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Current evolutionary theory conceptualizes fathers' childcare as highly facultative and likely contingent on a variety of local socioecological predictors. Much of the evolutionarily-motivated work on the predictors of paternal care has focused on smaller-scale societies, while similar, potentially complementary research in larger-scale societies has focused on theoretical frameworks from (e.g.) economics and developmental psychology. Due to the different emphases, relatively few studies have incorporated information on variables known to predict paternal care in one context with those known to predict it in the other. Here, we assess whether paternal care conforms to predictions derived from the facultative fathering hypothesis and life history theory in Cebu, the Philippines. We evaluated which of 6 variables—hours worked outside the home, age and number of children, number of other caregivers, family residence pattern, and fathers' educational attainment—predicted the number of hours fathers reported spending on 12 common caregiving tasks. Consistent with the basic premise of facultative fathering, men who worked more spent less time on childcare. Additionally, the time fathers spent on different types of caregiving reflected changes in demand as children age. However, alloparental care appears to be a complement to, rather than a substitute for, paternal care in this context, in contrast to the predictions of the facultative fathering hypothesis. We also found that men with more education reported spending more time on childcare, consistent with a caregiving strategy that emphasizes heavy investment in embodied capital across generations. Our data illustrate the context-specific nature of ‘facultative’ caregiving in humans, and highlight the importance of considering locally-relevant predictors when testing predictions derived from evolutionary theory.

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 father–son 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.

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.

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.

Partnered fathers often have lower testosterone than single non-parents, which is theorized to relate to elevated testosterone (T) facilitating competitive behaviors and lower T contributing to nurturing. Cultural- and individual-factors moderate the expression of such psychobiological profiles. Less is known about genetic variation's role in individual psychobiological responses to partnering and fathering, particularly as related to T. We examined the exon 1 CAG (polyglutamine) repeat (CAGn) within the androgen receptor (AR) gene. AR CAGn shapes T's effects after it binds to AR by affecting AR transcriptional activity. Thus, this polymorphism is a strong candidate to influence individual-level profiles of "androgenicity." While males with a highly androgenic profile are expected to engage in a more competitive-oriented life history strategy, low androgenic men are at increased risk of depression, which could lead to similar outcomes for certain familial dynamics, such as marriage stability and parenting. Here, in a large longitudinal study of Filipino men (n=683), we found that men who had high androgenicity (elevated T and shorter CAGn) or low androgenicity (lower T and longer CAGn) showed elevated likelihood of relationship instability over the 4.5-year study period and were also more likely be relatively uninvolved with childcare as fathers. We did not find that CAGn moderated men's T responses to the fatherhood transition. In total, our results provide evidence for invested fathering and relationship stability at intermediate levels of androgenicity and help inform our understanding of variation in male reproductive strategies and the individual hormonal and genetic differences that underlie it.

Circulating testosterone (T) follows a diurnal pattern with high waking levels that decline across the day. The hypothalamic-pituitary-adrenal axis produces cortisol in a similar manner but also undergoes an abrupt increase in hormone secretion immediately upon waking (a cortisol awakening response, CAR). Whether the hypothalamic-pituitary-gonadal axis, and circulating T levels, exhibit a similar post-waking response is unclear. Here we describe post-waking T changes in a sample of 108 young adult males from metropolitan Cebu City, the Philippines. As expected, salivary T was higher at waking than in the evening but, remarkably, 60 % of this diurnal decline occurred within 30 min of awakening. There was a strong inverse linear relationship between waking T and the post-waking T decline, such that men with higher waking T experienced a more rapid decline in the hormone. Even though fathers had lower waking T, they experienced a greater post-waking decline than non-fathers. Men with a larger positive CAR had modestly attenuated post-waking T declines. We speculate that these findings reflect a testosterone awakening response (TAR) that helps partition the target tissue effects of T by time of day. T rises overnight to facilitate muscle anabolism at a time when the hormone’s impacts on social behavior are limited. Upon waking, the rapid drop in T helps shift from anabolic to catabolic processes in support of physical activity, while also calibrating T levels in line with the competing social priorities of the individual, as determined by the current balance of behavioral investment towards mating and parental effort.

Objectives Research on the psychobiology of partnering and fathering has focused on testosterone (T), oxytocin, and prolactin (PRL) as mechanisms that potentially mediate life history trade-offs related to those roles. Less is known about other hormones that might be responsive to life history transitions and implicated in fathering, such as estradiol (E2). We examined how E2 changed during the transition to marriage and fatherhood, its correlation with fathers' caregiving, and its joint within-individual production with other hormones (T, PRL). Methods Data were collected from a total of 913 Filipino men (aged 25.9 years ± 0.3 SD at follow-up) enrolled in a longitudinal cohort study. Morning saliva samples collected at baseline (2005) and follow-up (2009) were assayed for T and E2 (n = 329), dried blood spots from baseline were assayed for PRL. Fathers reported on caregiving in 2009. Results When compared with men who remained single non-fathers over the study period, men who became married residential fathers experienced larger declines in E2. This effect was non-significant when we controlled for longitudinal changes in T. E2 was not significantly related to fathers' caregiving, controlling for T. In cross-sectional analyses for PRL, T, and E2, married residential fathers exhibited within-individual profiles of reduced T and elevated PRL, whereas single non-fathers exhibited the opposite profile of elevated T and reduced PRL. Conclusions Our findings point to the need for future research to consider the mutually regulatory dynamics and/or combinatorial implications of multiple physiological axes acting within individuals to underpin life history trade-offs and behavioral strategies.

Objective An increase in cortisol during human pregnancy helps coordinate the onset of parturition, and can have long-term effects on offspring biology. Maternal cortisol can also be transferred to offspring via breast milk during lactation. However, little is known about how diurnal cortisol profiles vary by trimester of pregnancy or during the postpartum period. Here, we describe diurnal cortisol profiles among a large cross-sectional sample of healthy Filipino young adult women varying in reproductive status and, during the postpartum period, in whether or not they are breastfeeding. Methods Salivary cortisol, anthropometric, and questionnaire data were obtained from participants in a birth cohort in metropolitan Cebu, Philippines (N = 741; age 20.8-22.4 years). Cortisol was assessed at waking, thirty minutes after waking (cortisol awakening response, CAR), and before bed. Results Compared with nulliparous women, morning cortisol was roughly 50% higher among women in late gestation, while evening cortisol was roughly 4-fold higher and the CAR was lower. Postpartum waking and evening cortisol were lower among currently breastfeeding women compared to nulliparous women, but were comparable in the absence of breastfeeding. The CAR was significantly lower among postpartum women compared to nulliparous women irrespective of breastfeeding status. Conclusions These findings are consistent with known alterations in hypothalamic-pituitary-adrenal axis function during reproduction, and in particular point to marked and progressive elevation in maternal cortisol during the course of gestation. Cortisol appears to return to nulliparous levels after parturition, with levels suppressed below nulliparous levels during lactation.

Early life cues of environmental harshness and unpredictability have been hypothesized to influence within-species variation in the timing of life history transitions and the dynamics of reproductive strategies, such as investments in mating and parenting. It is also believed that adolesence is an influential developmental period for male reproductive strategies, with those who achieve greater social and sexual success during that period maintaining faster life history strategies into adulthood. If correct, such early life and post-pubertal experiences could also help shape the psychobiological pathways that mediate reproductive strategies, including the well documented physiological shifts that occur when some men become parents. Drawing on a large sample of Filipino men (n=417), we evaluate whether men who experienced cues of harshness or unpredictability in childhood or have earlier ages at sexual debut have elevated testosterone (T) as fathers. We also test whether males who experienced a combination of early life experiences of harshness or unpredictability and had earlier ages of sexual debut during adolescence had the most elevated T as fathers. We found that fathers who experienced early life harshness and who engaged in sex at an earlier age had elevated waking T. Among men transitioning to fatherhood across the 4.5-year follow-up period of this study, those who experienced unpredictability and who engaged in sex at an earlier age showed attenuated declines in waking T between baseline and follow-up. Complementing these findings, we found that fathers who first engaged in sex at later ages had greater acute declines in T when they played with their toddlers. We suggest that these patterns could reflect programming effects of sociosexual experiences during the years following the marked biological transitions that accompany puberty, which occur along with the better-studied effects of earlier life exposures to stressors. Overall, our results support the hypothesis that early life circumstances and social and sexual experiences, from early life to young adulthood, help calibrate physiological axes as key mechanisms coordinating dynamic life history strategies.