Several researchers have proposed that facial width-to-height ratio (fWHR) is a sexually dimorphic signal that develops under the influence of pubertal testosterone (T); however, this hypothesis is currently under supported. Here we examine the association between fWHR and T during the period of the life span when facial growth is canalized--adolescence. To do so, we examine the association between T, known T-derived traits (i.e. strength and voice pitch), and craniofacial measurements in a sample of adolescent Tsimane males. If fWHR variation derives from pubertal T’s influence on craniofacial growth, several predictions can be made: 1) fWHR should increase with age as T increases, 2) fWHR should reflect adolescent T (rather than adult T per se), 3) fWHR should exhibit a growth spurt in parallel with T, 4) fWHR and T should correlate after controlling for potential confounds, and 5) fWHR should show a strong relationship to other T-derived traits. These effects were not observed. We also examined three additional facial masculinity ratios: facial width/lower face height, cheekbone prominence, and facial width/full face height. In contrast to fWHR, each of the three additional measures exhibit a strong age-related pattern of change and are associated with both T and T-dependent traits. In summary, our results challenge the status of fWHR as a sexually-selected signal of pubertal T and T-linked traits.The relationship between social status, body size, and salivary hormone levels among Garisakang forager-horticulturalist men of lowland Papua New Guinea
Social hierarchy is a robust phenomenon that exists within all human societies. Over the past several decades, a growing body of evidence from industrialized Western populations has suggested that social status is closely related to individual measures of stress, health, and many other fitness-related traits. Data regarding such relationships, however, remain rare among small-scale subsistence societies, preventing a clear understanding of the importance of social position for fitness cross-culturally. Here we contribute to this area of research by exploring the relationship between adult male social status, BMI, and levels of salivary testosterone and diurnal cortisol among Garisakang small-scale forager-horticulturalists of lowland Papua New Guinea (N = 32). Three measures of individual social status – Respect, Dominance and Prosociality – were extracted from principal components analysis of photo-rank data for locally valued male traits (e.g., sociability, hunting ability, community influence). Preliminary results from multiple regression models controlling for age suggest complex relationships between social status, body size, and salivary hormone levels among the Garisakang. Male Dominance is positively related to BMI (p < 0.05) but not with salivary hormone measures, while greater male Respect is associated with reduced salivary cortisol (p = 0.06) but not testosterone or BMI. Prosociality, conversely, is not significantly related to any evaluated measure. We discuss the evolutionary implications of our findings, with a focus on future directions for investigating the biocultural interface of health in this population.Men’s reproductive ecology and diminished hormonal regulation of skeletal muscle phenotype: An analysis of between- and within-individual variation among rural Polish men
Human life history is characterized by several distinctive features—sexual division of labor, prolonged care of altricial young, multiple dependents of different ages, and male provisioning. Testosterone has been suggested to mediate a trade-off between men’s reproduction and survival, through the regulation of sexually dimorphic musculature. This hypothesis predicts a relationship between testosterone and musculature in which mating effort, elevated testosterone, and dimorphic musculature covary positively. Testosterone is also posited to mediate a trade-off between mating and parenting effort, and accordingly, investing fathers show decreased testosterone production. Because men use their musculature not only in mating competition but also to support work demands, an important component of parenting effort, a relatively fixed relationship between testosterone and muscularity would seem maladaptive. We hypothesize that men’s parenting effort, speciﬁcally provisioning and subsistence activities, becomes a primary determinant of muscularity. Life history, anthropometric, and hormonal data were collected from 122 rural Polish men (at the Mogielica Human Ecology Study Site) during the summer harvest and for 103 of these participants in the winter. We found that fatherhood jointly predicted heavier workload and decreased testosterone, but positively predicted muscle mass and strength measures. Furthermore, within-individuals, men experienced intensiﬁed workload and suppressed testosterone during summer, along with a concomitant increase in muscularity and strength. These findings provide preliminary support for our model, termed the ‘Paternal Provisioning Hypothesis’. Between and within individuals, men’s provisioning and subsistence activities were robust predictors of muscular development and performance, whereas their testosterone levels had no appreciable effect on skeletal muscle phenotype.Testosterone, musculature, and development in Kanyawara chimpanzees and Tsimane forager-horticulturalists
Considerable evidence suggests that the steroid hormone testosterone mediates major life-history trade-offs in primates, promoting mating effort at the expense of parenting effort or survival. In many species, chronic shifts in testosterone production over the life course correlate with investment in male-male competition. Chimpanzees and humans represent interesting test cases, because although closely related, they maintain divergent mating systems. Chimpanzee males do not invest in pair bonds or paternal care. Consequently, across the lifespan, their testosterone levels are expected to track changes in (1) behavioral investment in dominance striving, and (2) investment in sexually dimorphic musculature employed in male-male competition. Humans, by contrast, are expected to show weaker associations between testosterone and musculature, because the latter is important not only for male competition, but for men’s work provisioning wives and children. We assayed >7000 chimpanzee and >3350 Tsimane urine samples for testosterone, creatinine, and specific gravity, in the same laboratory using the same assay methods. Male chimpanzees showed peak acceleration in testosterone increase at age 6, peak velocity at age 10, and peak deceleration at age 14, reaching adult levels by 15-16, when they began to challenge other adult males. Adult levels of testosterone were achieved 3 years later than in captivity, likely reflecting energetic constraints in the wild. Indirect measures of muscle mass followed a similar pattern, and were highly correlated with testosterone. As predicted, Tsimane men exhibited a weaker correlation, with testosterone accounting for half as much variance in the muscle mass measure as in the chimpanzee sample.Dads and cads? Male reproductive success, androgen profiles, and male-infant social bonds in wild mountain gorillas (Gorilla beringei beringei)
Male reproductive strategies are often reduced to a ‘dad versus cad’ dichotomy. When paternity certainty is high and mating opportunities scarce, theory predicts high levels of paternal investment; if paternity certainty is low and/or access to mating opportunities plentiful, male parenting is expected to be scarce. However, conflict between mating and parenting behavior is not equally strong across ecologies and social structures. Wild mountain gorillas (Gorilla beringei) have variable paternity certainty and a morphology suggestive of intense male contest competition. Despite this, relationships between males and infants are an important component of group structure, likely because males protect infants from infanticide and predation. Using data from gorilla groups monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center, we evaluated 1) the relationship between male-infant social bond strength and males’ reproductive success, and 2) the relationship between male-infant social bonds and males’ fecal androgen metabolite levels. Higher testosterone levels are generally correlated with increased aggression and mating activity, which are typically considered incompatible with parenting behavior. After controlling for male age and rank, males who had the strongest social bonds with infants were also the males with the highest reproductive success. There was no relationship between strength of male-infant social bonds and fecal androgen metabolite levels. Results demonstrate that reductive descriptions of male reproductive strategies may obscure important connections between mating and parenting effort, and highlight the need for additional data on the relationship between androgen activity, mating, and parenting in multimale/multifemale social systems.