Evidence for an adolescent sensitive period to family experiences influencing adult male testosterone production.

Across vertebrates, testosterone is an important mediator of reproductive trade-offs, shaping how energy and time are devoted to parenting versus mating/competition. Based on early environments, organisms often calibrate adult hormone production to adjust reproductive strategies. For example, favorable early nutrition predicts higher adult male testosterone in humans, and animal models show that developmental social environments can affect adult testosterone. In humans, fathers' testosterone often declines with caregiving, yet these patterns vary within and across populations. This may partially trace to early social environments, including caregiving styles and family relationships, which could have formative effects on testosterone production and parenting behaviors. Using data from a multidecade study in the Philippines (n = 966), we tested whether sons' developmental experiences with their fathers predicted their adult testosterone profiles, including after they became fathers themselves. Sons had lower testosterone as parents if their own fathers lived with them and were involved in childcare during adolescence. We also found a contributing role for adolescent father­son relationships: sons had lower waking testosterone, before and after becoming fathers, if they credited their own fathers with their upbringing and resided with them as adolescents. These findings were not accounted for by the sons' own parenting and partnering behaviors, which could influence their testosterone. These effects were limited to adolescence: sons' infancy or childhood experiences did not predict their testosterone as fathers. Our findings link adolescent family experiences to adult testosterone, pointing to a potential pathway related to the intergenerational transmission of biological and behavioral components of reproductive strategies.

Examining the dual hormone hypothesis in wild male mountain gorillas (Gorilla beringei beringei).

The Challenge Hypothesis is an influential framework for understanding how androgens are involved in the promotion of competitive behavior during mating-related challenges and has been tested extensively in studies across scientific disciplines. Mixed support in psychological research led scholars to develop the Dual Hormone Hypothesis as a potential path forward, which argues that glucocorticoids moderate the relationship between androgens and status-striving. In the current study, we examine the Challenge Hypothesis and the Dual Hormone Hypothesis in wild male mountain gorillas, representing the first time the latter hypothesis has been tested in a non-human primate. In a sample of 30 adult males comprising over 600 days of observation, we find some limited support for the Challenge Hypothesis. Greater daily rates of targeted aggression toward other adult males corresponded to higher fecal androgen metabolites 1-2 days following observations, though this pattern did not fully generalize to dominance rank or other competitive behaviors examined. However, we find no support for the Dual Hormone Hypothesis: neither dominance rank nor any category of competitive behavior was predicted by the interaction between androgens and glucocorticoids. We close by discussing how this initial investigation might be leveraged toward the development of an expanded Dual Hormone Hypothesis that draws on the large evidence base in primate behavioral ecology.

Effects of social environments on male primate HPG and HPA axis developmental programming.

Developmental plasticity is particularly important for humans and other primates because of our extended period of growth and maturation, during which our phenotypes adaptively respond to environmental cues. The hypothalamus-pituitary-gonadal (HPG) and hypothalamus-pituitary-adrenal (HPA) axes are likely to be principal targets of developmental "programming" given their roles in coordinating fitness-relevant aspects of the phenotype, including sexual development, adult reproductive and social strategies, and internal responses to the external environment. In social animals, including humans, the social environment is believed to be an important source of cues to which these axes may adaptively respond. The effects of early social environments on the HPA axis have been widely studied in humans, and to some extent, in other primates, but there are still major gaps in knowledge specifically relating to males. There has also been relatively little research examining the role that social environments play in developmental programming of the HPG axis or the HPA/HPG interface, and what does exist disproportionately focuses on females. These topics are likely understudied in males in part due to the difficulty of identifying developmental milestones in males relative to females and the general quiescence of the HPG axis prior to maturation. However, there are clear indicators that early life social environments matter for both sexes. In this review, we examine what is known about the impact of social environments on HPG and HPA axis programming during male development in humans and nonhuman primates, including the role that epigenetic mechanisms may play in this programming. We conclude by highlighting important next steps in this research area.

An empirical comparison of several commercial enzyme immunoassays for the non-invasive assessment of adrenocortical and gonadal function in mountain gorillas.

Wildlife researchers seeking to non-invasively examine endocrine function in their study species are presented with a dense and technical 'garden of forking paths' to navigate between collecting a biological sample and obtaining a final measurement. In particular, the choice of which enzyme immunoassay (EIA) to use with collected fecal samples, out of the many options offered by different manufacturers and research laboratories, may be one of the most consequential for final results. However, guidance for making this decision is still emerging. With this gap in mind, we performed a head-to-head comparison of results obtained from four different EIAs for fecal glucocorticoid metabolites (FGCMs), and three different EIAs for fecal androgen metabolites (FAMs), applied to the same set of fecal samples collected from the mountain gorillas (Gorilla beringei beringei) monitored by the Dian Fossey Gorilla Fund in Volcanoes National Park, Rwanda. We provide a) an analytical validation of the different EIAs via tests of parallelism and linearity; b) an estimate of inter-assay correlation between EIA kits designed for the same metabolites; and c) a test of the kits' ecological validity, in which we examine how well each captures endocrine changes following events that theory predicts should result in elevated FGCM and/or FAM concentrations. Our results show that kits differ to some degree in their performance; at the same time, nearly all assays exhibited at least moderate evidence of validity and covariance with others for the same analyte. Our findings, which differ somewhat from similar comparisons performed in other species, demonstrate the need to directly assess assay performance in a species- and context-specific manner as part of efforts to develop the burgeoning discipline of wildlife endocrinology.

Social bonds do not mediate the relationship between early adversity and adult glucocorticoids in wild baboons.

In humans and other animals, harsh conditions in early life can have profound effects on adult physiology, including the stress response. This relationship may be mediated by a lack of supportive relationships in adulthood. That is, early life adversity may inhibit the formation of supportive social ties, and weak social support is itself often linked to dysregulated stress responses. Here, we use prospective, longitudinal data from wild baboons in Kenya to test the links between early adversity, adult social bonds, and adult fecal glucocorticoid hormone concentrations (a measure of hypothalamic-pituitary-adrenal [HPA] axis activation and the stress response). Using a causal inference framework, we found that experiencing one or more sources of early adversity led to a 9 to 14% increase in females' glucocorticoid concentrations across adulthood. However, these effects were not mediated by weak social bonds: The direct effects of early adversity on adult glucocorticoid concentrations were 11 times stronger than the effects mediated by social bonds. This pattern occurred, in part, because the effect of social bonds on glucocorticoids was weak compared to the powerful effects of early adversity on glucocorticoid levels in adulthood. Hence, in female baboons, weak social bonds in adulthood are not enough to explain the effects of early adversity on glucocorticoid concentrations. Together, our results support the well-established notions that early adversity and weak social bonds both predict poor adult health. However, the magnitudes of these two effects differ considerably, and they may act independently of one another.

Pathways to paternal care in primates.

Natural selection will favor male care when males have limited alternative mating opportunities, can invest in their own offspring, and when care enhances males' fitness. These conditions are easiest to fulfill in pair-bonded species, but neither male care nor stable "breeding bonds" that facilitate it are limited to pair-bonded species. We review evidence of paternal care and extended breeding bonds in owl monkeys, baboons, Assamese macaques, mountain gorillas, and chimpanzees. The data, which span social/mating systems and ecologies, suggest that there are multiple pathways by which conditions conducive to male care can arise. This diversity highlights the difficulty of making inferences about the emergence of male care in early hominins based on single traits visible in the fossil record. We discuss what types of data are most needed and the questions yet to be answered about the evolution of male care and extended breeding bonds in the primate order.

Neither environmental unpredictability nor harshness predict reliance on alloparental care among families in Cebu, Philippines.

Alloparental caregiving is key to humans' highly flexible reproductive strategies. Across species and across societies, alloparental care is more common in harsh and/or unpredictable environments (HUEs). Currently, however, it is unclear whether HUEs predict intra-population variation in alloparental care, or whether early life HUEs might predict later alloparental care use in adulthood, consistent with adaptive developmental plasticity. We test whether harshness measures (socioeconomic status (SES), environmental hygiene, crowding) and unpredictability measures (parental unemployment, paternal absence, household moves) predicted how much alloparental assistance families in Cebu, Philippines received, in a multigenerational study with data collected across four decades. Though worse environmental hygiene predicted more concurrent alloparental care in 1994, we found little evidence that HUEs predict within-population variation in alloparental care in this large-scale, industrialized society. Indeed, less-crowded conditions and higher SES predicted more alloparental care, not less, in the 1980s and in 2014 respectively, while paternal absence in middle childhood predicted less reliance on alloparental care in adulthood. In this cultural context, our results generally do not provide support for the translation of interspecific or intersocietal patterns linking HUEs and alloparental care to intra-population variation in alloparental care, nor for the idea that a reproductive strategy emphasizing alloparental care use may be preceded by early life HUEs.

Group structure, but not dominance rank, predicts fecal androgen metabolite concentrations of wild male mountain gorillas (Gorilla beringei beringei).

Androgens are important mediators of male-male competition in many primate species. Male gorillas' morphology is consistent with a reproductive strategy that relies heavily on androgen-dependent traits (e.g., extreme size and muscle mass). Despite possessing characteristics typical of species with an exclusively single-male group structure, multimale groups with strong dominance hierarchies are common in mountain gorillas. Theory predicts that androgens should mediate their dominance hierarchies, and potentially vary with the type of group males live in. We validated the use of a testosterone enzyme immunoassay (T-EIA R156/7, CJ Munro, UC-Davis) for use with mountain gorilla fecal material by (1) examining individual-level androgen responses to competitive events, and (2) isolating assay-specific hormone metabolites via high-performance liquid chromatography. Males had large (2.6- and 6.5-fold), temporary increases in fecal androgen metabolite (FAM) after competitive events, and most captured metabolites were testosterone or 5α-dihydrotestosterone-like androgens. We then examined the relationship between males' dominance ranks, group type, and FAM concentrations. Males in single-male groups had higher FAM concentrations than males in multimale groups, and a small pool of samples from solitary males suggested they may have lower FAM than group-living peers. However, data from two different time periods (n = 1610 samples) indicated there was no clear relationship between rank and FAM concentrations, confirming results from the larger of two prior studies that measured urinary androgens. These findings highlight the need for additional research to clarify the surprising lack of a dominance hierarchy/androgen relationship in mountain gorillas.

Evolutionary life history theory as an organising framework for cohort studies: insights from the Cebu Longitudinal Health and Nutrition Survey.

By tracking a group of individuals through time, cohort studies provide fundamental insights into the developmental time course and causes of health and disease. Evolutionary life history theory seeks to explain patterns of growth, development, reproduction and senescence, and inspires a range of hypotheses that are testable using the longitudinal data from cohort studies. Here we review two decades of life history theory-motivated work conducted in collaboration with the Cebu Longitudinal Health and Nutrition Survey (CLHNS), a birth cohort study that enrolled more than 3000 pregnant women in the Philippines in 1983 and has since followed these women, their offspring and grandoffspring. This work has provided evidence that reproduction carries "costs" to cellular maintenance functions, potentially speeding senescence, and revealed an unusual form of genetic plasticity in which the length of telomeres inherited across generations is influenced by reproductive timing in paternal ancestors. Men in Cebu experience hormonal and behavioural changes in conjunction with changes in relationship and fatherhood status that are consistent with predictions based upon other species that practice bi-parental care. The theoretical expectation that early life cues of mortality or environmental unpredictability will motivate a "fast" life history strategy are confirmed for behavioural components of reproductive decision making, but not for maturational tempo, while our work points to a broader capacity for early life developmental calibration of systems like immunity, reproductive biology and metabolism. Our CLHNS findings illustrate the power of life history theory as an integrative, lifecourse framework to guide longitudinal studies of human populations.

Does a man's testosterone "rebound" as dependent children grow up, or when pairbonds end? A test in Cebu, Philippines.

OBJECTIVE: Cross-culturally, men's T declines in response to pairbonding and fatherhood, but less is known about what happens to T during and after life history transitions that theoretically lead to renewed mating effort. We tested whether men's T rises (or declines less with age) as their children age, or when pairbonds end, independent of changes in fatherhood-related variables such as co-residence with children. METHODS: We used demographic, behavioral, and salivary hormone data (waking and pre-bed T) collected in 2009 and 2014 for the Cebu Longitudinal Health and Nutrition Survey (n = 571 men). RESULTS: Fathers with older children tended to have attenuated decline in pre-bedtime T between 2009 and 2014 compared to men with younger children, after controlling for pairbonding (ß = 1.58, SE = 0.88, P = 0.074). Separated men had higher pre-bedtime T than pairbonded men, controlling for fatherhood-related variables (ß = 11.74, SE = 4.33, P = 0.007). Change in T did not significantly differ for men who separated between the two surveys, relative to men who remained pairbonded throughout. CONCLUSION: We found modest support for the prediction that men experience less of an age-related drop in T as their youngest child ages, a trend that might strengthen as children age further. We also replicate the finding that separated men have higher T, although longitudinal changes in the hormone were not significantly different in these men. Our data suggest that, of two life history transitions that may predict renewed mating effort, pair bond loss is more strongly endocrine mediated than potential mating effort shifts related to the aging of children.

The effects of collection and storage conditions in the field on salivary testosterone, cortisol, and sIgA values.

AIM: To determine if field-typical storage and collection conditions are related to salivary testosterone (T), cortisol (cort) and secretory immunoglobulin A (sIgA) as measured using commercially available kits. SUBJECTS AND METHODS: This study assessed whether storage time (∼6 months to 1.5 years) at -35 °C impacted levels of the measured biomarkers (n = 10,247 samples). For a sub-set of T samples (n = 2954), we also evaluated the impact of collection conditions, such as time spent at room temperature in participants' homes (0-39 hours) and time spent in coolers (0.3-10 hours) in transit. RESULTS: T was unrelated to storage and collection variables and there was no evidence of reduced sample fidelity at longer storage times. Cort samples stored at -35 °C for longer had significantly lower values, but the effect was small (ß = -0.003 nmol/L/day, SE <0.001, p = 0.005). sIgA trended higher with longer storage at -35 °C (ß = 0.84 µg/mL/day, SE = 0.45, p = 0.063). Collection and storage time variables did not improve the fit of any of the models except the one that evaluated cortisol. CONCLUSIONS: Salivary T was unaffected by extended storage at -35 °C and only a weak relationship was found between storage time and salivary cort or sIgA. Findings underscore the robustness of these biomarkers under field-typical freezer storage conditions.

Men's physical health and life history transitions in the Philippines: Evidence for 'marital selection' but not protective effects of partnering and fatherhood.

In Euro-American societies, married people typically have lower overall risks for total mortality and for certain chronic conditions compared to non-married people. However, people becoming partnered and parents also tend to gain weight in Euro-American settings. Few studies have tested whether links between physical health and life history status translate to other cultural contexts where the socio-ecological dynamics of family life may differ. This limits the application of these insights to men's well-being in global public health. To help address this gap, we drew on a large, long-running birth cohort study of Filipino men, using data collected at three waves between 2005 and 2014 when men were 21.5-30.5 years old (N = 607, obs. = 1760). We tested for the effects of the transition to partnering (marriage/cohabitation) and fatherhood on men's physical health (waist circumference, fat-free mass index, and grip strength). Men becoming partnered or partnered fathers (P/PF) had comparable longitudinal physical health trajectories to men remaining single non-fathers. However, men who became P/PF by their mid 20s had higher fat-free mass index values than single non-fathers at each wave, with the largest effect observed when all men were single non-fathers at baseline. Men who became P/PF by their early 30s were also stronger than the reference group at baseline. Thus, men who were more muscular and stronger at baseline were more likely to transition to P/PF status, consistent with a 'marital selection' model. In complementary analyses, men did not exhibit adverse health effects when they became partnered fathers as young adults or parents to infants, respectively. These findings suggest that links between health and life history transitions in this setting differ from those commonly observed in Euro-American societies. While transitions to marriage and fatherhood are promising windows for interventions to improve men's health, our results highlight the importance of tailoring such approaches to local dynamics.

Testing frameworks for early life effects: the developmental constraints and adaptive response hypotheses do not explain key fertility outcomes in wild female baboons.

In evolutionary ecology, two classes of explanations are frequently invoked to explain "early life effects" on adult outcomes. Developmental constraints (DC) explanations contend that costs of early adversity arise from limitations adversity places on optimal development. Adaptive response (AR) hypotheses propose that later life outcomes will be worse when early and adult environments are poorly "matched." Here, we use recently proposed mathematical definitions for these hypotheses and a quadratic-regression based approach to test the long-term consequences of variation in developmental environments on fertility in wild baboons. We evaluate whether low rainfall and/or dominance rank during development predict three female fertility measures in adulthood, and whether any observed relationships are consistent with DC and/or AR. Neither rainfall during development nor the difference between rainfall in development and adulthood predicted any fertility measures. Females who were low-ranking during development had an elevated risk of losing infants later in life, and greater change in rank between development and adulthood predicted greater risk of infant loss. However, both effects were statistically marginal and consistent with alternative explanations, including adult environmental quality effects. Consequently, our data do not provide compelling support for either of these common explanations for the evolution of early life effects.

The promise of great apes as model organisms for understanding the downstream consequences of early life experiences.

Early life experiences have a significant influence on adult health and aging processes in humans. Despite widespread interest in the evolutionary roots of this phenomenon, very little research on this topic has been conducted in humans' closest living relatives, the great apes. The longitudinal data sets that are now available on wild and captive great ape populations hold great promise to clarify the nature, evolutionary function, and mechanisms underlying these connections in species which share key human life history characteristics. Here, we explain features of great ape life history and socioecologies that make them of particular interest for this topic, as well as those that may limit their utility as comparative models; outline the ways in which available data are complementary to and extend the kinds of data that are available for humans; and review what is currently known about the connections among early life experiences, social behavior, and adult physiology and biological fitness in our closest living relatives. We conclude by highlighting key next steps for this emerging area of research.

Conceptual and analytical approaches for modelling the developmental origins of inequality.

In many species, individuals that experience harsh conditions during development have poor health and fitness outcomes in adulthood, compared with peers that do not. These early-life contributions to inequality are often attributed to two classes of evolutionary hypotheses: Developmental Constraints (DC) models, which focus on the deleterious effects of low-quality early-life environments, and Predictive Adaptive Response (PAR) hypotheses, which emphasize the costs individuals incur when they make incorrect predictions about conditions in adulthood. Testing these hypotheses empirically is difficult for conceptual and analytical reasons. Here, we help resolve some of these difficulties by providing mathematical definitions for DC, PAR (particularly focusing on 'external' PAR) and related concepts. We propose a novel, quadratic regression-based statistical test derived from these definitions. Our simulations show that this approach markedly improves the ability to discriminate between DC and PAR hypotheses relative to the status quo approach, which uses interaction effects. Simulated data indicate that the interaction effects approach often conflates PAR with DC, while the quadratic regression approach yields high sensitivity and specificity for detecting PAR. Our results highlight the value of linking verbal and visual models to a formal mathematical treatment for understanding the developmental origins of inequitable adult outcomes. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Cumulative early-life adversity does not predict reduced adult longevity in wild gorillas.

Extensive research across fields has repeatedly confirmed that early-life adversity (ELA) is a major selective force for many taxa, in part via its ties to adult health and longevity.1,2,3 Negative effects of ELA on adult outcomes have been documented in a wide range of species, from fish to birds to humans.4 We used 55 years of long-term data collected on 253 wild mountain gorillas to examine the effects of six putative sources of ELA on survival, both individually and cumulatively. Although cumulative ELA was associated with high mortality in early life, we found no evidence that it had detrimental consequences for survival later in life. Experiencing three or more forms of ELA was associated with greater longevity, with a 70% reduction in the risk of death across adulthood, driven specifically by greater longevity in males. Although this higher survival in later life is likely a consequence of sex-specific viability selection5 during early life due to the immediate mortality consequences of adverse experiences, patterns in our data also suggest that gorillas have significant resilience to ELA. Our findings demonstrate that the detrimental consequences of ELA on later life survival are not universal, and indeed largely absent in one of humans' closest living relatives. This raises important questions about the biological roots of sensitivity to early experiences and the protective mechanisms that contribute to resiliency in gorillas, which could be critical for understanding how best to encourage similar resiliency to early-life shocks in humans.

Mountain gorillas maintain strong affiliative biases for maternal siblings despite high male reproductive skew and extensive exposure to paternal kin.

Evolutionary theories predict that sibling relationships will reflect a complex balance of cooperative and competitive dynamics. In most mammals, dispersal and death patterns mean that sibling relationships occur in a relatively narrow window during development and/or only with same-sex individuals. Besides humans, one notable exception is mountain gorillas, in which non-sex-biased dispersal, relatively stable group composition, and the long reproductive tenures of alpha males mean that animals routinely reside with both maternally and paternally related siblings, of the same and opposite sex, throughout their lives. Using nearly 40,000 hr of behavioral data collected over 14 years on 699 sibling and 1235 non-sibling pairs of wild mountain gorillas, we demonstrate that individuals have strong affiliative preferences for full and maternal siblings over paternal siblings or unrelated animals, consistent with an inability to discriminate paternal kin. Intriguingly, however, aggression data imply the opposite. Aggression rates were statistically indistinguishable among all types of dyads except one: in mixed-sex dyads, non-siblings engaged in substantially more aggression than siblings of any type. This pattern suggests mountain gorillas may be capable of distinguishing paternal kin but nonetheless choose not to affiliate with them over non-kin. We observe a preference for maternal kin in a species with a high reproductive skew (i.e. high relatedness certainty), even though low reproductive skew (i.e. low relatedness certainty) is believed to underlie such biases in other non-human primates. Our results call into question reasons for strong maternal kin biases when paternal kin are identifiable, familiar, and similarly likely to be long-term groupmates, and they may also suggest behavioral mismatches at play during a transitional period in mountain gorilla society.

Social and ecological factors alter stress physiology of Virunga mountain gorillas (Gorilla beringei beringei).

Living in a rapidly changing environment can alter stress physiology at the population level, with negative impacts on health, reproductive rates, and mortality that may ultimately result in species decline. Small, isolated animal populations where genetic diversity is low are at particular risks, such as endangered Virunga mountain gorillas (Gorilla beringei beringei). Along with climate change-associated environmental shifts that are affecting the entire population, subpopulations of the Virunga gorillas have recently experienced extreme changes in their social environment. As the growing population moves closer to the forest's carrying capacity, the gorillas are coping with rising population density, increased frequencies of interactions between social units, and changing habitat use (e.g., more overlapping home ranges and routine ranging at higher elevations). Using noninvasive monitoring of fecal glucocorticoid metabolites (FGM) on 115 habituated Virunga gorillas, we investigated how social and ecological variation are related to baseline FGM levels, to better understand the adaptive capacity of mountain gorillas and monitor potential physiological indicators of population decline risks. Generalized linear mixed models revealed elevated mean monthly baseline FGM levels in months with higher rainfall and higher mean maximum and minimum temperature, suggesting that Virunga gorillas might be sensitive to predicted warming and rainfall trends involving longer, warmer dry seasons and more concentrated and extreme rainfall occurrences. Exclusive use of smaller home range areas was linked to elevated baseline FGM levels, which may reflect reduced feeding efficiency and increased travel efforts to actively avoid neighboring groups. The potential for additive effects of stress-inducing factors could have short- and long-term impacts on the reproduction, health, and ultimately survival of the Virunga gorilla population. The ongoing effects of environmental changes and population dynamics must be closely monitored and used to develop effective long-term conservation strategies that can help address these risk factors.

With a little help from her friends (and family) part I: the ecology and evolution of non-maternal care in mammals.

In the class Mammalia, most young are cared for exclusively by their mothers. In species where mothers receive help, however, non-maternal caregivers may play a crucial role in development and life history trajectories. In turn, recipients of such care may have important impacts on caregivers of all types. In Part I of this overview, we briefly review the evolutionary barriers to widespread non-maternal care in mammals, and explain why the exceptions are of particular theoretical importance. We also summarize the current understanding of the selective forces leading to non-maternal care, and the taxa and types of caretakers amongst which it occurs. Finally, we argue for a fresh look at the categorization schemes that have traditionally been used to separate various types of mammalian non-maternal caregivers. This two-part introduction is aimed at scientists from multiple disciplines who study diverse organismal systems. It draws from the social and biological sciences literatures to provide an overview of this special issue of Physiology and Behavior's suite of methodological offerings and theoretical underpinnings.

With a little help from her friends (and family) part II: Non-maternal caregiving behavior and physiology in mammals.

The diversity of competing frameworks for explaining the evolution of non-maternal care in mammals (Part I, this issue) reflects the vast range of behaviors and associated outcomes these theories attempt to subsume. Caretaking comprises a wide variety of behavioral domains, and is mediated by an equally large range of physiological systems. In Part II, we provide an overview of how non-maternal care in mammals is expressed, the ways in which it is regulated, and the many effects such care has on both recipients and caretakers. We also discuss the two primary ways in which closer integration of ultimate and proximate levels of explanation can be useful when addressing questions about non-maternal caretaking. Specifically, proximate mechanisms provide important functional clues, and are key to testing theory concerning evolutionary tradeoffs. Finally, we highlight a number of methodological and publication biases that currently shape the literature, which provide opportunities for knowledge advancement in this domain going forward. In this conclusion to our two-part introduction, we provide a broad survey of the behavior and physiology that the contributions to this special issue represent.