maternal profiles presenting distinctive patterns of association between mothers' characteristics and care practices displayed to the youngest child were identified. We conclude that maternal care is a multi-determined phenomenon and that the method employed in this study can give insights into how the combination of diverse social-biological factors can result in a set of childcare practices.

The foundation of kinship: households

Men's hunting has dominated the discourse on energy capture and flow in the past decade or so. We turn to women's roles as critical to household formation, pair-bonding, and intergenerational bonds. Their pivotal contributions in food processing and distribution likely promoted kinship, both genetic and affinal, and appear to be the foundation from which households evolved. With conscious recognition of household social units, variable cultural constructions of human kinship systems that were sensitive to environmental and technological conditions could emerge. Kinship dramatically altered the organization of resource access for our species, creating what we term "kinship ecologies." We present simple mathematical models to show how hunting leads to dependence on women's contributions, bonds men to women, and bonds generations together. Kinship, as it organized transfers of food and labor energy centered on women, also became integrated with the biological evolution of human reproduction and life history.

Modeling the effects of weaning age on length of female reproductive period: implications for the evolution of human life history

OBJECTIVES

Human life history is unique among primates. Despite our extended lifespan, the length of the female reproductive period is shorter in humans than in our closest living relatives, chimpanzees. Here, we investigate whether this difference could be explained by another unique aspect of human life history-a young weaning age.

METHODS

Age-dependent female fertility is modeled with the Brass polynomial. We model female reproductive period length as single locus with multiple alleles. Selection acts on the length of the female reproductive period in an evolutionary agent-based simulation. We quantify the effect of weaning age on the optimal length of the female reproductive period under a range of adult mortality rates.

RESULTS

Females sacrifice a smaller proportion of their reproductive potential due to nursing by weaning their offspring at younger ages. As a consequence, the optimal length of the female reproductive period decreases as weaning age decreases, even when adult mortality is low.

CONCLUSIONS

Natural selection will favor mutations or strategies that can decrease weaning age without incurring fitness costs. In the presence of younger weaning ages, selection favors a shorter female reproductive period. To the extent that allocare can decrease weaning age without decreasing fitness, its ubiquity in human societies and near absence in other primate societies may explain why women have a shorter reproductive period. Furthermore, allocare may have provided human ancestors with an avenue to decreased weaning age-and, ultimately, a shorter female reproductive period-that was unavailable to their hominoid contemporaries.

Parental precaution: neurobiological means and adaptive ends

Humans invest precious reproductive resources in just a few offspring, who remain vulnerable for an extended period of their lifetimes relative to other primates. Therefore, it is likely that humans evolved a rich precautionary psychology that assists in the formidable task of protecting offspring. In this review, we integrate precautionary behaviors during pregnancy and postpartum with the adaptive functions they may serve and what is known of their biological mediators, particularly brain systems motivating security and attachment. We highlight the role of reproductive hormones in (i) priming parental affiliation with young to incentivize offspring protection, (ii) focusing parental attention on cues of potential threat, and (iii) facilitating maternal defense against potentially dangerous conspecifics and predators. Throughout, we center discussion on adaptive responses to threats of disease, accident and assault as common causes of child mortality in the ancestral past.

The goals of direct paternal care among a South Amerindian population

Human males are remarkable among mammals in the level of investment they provide to their wives and children. However, there has been debate as to the degree to which men actually invest and through which fitness pathways the benefits of familial investment are realized. Much of the previous research exploring these issues has focused on men's roles as providers, but few have explored correlates of men's direct parental care. Although this is reasonable given men's parental emphasis on provisioning, the providing of direct care is more straightforward with a clear provider and recipient and little ambiguity as to the care-giver's intent. Here, we explore contextual correlates of men's direct care among the Tsimane of Bolivia to determine the extent to which such care is patterned to enhance its effectiveness in increasing child wellbeing and the efficient functioning of the family. We also explore whether Tsimane fathers provide care in ways that enhance the positive effect it has on the wife's perception of the care provider. Overall, we find that Tsimane men appear responsive to the needs of children and the family, but show that there is little evidence that men respond to factors expected to increase the impact that men's care has on their reputations with their wives.

Maternal effects in cooperative breeders: from hymenopterans to humans

The environment that an offspring experiences during its development can have lifelong consequences for its morphology, anatomy, physiology and behaviour that are strong enough to span generations. One aspect of an offspring's environment that can have particularly pronounced and long-lasting effects is that provided by its parent(s) (maternal effects). Some disciplines in biology have been quicker to appreciate maternal effects than others, and some organisms provide better model systems for understanding the causes and consequences of the maternal environment for ecology and evolution than others. One field in which maternal effects has been poorly represented, and yet is likely to represent a particularly fruitful area for research, is the field of cooperative breeding (i.e. systems where offspring are reared by carers in addition to parent(s)). Here, we attempt to illustrate the scope of cooperative breeding systems for maternal effects research and, conversely, highlight the importance of maternal effects research for understanding cooperative breeding systems. To this end, we first outline why mothers will commonly benefit from affecting the phenotype of their offspring in cooperative breeding systems, present potential strategies that mothers could employ in order to do so and offer predictions regarding the circumstances under which different types of maternal effects might be expected. Second, we highlight why a neglect of maternal strategies and the effects that they have on their offspring could lead to miscalculations of helper/worker fitness gains and a misunderstanding of the factors selecting for the evolution and maintenance of cooperative breeding. Finally, we introduce the possibility that maternal effects could have significant consequences for our understanding of both the evolutionary origins of cooperative breeding and the rise of social complexity in cooperative systems.