Testosterone and reproductive effort in male primates

Effects of combined postweaning social isolation and ketamine administration on schizophrenia-like behaviour in male Sprague Dawley rats

The pathophysiology behind negative and cognitive symptoms of schizophrenia is not well understood, thus limiting the effectiveness of treatment on these symptoms. Developing reliable animal model of schizophrenia is vital to advance our understanding on the neurobiological basis of the disorder. Double hit is used to refer to the use of two schizophrenia inducing interventions viz ketamine exposure and social isolation. In this study we aim to investigate the robustness of double hit model of schizophrenia in inducing negative and cognitive symptoms of schizophrenia. On postnatal day (PND) 23, thirty-two male Sprague Dawley rats were randomly grouped into four equal groups as follows: group housed + saline (GH), group housed + ketamine (GHK), isolated + saline (SI), and isolated + ketamine (SIK). A single ketamine dose (16 mg/kg) was administered 3 times a week for four weeks. Isolated animals were housed singly throughout the study. The following behavioural tests were carried out: elevated plus maze, three chamber social interaction, resident intruder tests, and novel object recognition (NOR). The SIK group exhibited high anxiety levels, with increased ACTH, corticosterone and norepinephrine concentration when compared to the other groups. The SIK animals also presented with reduced social interaction and decreased oxytocin concentration. SIK rats were more aggressive towards a juvenile intruder but had low testosterone concentration. The SIK group or double hit model showed impaired visual learning and memory and increased expression of proinflammatory cytokines. This suggest that the double hit model is more robust in inducing negative and cognitive symptoms of schizophrenia than each treatment alone.

Sirt1 m6A modification-evoked Leydig cell senescence promotes Cd-induced testosterone decline

Diminished testosterone levels have been documented as a key factor in numerous male health disorders. Both human and animal studies have consistently demonstrated that cadmium (Cd), a pervasive environmental heavy metal, results in decreased testosterone levels. However, the exact mechanism through which Cd interferes with testosterone synthesis remains incompletely elucidated. This research sought to examine the impact of cellular senescence on Cd-suppressed testosterone synthesis. We also investigated the related m6A modification mechanism. The results demonstrated that Cd (100 mg/L) led to a decrease in testosterone levels, along with downregulated expression of testosterone synthase in C57BL/6 N male mice. Furthermore, Cd significantly increased β-galactosidase staining intensity, senescence-related proteins, and senescence-related secretory phenotypes in mouse testicular Leydig cells. Subsequent investigations revealed that Cd decreased the mRNA and protein levels of NAD-dependent deacetylase Sirtuin-1 (SIRT1) in Leydig cells. Mechanistically, mice treated with resveratrol (50 mg/kg), a specific SIRT1 activator, mitigated Leydig cell senescence and reversed Cd-reduced testosterone levels in mouse testes. These effects were also restored by SIRT1 overexpression in Leydig cells. Additionally, we found that Cd increased the level of methyltransferase enzyme METTL3 and Sirt1 m6A modification in Leydig cells. Mettl3 siRNA effectively restored Cd-enhanced Sirt1 m6A level and reversed Cd-downregulated Sirt1 mRNA expression in Leydig cells. Overall, our findings suggest that Cd exposure inhibits testosterone synthesis via Sirt1 m6A modification-mediated senescence in mouse testes. These results offer an experimental basis for investigating the causes and potential treatments of hypotestosteronemia induced by environmental factors.

The relationship between remnant cholesterol and the risk of testosterone deficiency in US adults: a cross-sectional study based on the NHANES database

Background

Testosterone deficiency (TD) is an urgent health issue that requires attention, associated with various adverse health outcomes including cardiovascular diseases (CVD) and metabolic syndrome. Remnant cholesterol (RC) has emerged as a potential biomarker for cardiovascular risk, but its relationship with testosterone levels and TD has not been thoroughly investigated. This study aims to explore the association between RC and TD in adult American males using data from the National Health and Nutrition Examination Survey (NHANES).

Methods

This cross-sectional study utilized data from three NHANES cycles (2011-2016), including 2,848 adult male participants. RC was calculated as total cholesterol minus high-density lipoprotein cholesterol (HDL) and low-density lipoprotein cholesterol (LDL). TD was defined as total testosterone levels below 300 ng/dL. Multivariable linear and logistic regression analyses, as well as smooth curve fitting and generalized additive models, were performed to assess the associations between RC and total testosterone levels and TD, adjusting for potential confounders. Subgroup analyses were conducted based on age, BMI, smoking status, diabetes, hypertension, CVD, and chronic kidney disease (CKD).

Results

Higher RC levels were significantly associated with lower total testosterone levels (β = -53.87, 95% CI: -77.69 to -30.06, p<0.001) and an increased risk of TD (OR = 1.85, 95% CI: 1.29 to 2.66, p=0.002) in fully adjusted models. When RC was analyzed as quartiles, participants in the highest quartile (Q4) had significantly lower total testosterone levels (β = -62.19, 95% CI: -93.62 to -30.76, p<0.001) and higher odds of TD (OR = 2.15, 95% CI: 1.21 to 3.84, p=0.01) compared to those in the lowest quartile (Q1). Subgroup analyses revealed consistent associations across different age groups, particularly strong in participants over 60 years, and in never smokers. The associations remained significant in both hypertensive and non-hypertensive groups, as well as in those with and without CKD. No significant interactions were found across subgroups.

Conclusion

This study demonstrates a significant inverse association between RC levels and total testosterone levels, along with a positive association with the risk of TD. These findings suggest that RC could serve as a valuable biomarker for early identification of individuals at risk for TD. Future longitudinal studies are needed to confirm these findings and explore the underlying mechanisms.

Reproductive endocrinology and aging in human males: An evolutionary perspective

Due to its important role in fertility, reproductive endocrine function has been subject to natural selection in all organisms including human males. Moreover, reproductive endocrine function is subject to change as males age. Indeed, the biology of aging is also subject to natural selection. As males age, hormone function such as variation in testosterone can change as the result of general somatic degradation. However these changes are not universal and can differ between human male populations depending on lifestyle and ecological context. The degree to which this variation is adaptive remains an open question but recent evolutionary anthropology research has provided some clarity. While knowledge of evolutionary approaches has limitations, the benefits of understanding the origins and comparative context of reproductive endocrine function in older human males are significant. This paper discusses our present comprehension of reproductive endocrinology and aging in human males, with a focus on human diversity across varied lifestyles, ecologies, and environments. In addition, comparative great ape research is examined. Current research challenges and future directions related to the importance of evolutionary biology and human diversity for understanding human male aging are discussed.

Activational and organizational effects of testosterone on the number of mating partners and reproductive success in males of a social rodent

The timing of exposure to the steroid hormone, testosterone, produces activational and organizational effects in vertebrates. These activational and organizational effects are hypothesized to relate with the number of female mating partners and reproductive success in males. We tested this hypothesis by examining 151 wild degu (Octodon degus) males across a 10-year study. We quantified the association between adult serum testosterone levels (i.e., an indirect index of adult activational effects) and anogenital distance (AGD) length (i.e., a direct index of fetal organizational effects), and their interaction on the number of female mating partners and reproductive success. We found no evidence of an association between adult male serum testosterone levels and the number of female mating partners, or between adult male serum testosterone levels and reproductive success. However, male AGD was positively associated with reproductive success, but not so with the number of female mating partners. Additionally, the positive association between male AGD and male reproductive success was mediated by the number of mates. Our findings do not support major roles of activational or organizational effects of testosterone on the number of female mating partners and its consequences on male reproductive success. Instead, our results suggest that compared with individual male attributes, the female social environment plays a more important role in driving male reproductive success.

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.

Association between life's essential 8 and testosterone deficiency in US men: findings from national health and nutrition examination survey (NHANES) 2011-2016

Background

Testosterone deficiency (TD) is closely associated with cardiovascular diseases (CVD). We intended to explore the association of Life's Essential 8 (LE8), the recently updated measurement of cardiovascular health, with the prevalence of TD among US male adults.

Methods

The population-based cross-sectional study selected male adults aged 20 years or older from the National Health and Nutrition Examination Survey (NHANES) from 2011 to 2016. According to the American Heart Association definitions, the LE8 score was measured on a scale of 0-100, and divided into health behavior and health factor scores, simultaneously. Furthermore, these scores were categorized into low (0-49), moderate (50-79), and high (80-100) classifications. TD is defined as a total testosterone level below 300ng/dL. Correlations were investigated by weighted multivariable logistic regression, and the robustness of the results were verified by subgroup analysis.

Results

A total of 4971 male adults with an average age of 47.46 ± 0.41 years were eligible for the final analyses, of whom 1372 were determined to have TD. The weighted mean LE8 score of the study population was 68.11 ± 0.41. After fully adjusting potential confounders, higher LE8 scores were significantly associated with low risk of TD (odd ratio [OR] for each 10-point increase, 0.79; 95% CI, 0.71-0.88) in a linear dose-response relationship. Similar patterns were also identified in the association of health factor scores with TD (OR for each 10-point increase, 0.74; 95% CI, 0.66-0.83). These results persisted when LE8 and health factor scores was categorized into low, moderate, and high groups. The inversed association of LE8 classifications and TD remained statistically significant among older, obese, and men without CVD.

Conclusions

LE8 and its health factor subscales scores were negatively associated with the presence of TD in linear fashions. Promoting adherence to optimal cardiovascular health levels may be advantageous to alleviate the burden of TD.

Association between weight-adjusted waist index and testosterone deficiency in adult American men: findings from the national health and nutrition examination survey 2013-2016

BACKGROUND

Testosterone deficiency (TD) and obesity are globally recognized health concerns, with a bidirectional causal relationship between them. And a newly discovered obesity indicator, the Weight-Adjusted-Waist Index (WWI), has been proposed, demonstrating superior adiposity identification capability compared to traditional body mass index (BMI) and waist circumference (WC) indicators. Therefore, we present the inaugural investigation into the associations of WWI with total testosterone levels and the risk of TD.

METHODS

Data restricted to the National Health and Nutrition Examination Survey (NHANES) between 2013 and 2016 were analyzed. Only males aged > 20 years who completed body measures and underwent serum sex hormone testing were potentially eligible for analysis. Weighted multivariable linear regression and logistic regression analyses were employed to investigate the relationships between WWI and total testosterone levels, and the risk of TD, respectively. Smooth curve fittings and weighted generalized additive model (GAM) regression were conducted to examine the linear relationship among them. Additionally, subgroup analyses with interaction tests were performed to assess the stability of the results.

RESULTS

Finally, a total of 4099 participants with complete data on testosterone and WWI were included in the formal analysis. The mean age of study participants was 46.74 ± 0.35 years with a TD prevalence of 25.54%. After adjusting all potential confounders, the continuous WWI displayed a negative linear relationship with total testosterone levels (β=-61.41, 95%CI: -72.53, -50.29, P < 0.0001) and a positive linear relationship with risk of TD (OR = 1.88, 95%CI: 1.47, 2.39, P < 0.0001). When WWI was transformed into quartiles as a categorical variable, participants in Q4 exhibited lower total testosterone levels (β=-115.4, 95%CI: -142.34, -88.45, P < 0.0001) and a higher risk of TD (OR = 3.38, 95% CI: 2.10, 5.44, P < 0.001). These associations remained stable in subgroup analyses without significant interaction (all P for interaction > 0.05).

CONCLUSIONS

This investigation firstly unveiled a negative linear association between WWI and total testosterone levels, coupled with a positive linear relationship with the prevalence of TD in U.S. male adults aged 20 years and older. Further studies are needed to validate the potential utility of WWI for the early identification and timely intervention of TD.

Association between life's essential 8 and testosterone deficiency in men: NHANES 2011-2016

Background

Serum testosterone is intrinsically associated to cardiovascular disease. Our aim is to explore the relationship between the recently updated cardiovascular health measurement, known as Life's Essential 8 (LE8), and the prevalence of testosterone deficiency (TD) in adult males in the United States.

Methods

Study data was obtained from the National Health and Nutrition Examination Survey (NHANES) from 2011 to 2016. A weighted multivariate logistic regression model was applied to evaluate the correlation between LE8 and testosterone deficiency. Restricted Cubic Spline (RCS) was employed to explore its non-linear relationship. In addition, a stratified analysis was conducted.

Results

The final analysis included 2332 participants from NHANES from 2011 to 2016. After adjusting for confounding factors, the odds ratios (ORs) and 95% confidence intervals (CIs) for testosterone deficiency in participants with moderate and higher LE8 scores compared to the lowest LE8 scores were 0.59 (0.38-0.92) and 0.38 (0.19-0.76), respectively. The results of subgroup analysis showed that LE8 score was significantly associated with TD among young and middle-aged participants.

Conclusion

A lower LE8 score is related to a higher incidence of testosterone deficiency, especially in young and middle-aged men. Further research is necessary to explore the potential mechanisms between them.

Negative correlation between metabolic score for insulin resistance index and testosterone in male adults

BACKGROUND

Insulin resistance (IR) is strongly correlated with the decreased deficiency of testosterone levels in males. The metabolic score for insulin resistance (METS-IR) index is regarded as an innovative measure for the assessment on IR. The research aims to explore the correlation between the METS-IR index and the testosterone levels in males.

METHODS

In this study, a cross-sectional design was made through the data obtained from the National Health and Nutrition Examination Survey (NHANES) from 2013 to 2020. Besides, the METS-IR index was derived from serum triglyceride levels, fasting plasma glucose, HDL-C and BMI.

RESULTS

A total of 2082 participants were included in the final analysis. After controlling for confounding variables, it was found that METS-IR was independently and negatively correlated with testosterone levels (β = -3.88, 95% CI = -4.49, -3.27, P < 0.001). As shown by the generalized smooth curve fitting, METS-IR had a linear correlation with testosterone levels without threshold or saturation effects, which was consistently observed across all subgroups through stratified analysis (all P > 0.05). As revealed by the analysis on the ROC curve, the area under the curve (AUC) for the METS-IR index (0.732, 95% CI = 0.705, 0.760) was significantly larger than that of homeostatic model assessment of insulin resistance (HOMA-IR), TG/HDL ratio, triglyceride-glucose index (TyG) and body mass index (BMI).

CONCLUSION

The findings suggest a negative relationship between the METS-IR index and the testosterone levels in male adults. Furthermore, the METS-IR index demonstrates superior predictive ability for testosterone deficiency in comparison to HOMA-IR, TG/HDL ratio, TyG and BMI.

The role of exogenous testosterone and social environment on the expression of sociosexuality and status-seeking behaviors in young Chilean men

Testosterone plays an important role as a social hormone. Current evidence suggests that testosterone is positively related to sociosexuality increasing the psychological attitudes toward investing in short-term versus long-term mating and promotes status-seeking behaviors both by dominance and prestige. In addition, the social environment may play an important role in the expression of mating effort through changes in sociosexuality and status-seeking behaviors. However, the causal relationships among the mentioned variables are still debated. We employed a double-blind, placebo-controlled within-individual design, in order to test and integrate the proposed causal relationships between testosterone and social environment over short-term and long-term mating orientation and dominant and prestigious status-seeking behaviors in a sample of 95 young Chilean men. We did not find evidence that the administration of exogenous testosterone increased short-term or decreased long-term mating orientation as expected. Moreover, exogenous testosterone did not affect either aggressive or cooperative behavior failing to support the social status hypothesis. We also did not find any relationship between short or long-term mating orientation with status-seeking behaviors. Finally, we found support for the effect of social environment on sociosexual attitudes but not over status-seeking behaviors. Thus, men reported higher levels of short-term mating orientation in the presence of a woman compared to a man and no differences were found for long-term mating orientation. We argue that sociosexuality may be expressed flexibly, but contextual factors such as the presence of women seem more important than changes in testosterone levels.

Toxoplasma gondii infection and testosterone alteration: A systematic review and meta-analyses

BACKGROUND

Toxoplasma gondii (T. gondii) is a worldwide distributed protozoan parasite which has infected a wide range of warm-blooded animals and humans. The most common form of T. gondii infection is asymptomatic (latent); nevertheless, latent toxoplasmosis can induce various alterations of sex hormones, especially testosterone, in infected humans and animals. On the other hand, testosterone is involved in behavioral traits and reproductive functions in both sexes. Hence, the purpose of this systematic review is to summarize the available evidence regarding the association between T. gondii infection and testosterone alteration.

METHODS

In the setting of a systematic review, an electronic search (any date to 10 January 2023) without language restrictions was performed using Science Direct, Web of Science, PubMed, Scopus, and Google Scholar. The PRISMA guidelines were followed. Following the initial search, a total of 12,306 titles and abstracts were screened initially; 12,281 were excluded due to the lack of eligibility criteria or duplication. Finally, 24 articles met the included criteria. A mean±standard deviation (SD) was calculated to assess the difference of testosterone between T. gondii positive and T. gondii negative humans. The possibility of publication bias was assessed using Egger's regression. P-value < 0.05 was considered statistically significant.

RESULTS

This systematic review identified 24 articles (18 studies in humans and six studies in animals). Most human studies (13 out of 19) reported an increased level of testosterone following latent toxoplasmosis in males, while three studies reported decreased levels and two studies reported an insignificant change. Eleven articles (seven datasets in males and seven datasets in females) were eligible to be included in the data synthesis. Based on the random-effects model, the pooled mean± SD of testosterone in T. gondii positive than T. gondii negative was increased by 0.73 and 0.55 units in males and females, respectively. The Egger's regression did not detect a statistically significant publication bias in males and females (p = value = 0.95 and 0.71), respectively. Three studies in male animals (rats, mice, and spotted hyenas) and two studies in female animals (mice and spotted hyenas) reported a decline in testosterone in infected compared with non-infected animals. While, one study in female rats reported no significant changes of testosterone in infected than non-infected animals. Moreover, two studies in male rats reported an increased level of testosterone in infected than non-infected animals.

CONCLUSIONS

This study provides new insights about the association between T. gondii infection and testosterone alteration and identifies relevant data gaps that can inform and encourage further studies. The consequence of increased testosterone levels following T. gondii infection could partly be associated with increased sexual behavior and sexual transmission of the parasite. On the other hand, declining testosterone levels following T. gondii infection may be associated with male reproductive impairments, which were observed in T. gondii-infected humans and animals. Furthermore, these findings suggest the great need for more epidemiological and experimental investigations in depth to understand the relationship between T. gondii infection and testosterone alteration alongside with future consequences of testosterone alteration.

Dichloroacetic acid and trichloroacetic acid as disinfection by-products in drinking water are endocrine-disrupting chemicals

Dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) are two typical non-volatile disinfection by-products (DBPs) found in drinking water. Increasing evidence has demonstrated that they show reproductive toxicity. However, whether they might have endocrine disrupting properties remains largely unknown. To discover this, we treated male mice or pregnant mice with 0, 1-, 102-, 103-, 104-, or 5 × 104-fold maximal concentration level (MCL) of DCAA or TCAA in drinking water. In male mice, the levels of testosterone in serum and androgen receptor (AR) in testis were declined with ≥ 103-fold MCL of DCAA (26.4 mg/kg/d) or TCAA (52.7 mg/kg/d). In pregnant mice, miscarriage rates were increased with ≥ 104-fold MCL of DCAA (264 mg/kg/d) or ≥ 103-fold MCL of TCAA. The levels of FSH in serum were increased and those of estradiol and progesterone were reduced with ≥ 103-fold MCL of DCAA or TCAA. The protein levels of estrogen receptors (ERα and ERβ) in ovary were reduced with ≥ 102-fold MCL of DCAA (2.64 mg/kg/d) or TCAA (5.27 mg/kg/d). Exposure to some certain fold MCL of DCAA or TCAA also altered the protein levels of ERα and ERβ in uterus and placenta. Exposure to 5 × 104-fold MCL of both DCAA and TCAA showed the combined effects. Therefore, both DCAA and TCAA could be considered as novel reproductive endocrine disrupting chemicals, which might be helpful for further assessment of the toxicological effects of DCAA and TCAA and the awareness of reproductive endocrine disrupting properties caused by DCAA and TCAA in drinking water.

Androgen receptors and sociosexual behaviors in mammals: The limits of generalization

Circulating testosterone is easily aromatized to estradiol and reduced to dihydrotestosterone in target tissues and elsewhere in the body. Thus, the actions of testosterone can be mediated either by the estrogen receptors, the androgen receptor or by simultaneous action at both receptors. To determine the role of androgens acting at the androgen receptor, we need to eliminate actions at the estrogen receptors. Alternatively, actions at the androgen receptor itself can be eliminated. In the present review, I will analyze the specific role of androgen receptors in male and female sexual behavior as well as in aggression. Some comments about androgen receptors and social recognition are also made. It will be shown that there are important differences between species, even between strains within a species, concerning the actions of the androgen receptor on the behaviors mentioned. This fact makes generalizations from one species to another or from one strain to another very risky. The existence of important species differences is often ignored, leading to many misunderstandings and much confusion.

Evidence for adolescent length growth spurts in bonobos and other primates highlights the importance of scaling laws

Adolescent growth spurts (GSs) in body length seem to be absent in non-human primates and are considered a distinct human trait. However, this distinction between present and absent length-GSs may reflect a mathematical artefact that makes it arbitrary. We first outline how scaling issues and inappropriate comparisons between length (linear) and weight (volume) growth rates result in misleading interpretations like the absence of length-GSs in non-human primates despite pronounced weight-GSs, or temporal delays between length- and weight-GSs. We then apply a scale-corrected approach to a comprehensive dataset on 258 zoo-housed bonobos that includes weight and length growth as well as several physiological markers related to growth and adolescence. We found pronounced GSs in body weight and length in both sexes. Weight and length growth trajectories corresponded with each other and with patterns of testosterone and insulin-like growth factor-binding protein 3 levels, resembling adolescent GSs in humans. We further re-interpreted published data of non-human primates, which showed that aligned GSs in weight and length exist not only in bonobos. Altogether, our results emphasize the importance of considering scaling laws when interpreting growth curves in general, and further show that pronounced, human-like adolescent length-GSs exist in bonobos and probably also many other non-human primates.

Testosterone administration modulates inequality aversion in healthy males: evidence from computational modeling

Fairness concerns play a prominent role in promoting cooperation in human societies. Social preferences involving fairness concern have been associated with individual testosterone levels. However, the causal effects of testosterone administration on fairness-related decision making remain to be elucidated. Here, we used a randomized, double-blind, between-participant design and administered testosterone or placebo gel to 120 healthy young men. Three hours after administration, participants performed a modified Dictator Game from behavioral economics, in which they were asked to choose one of two monetary allocations between themselves and anonymous partners. Participants were either in a position of advantageous inequality (i.e., endowed with more than others) or disadvantageous inequality (i.e., endowed with less than others). Computational modeling showed that inequality-related preferences explained behavior better than competing models. Importantly, compared with the placebo group, the testosterone group showed significantly reduced aversion to advantageous inequality but enhanced aversion to disadvantageous inequality. These findings suggest that testosterone facilitates decisions that prioritize selfish economic motives over fairness concerns, which in turn may boost status-enhancing behaviors.

The Ancestral Modulation Hypothesis: Predicting Mechanistic Control of Sexually Heteromorphic Traits Using Evolutionary History

AbstractAcross the animal kingdom there are myriad forms within a sex across, and even within, species, rendering concepts of universal sex traits moot. The mechanisms that regulate the development of these trait differences are varied, although in vertebrates, common pathways involve gonadal steroid hormones. Gonadal steroids are often associated with heteromorphic trait development, where the steroid found at higher circulating levels is the one involved in trait development for that sex. Occasionally, there are situations in which a gonadal steroid associated with heteromorphic trait development in one sex is involved in heteromorphic or monomorphic trait development in another sex. We propose a verbal hypothesis, the ancestral modulation hypothesis (AMH), that uses the evolutionary history of the trait-particularly which sex ancestrally possessed higher trait values-to predict the regulatory pathway that governs trait expression. The AMH predicts that the genomic architecture appears first to resolve sexual conflict in an initially monomorphic trait. This architecture takes advantage of existing sex-biased signals, the gonadal steroid pathway, to generate trait heteromorphism. In cases where the other sex experiences evolutionary pressure for the new phenotype, that sex will co-opt the existing architecture by altering its signal to match that of the original high-trait-value sex. We describe the integrated levels needed to produce this pattern and what the expected outcomes will be given the evolutionary history of the trait. We present this framework as a testable hypothesis for the scientific community to investigate and to create further engagement and analysis of both ultimate and proximate approaches to sexual heteromorphism.

Sex differences in distribution and identity of aromatase gene expressing cells in the young adult rat brain

BACKGROUND

Aromatase catalyzes the synthesis of estrogens from androgens. Knowledge on its regional expression in the brain is of relevance to the behavioral implications of these hormones that might be linked to sex differences in mental health. The present study investigated the distribution of cells expressing the aromatase coding gene (Cyp19a1) in limbic regions of young adult rats of both sexes, and characterized the cell types expressing this gene.

METHODS

Cyp19a1 mRNA was mapped using fluorescent in situ hybridization (FISH). Co-expression with specific cell markers was assessed with double FISH; glutamatergic, gamma-aminobutyric acid (GABA)-ergic, glial, monoaminergic, as well as interneuron markers were tested. Automated quantification of the cells expressing the different genes was performed using CellProfiler. Sex differences in the number of cells expressing Cyp19a1 was tested non-parametrically, with the effect size indicated by the rank-biserial correlation. FDR correction for multiple testing was applied.

RESULTS

In the male brain, the highest percentage of Cyp19a1+ cells was found in the medial amygdaloid nucleus and the bed nucleus of stria terminalis, followed by the medial preoptic area, the CA2/3 fields of the hippocampus, the cortical amygdaloid nucleus and the amygdalo-hippocampal area. A lower percentage was detected in the caudate putamen, the nucleus accumbens, and the ventromedial hypothalamus. In females, the distribution of Cyp19a1+ cells was similar but at a lower percentage. In most regions, the majority of Cyp19a1+ cells were GABAergic, except for in the cortical-like regions of the amygdala where most were glutamatergic. A smaller fraction of cells co-expressed Slc1a3, suggesting expression of Cyp19a1 in astrocytes; monoaminergic markers were not co-expressed. Moreover, sex differences were detected regarding the identity of Cyp19a1+ cells.

CONCLUSIONS

Females show overall a lower number of cells expressing Cyp19a1 in the limbic brain. In both sexes, aromatase is expressed in a region-specific manner in GABAergic and glutamatergic neurons. These findings call for investigations of the relevance of sex-specific and region-dependent expression of Cyp19a1 in the limbic brain to sex differences in behavior and mental health.

The Impact of Visceral Adiposity on Testosterone Levels in American Adult Men: A Cross-Sectional Analysis

BACKGROUND Testosterone decline and deficiency importantly affect men's health, and may be associated with excessive deposition of visceral adipose tissue. This study was conducted to explore the association between visceral adiposity index (VAI) and testosterone level. MATERIAL AND METHODS A total of 1551 participants from the NHANES 2013-2013 cycle and 2015-2016 cycle were selected for our analyses. The VAI index was calculated based on waist circumference (WC), body mass index (BMI), triglyceride (TG), and high-density lipoprotein cholesterol (HDL-c), and serum testosterone was measured by isotope dilution liquid chromatography tandem mass spectrometry. Multivariable adjusted linear and logistic regression were utilized to investigate the associations between VAI index and testosterone level and testosterone deficiency, respectively. Additionally, subgroup analyses were performed to identify sensitive populations. RESULTS A total of 1551 participants with mean VAI index of 1.95±0.08 were eligible for our analysis. After adjusting for all potential cofounders, men with higher VAI index displayed a lower level of total testosterone level (ß: -11.74, 95% CI: -17.33, -6.15, P<0.0001), and higher risk of testosterone deficiency (OR: 1.24, 95% CI: 1.09, 1.40, P=0.0022). Comparing to VAI quartile 1, quartile 4 showed the most decreased testosterone level (ß: -94.59, 95% CI: -130.04, -59.14, P<0.0001), and highest risk of testosterone deficiency (OR: 5.07, 95% CI: 2.41,10.63, P<0.0001). Subgroup analysis demonstrated that VAI index was strongly related to testosterone level and testosterone deficiency in aged and obese men. CONCLUSIONS Men with higher VAI index displayed lower testosterone levels and higher risk of testosterone deficiency, especially in aged men and obese men.

Lean muscle mass, not aggression, mediates a link between dominance rank and testosterone in wild male chimpanzees

Testosterone promotes mating effort, which involves intraspecific aggression for males of many species. Therefore, males with higher testosterone levels are often thought to be more aggressive. For mammals living in multimale groups, aggression is hypothesized to link male social status (i.e. dominance rank) and testosterone levels, given that high status predicts mating success and is acquired partly through aggressive intragroup competition. In male chimpanzees, Pan troglodytes, dominance rank has been repeatedly linked to interindividual variation in testosterone levels, but evidence directly linking interindividual variation in testosterone and aggression is lacking. In the present study, we test both aggression levels and lean muscle mass, as measured by urinary creatinine, as links between dominance rank and testosterone levels in a large sample of wild male chimpanzees. Multivariate analyses indicated that dominance rank was positively associated with total rates of intragroup aggression, average urinary testosterone levels and average urinary creatinine levels. Testosterone was positively associated with creatinine levels but negatively associated with total aggression rates. Furthermore, mediation analyses showed that testosterone levels facilitated an association between dominance rank and creatinine levels. Our results indicate that (1) adult male chimpanzees with higher average testosterone levels are often higher ranking but not more aggressive than males with lower testosterone and (2) lean muscle mass links dominance rank and testosterone levels in Ngogo males. We assert that aggression rates are insufficient to explain links between dominance rank and testosterone levels in male chimpanzees and that other social variables (e.g. male-male relationship quality) may regulate testosterone's links to aggression.

Hormonal responses to mating competition in male Tonkean macaques

Glucocorticoid and androgen hormones play a prominent role in male reproductive effort. Their production usually increases in non-human primates during mating competition, which may include rivalry for access to receptive females, struggles for high dominance rank, or social pressure on low-ranking individuals. It is generally assumed that glucocorticoids and androgens are associated with mating challenges rather than dominance status, but the involvement of multiple factors makes it difficult to disentangle the two. In this regard, Tonkean macaques provide a suitable model because they are characterized by relaxed dominance and year-round breeding, meaning that there is typically no more than one receptive female in a group, and thus first-ranking males can easily monopolize her. We studied two captive groups of Tonkean macaques over an 80-month period, recording the reproductive status of females, collecting urine from males and sampling behaviors in both sexes. Male urinary hormone concentrations could be affected by increased competition caused by the mating period, the number of males and the degree of female attractiveness. The highest increases in androgens were recorded in males performing female mate-guarding. Despite the importance of dominance status in determining which males can mate, we found no significant effect of male rank on glucocorticoids and only a marginal effect on androgens during mate-guarding. Both types of hormones were more directly involved in the mating effort of males than in their dominance status. Our results show that their function can be understood in light of the particular competitive needs generated by the species-specific social system.

Tradeoffs between mating effort and parenting effort in a polygynandrous mammal

Reproductive strategies are defined by expenditures of time and energy devoted to mating effort, which increases mating opportunities, and parenting effort, which enhances the survival of offspring. We examine tradeoffs between mating effort and parenting effort in male olive baboons, Papio anubis, a species in which males compete for mating opportunities, but also form ties to lactating females (primary associations) that represent a form of parenting effort. Males that are involved in more primary associations invest less in mating effort than males who are involved in fewer primary associations. Males that are involved in more primary associations play a smaller role in establishing proximity to their primary associates than other males, suggesting that males operate under temporal constraints. There is also some evidence that involvement in primary associations negatively affects paternity success. Taken together, the data suggest that males face tradeoffs between mating effort and parenting effort.

A negative association between triglyceride glucose-body mass index and testosterone in adult males: a cross-sectional study

Background and objectives

Insulin resistance (IR) is closely related to the decline or deficiency of testosterone in males. Triglyceride glucose-body mass (TyG-BMI) is considered to be a novel indicator of IR. We conducted this analysis to investigate the association between TyG-BMI and male testosterone, and to explore whether its ability to predict testosterone deficiency is superior to HOMA-IR and TyG.

Methods

This was a cross-sectional study using data from the National Health and Nutrition Examination Survey (NHANES, 2011-2016). The TyG-BMI index was calculated from serum triglyceride, fasting plasma glucose and BMI. The association of TyG-BMI with male testosterone was estimated by weighted multivariable regression.

Results

We included 3394 participants for the final analysis. After adjusting for confounders, TyG-BMI was found to show an independent negative association with testosterone (β=-1.12, 95%CI: -1.50, -0.75, P<0.0001). Multivariate-adjusted beta also showed testosterone levels were significantly lower in the two highest TyG-BMI group (Q3, Q4) compared to the lowest group (Q1). Similar results were seen in all of the subgroup populations by stratified analysis (all P-interaction >0.05). Furthermore, ROC curve analysis indicated that the area under the curve of TyG-BMI index (0.73, 95% CI: 0.71, 0.75) was larger than that of HOMA-IR index (0.71, 95% CI: 0.69, 0.73) and TyG index (0.66, 95% CI: 0.64, 0.68).

Conclusion

Our result suggested a negative association between TyG-BMI index and testosterone in adult males. The predictability of the TyG-BMI index for testosterone deficiency is better than that of HOMA-IR index and TyG index.

Testing the Challenge Hypothesis in Stumptail Macaque Males: The Role of Testosterone and Glucocorticoid Metabolites in Aggressive and Mating Behavior

The "challenge hypothesis" predicts higher male-male aggressive behavior along with increases in testosterone levels during times of reproductive challenges and social instability. In addition, in some primate species, higher glucocorticoid levels can be observed as well, but this is usually modulated by dominance rank. We studied rank-related aggressive behavior, mating activity, and fecal testosterone and glucocorticoid metabolites (fTm and fGCm) in male stumptail macaques (Macaca arctoides) in order to test some predictions of the "challenge hypothesis". Over a 20-month period, we collected data on aggressive behavior and copulation, as well as fecal samples (n = 700) to quantify fTm and fGCm in seven adult stumptail males living in captivity. During periods of mating activity, male-to-male aggression increased in higher- and middle-ranking males. Neither fTm nor fGCm levels predicted male-to-male aggression. fGCm levels (but not fTm) were positively associated with male-to-female aggression; however, this association was pronounced during periods of mating activity. fGCm levels differed according to social rank, with middle-ranking males having the highest levels. Both hormones were higher during periods of mating activity, but only in higher- and middle-ranking males. Taken together, our findings partially support the challenge hypothesis in a non-seasonal primate and shed some light on the unique social and mating system of the stumptail macaque.

No Compelling Evidence that Self-Reported Personality Traits Explain Basal Testosterone and Cortisol’s Associations with Status-Relevant Behavior

Objective

A goal of behavioral neuroendocrinology is to understand how basal hormone levels relate to behavior. Studies of human participants sometimes measure self-reported personality traits, in addition to or instead of direct behavioral observation. Although personality traits often predict their respective behaviors, whether personality explains hormone-behavior relationships remains unclear.

Methods

We obtained data from eight previous studies (total N = 985) that examined baseline testosterone and cortisol as predictors of status-relevant behavior (competitiveness, dominance, risk-taking, aggression, affiliation, and social status). We tested whether the previously reported hormone-behavior relationships are mediated by self-reported personality traits (e.g., trait dominance, prestige, extraversion). As a secondary research question, we also tested whether trait dominance moderated the testosterone-behavior relationships.

Results

As expected, self-reported personality traits often predicted status-relevant behaviors, but there was little evidence that traits also correlated with basal testosterone or the testosterone × cortisol interaction. Across all eight studies, personality traits did not significantly mediate hormone-behavior relationships. Indeed, the effect sizes of the hormone-behavior relationships were robust to the inclusion of personality traits as covariates. Further, we did not find strong or consistent evidence that trait dominance moderates the testosterone-behavior association.

Conclusion

Results suggest that basal testosterone and cortisol predict status-related behavior independent of self-reported personality. We discuss how these results may have broader implications for the physiological mechanisms by which testosterone and cortisol influence behavior, a process that could be unconscious and automatic. We also discuss alternative explanations, limitations, and future directions.

Effects of testis testosterone deficiency on gene expression in the adrenal gland and skeletal muscle of ducks

1. Testosterone has an anabolic effect on skeletal muscle. The testes produce most of the testosterone in vivo, while the adrenal glands contribute smaller amounts. When testis testosterone is deficient the adrenal gland increases steroid hormone synthesis, which is referred to as compensatory testicular adaptation (CTA).2. To reveal the effects of testis testosterone deficiency on adrenal steroid hormones synthesis and skeletal muscle development, gene expression related to adrenal steroid hormones synthesis and skeletal muscle development were determined by RNA-seq.3. The results showed that castrating male ducks had significant effects on their body weight but no significant impact on cross-sectional area (CSA) or density of pectoral muscle fibres. In skeletal muscle protein metabolism, expression levels of the catabolic gene atrogin1/MAFbx and the anabolic gene eEF2 were significantly higher, with concomitant increases after castration. The adrenal glands' alteration of the steroid hormone 11β-hydroxylase (CYP11B1) was significantly lower following castration.4. Expression pattern analysis showed that the adrenal glands' glucocorticoid receptor (NR3C1/GR) had a potential regulatory relationship with the skeletal muscle-related genes (Pax7, mTOR, FBXO32, FOXO3, and FOXO4).5. The data showed that castration affected muscle protein metabolism, adrenal steroid and testosterone synthesis. In addition, it was speculated that, after castration, steroid hormones produced by the adrenal gland could have a compensatory effect, which might mediate the changes in skeletal muscle protein metabolism and development.

The Triglycerides and Glucose Index is not superior to HOMA-IR in predicting testosterone deficiency among adult males

BACKGROUND

Testosterone decline and deficiency in males have aroused increased attention in male health management, which might have a close relationship with insulin resistance (IR).

OBJECTIVES

We utilized a novel and practical IR indicator, the triglyceride-glucose (TyG) index, to investigate the association between the TyG index and serum testosterone in US adult males.

MATERIALS AND METHODS

This was a cross-sectional study based on data from the National Health and Nutrition Examination Survey (NHANES) 2013-2014 and 2015-2016. The TyG index was calculated from fasting plasma glucose and serum triglyceride, and serum testosterone was measured by isotope dilution liquid chromatography tandem mass spectrometry in NHANES.

RESULTS

A total of 2186 male participants aged more than 20 years old were included, and the mean TyG index was 8.72 ± 0.71. Participants with a higher TyG index showed a lower level of total testosterone (β = -45.83, 95% CI: -58.50, -33.15, p < 0.0001) and a higher risk of testosterone deficiency (OR = 1.80, 95% CI: 1.46, 2.21, p < 0.0001) after we adjusted for all potential cofounders. Males in TyG index tertile 3 had a 113% (95% CI: 1.50, 3.02, p < 0.0001) higher risk of testosterone deficiency than those in tertile 1. Subgroup analysis stratified by diabetes condition indicated that the TyG index might be a good predictor of testosterone decline or deficiency in either people with or without diabetes. However, ROC cure analysis indicated that a larger area under the curve was found in the homeostasis model assessment of insulin resistance (HOMA-IR) (0.71, 95% CI: 0.68,0.74) than in the TyG index (0.67, 95% CI: 0.64, 0.70).

CONCLUSIONS

Males with a higher TyG index tended to have a higher risk of testosterone decline or even testosterone deficiency. However, the predictability of the TyG index for testosterone deficiency was not better than that of HOMA-IR. More well-designed studies are still needed to validate this relationship.

Lower testosterone levels are associated with higher risk of death in men

Background and Objectives

Testosterone plays an important role in regulating male development, reproduction and health. Declining levels across the lifespan may reflect, or even contribute to, chronic disease and mortality in men.

Methodology

Relationships between testosterone levels and male mortality were analyzed using data from multiple samples of the cross-sectional National Health and Nutrition Examination Survey (n = 10 225). Target outcomes included known deaths from heart disease, malignant neoplasms, chronic lower respiratory diseases, cerebrovascular diseases, Alzheimer's disease, diabetes mellitus, influenza and pneumonia, kidney diseases, and accidents or unintentional injuries.

Results

Results of discrete-time hazard models revealed that lower levels of testosterone were related to higher mortality for the majority of disease categories in either an age-dependent or age-independent fashion. Analysis of all-cause mortality-which included deaths from any known disease-also revealed greater general risk for those with lower testosterone levels. For most disease categories, the hazard associated with low testosterone was especially evident at older ages when mortality from that particular ailment was already elevated. Notably, testosterone levels were not related to mortality risk for deaths unrelated to chronic disease (i.e. accidents and injuries).

Conclusions and Implications

While the causal direction of relationships between testosterone and mortality risk remains unclear, these results may reflect the decline in testosterone that accompanies many disease states. Accordingly, the relationship between testosterone and male mortality may be indirect; ill individuals are expected to have both lower testosterone and higher mortality risk.

Evaluating drivers of female dominance in the spotted hyena

Introduction

Dominance relationships in which females dominate males are rare among mammals. Mechanistic hypotheses explaining the occurrence of female dominance suggest that females dominate males because (1) they are intrinsically more aggressive or less submissive than males, and/or (2) they have access to more social support than males.

Methods

Here, we examine the determinants of female dominance across ontogenetic development in spotted hyenas (Crocuta crocuta) using 30 years of detailed behavioral observations from the Mara Hyena Project to evaluate these two hypotheses.

Results

Among adult hyenas, we find that females spontaneously aggress at higher rates than males, whereas males spontaneously submit at higher rates than females. Once an aggressive interaction has been initiated, adult females are more likely than immigrant males to elicit submission from members of the opposite sex, and both adult natal and immigrant males are more likely than adult females to offer submission in response to an aggressive act. We also find that adult male aggressors are more likely to receive social support than are adult female aggressors, and that both adult natal and immigrant males are 2–3 times more likely to receive support when attacking a female than when attacking another male. Across all age classes, females are more likely than males to be targets of aggressive acts that occur with support. Further, receiving social support does slightly help immigrant males elicit submission from adult females compared to immigrant males acting alone, and it also helps females elicit submission from other females. However, adult females can dominate immigrant males with or without support far more often than immigrant males can dominate females, even when the immigrants are supported against females.

Discussion

Overall, we find evidence for both mechanisms hypothesized to mediate female dominance in this species: (1) male and female hyenas clearly differ in their aggressive and submissive tendencies, and (2) realized social support plays an important role in shaping dominance relationships within a clan. Nevertheless, our results suggest that social support alone cannot explain sex-biased dominance in spotted hyenas. Although realized social support can certainly influence fight outcomes among females, adult females can easily dominate immigrant males without any support at all.

Using an on-site laboratory for fecal steroid analysis in wild white-faced capuchins

Hormone laboratories located "on-site" where field studies are being conducted have a number of advantages. On-site laboratories allow hormone analyses to proceed in near-real-time, minimize logistics of sample permits/shipping, contribute to in-country capacity-building, and (our focus here) facilitate cross-site collaboration through shared methods and a shared laboratory. Here we provide proof-of-concept that an on-site hormone laboratory (the Taboga Field Laboratory, located in the Taboga Forest Reserve, Costa Rica) can successfully run endocrine analyses in a remote location. Using fecal samples from wild white-faced capuchins (Cebus imitator) from three Costa Rican forests, we validate the extraction and analysis of four steroid hormones (glucocorticoids, testosterone, estradiol, progesterone) across six assays (DetectX® and ISWE, all from Arbor Assays). Additionally, as the first collaboration across three long-term, wild capuchin field sites (Lomas Barbudal, Santa Rosa, Taboga) involving local Costa Rican collaborators, this laboratory can serve as a future hub for collaborative exchange.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Oxytocin and testosterone administration amplify viewing preferences for sexual images in male rhesus macaques

Social stimuli, like faces, and sexual stimuli, like genitalia, spontaneously attract visual attention in both human and non-human primates. Social orienting behaviour is thought to be modulated by neuropeptides as well as sex hormones. Using a free viewing task in which paired images of monkey faces and anogenital regions were presented simultaneously, we found that male rhesus macaques overwhelmingly preferred to view images of anogenital regions over faces. They were more likely to make an initial gaze shift towards, and spent more time viewing, anogenital regions compared with faces, and this preference was accompanied by relatively constricted pupils. On face images, monkeys mostly fixated on the forehead and eyes. These viewing preferences were found for images of both males and females. Both oxytocin (OT), a neuropeptide linked to social bonding and affiliation, and testosterone (TE), a sex hormone implicated in mating and aggression, amplified the pre-existing orienting bias for female genitalia over female faces; neither treatment altered the viewing preference for male anogenital regions over male faces. Testosterone but not OT increased the probability of monkeys making the first gaze shift towards female anogenital rather than face pictures, with the strongest effects on anogenital images of young and unfamiliar females. Finally, both OT and TE promoted viewing of the forehead region of both female and male faces, which display sexual skins, but decreased the relative salience of the eyes of older males. Together, these results invite the hypothesis that both OT and TE regulate reproductive behaviours by acting as a gain control on the visual orienting network to increase attention to mating-relevant signals in the environment.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

The Older the Better: Infanticide Is Age-Related for Both Victims and Perpetrators in Captive Long-Tailed Macaques

In wild primates, infanticide is a risk that is especially prevalent when a new male takes over the alpha position. Insight into risk factors related to infanticide may decrease the incidence of infanticide in captivity during male introductions. We investigated several risk factors of infanticide derived from hypotheses explaining infanticide in the wild and tested this in captive long-tailed macaques (Macaca fascicularis) using demographic data spanning a 25.5-year period. Factors that are related to infanticide in the wild explained a large proportion, but not all incidences, of infanticide in captivity. Consistent with the wild data, infants young enough to decrease the interbirth interval (<215 days) were at risk of being killed. In contrast to studies from the wild, infanticidal males were more than 2.5 years younger than non-infanticidal males. This indicates that captive settings can lead to new risks since relatively young males may gain the alpha position, promoting infanticide. Therefore, we propose the adolescent male risk hypothesis as a captive risk factor in which subadult males pose a risk of infanticide. In conclusion, the ages of both males and infants are related to infanticide in captivity and have to be taken into account during male introductions.

The piRNAs present in the developing testes of Chinese indigenous Xiang pigs

The piRNA pathway plays an essential role in defense against transposable elements in the germline tissues of animals and contributes to post-transcriptional regulation of genes. Xiang pigs present an earlier sexual maturation compared with most European pig breeds, but the role that the piRNA pathway plays in the development of Xiang pigs is currently not understood. In this study, we sequenced and analyzed piRNAs expressed in the testes of Xiang pigs at four different ages, and identified endogenous piRNAs which were highly abundant at each time point. The lengths of the identified piRNAs ranged from 24 to 34 nucleotides (nt), with the most abundant length being 29 nt. Additionally, there was a strong bias for uracil at the first position, a slight bias for adenine at position 10 and frequent 5'-10 nt complementary sequences, suggesting that ping-pong-mediated silencing is present in the Xiang pig germline. We observed that the piRNA composition changed from TE-associated piRNAs in two- and three-month-old testes to predominantly gene-derived and intergenic piRNAs in six- and twelve-month-old testes, with a gradual increase in the expression level of piRNAs over the course of testis development. And more than half of piRNA reads mapped to just a few of 473 predicted piRNA clusters. Additionally, we found that several genes were highly enriched by piRNA reads, including CYP19A1, PRMT8, SUZ12, WWOX, SGSM1 and MIF. The functions of these genes are primarily associated with steroidogenesis and histone modification. Changes in piRNA composition and widespread expression patterns during spermatid development indicate that these small ncRNAs may be responsible not only for transposon suppression but also for post-transcriptional regulation of several protein-coding genes essential for normal spermatogenesis.

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 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.

Number of sexual partners and serum testosterone levels in a population-based sample of unpartnered heterosexual men

BACKGROUND

The relationship between men's testosterone levels and their sexual behaviors is unclear as existing studies find mixed results.

OBJECTIVES

The key objective of this study is to assess whether men's testosterone levels are affected by sexual behaviors, and to explore whether this relationship varies by age. Specifically, this study addresses the question: Are men's testosterone levels affected by the number of sexual partners they accrue?

METHODS

This analysis was based on a nationally-representative sample of 977 unpartnered heterosexual men between the ages of 20 and 65 in the United States. Serum testosterone levels were measured from blood samples using isotope dilution liquid chromatography tandem mass spectrometry (ID-LC-MS/MS) and the number of sexual partners in the past 12 months was ascertained from an Audio Computer-Assisted Personal Self-Interview (ACASI). Using ordinary least squares regression, serum testosterone levels were predicted as a function of the number of sexual partners that men reported, sociodemographic characteristics, and factors known to affect endocrine regulation. An interaction term between the number of sexual partners in the past 12 months and age was included.

RESULTS

The correlation between the number of sexual partners in the past 12 months and testosterone levels is contingent upon age: The relationship is positive for younger men but negative for older men. Among 20-year-old unpartnered heterosexual men, an additional sexual partner in the past 12 months is associated with an increase of 5.55 ng/dL of testosterone (95% CI: 2.92, 8.19). This initial positive effect at age 20 is reduced by 0.25 ng/dL of testosterone (95% CI: -0.44, -0.06) with each year that men age.

CONCLUSION

Testosterone levels in men are related to reproductive behaviors, but the direction of this relationship changes as men age. This article is protected by copyright. All rights reserved.

Male Bowhead Whale Reproductive Histories Inferred from Baleen Testosterone and Stable Isotopes

Male mammals of seasonally reproducing species typically have annual testosterone (T) cycles, with T usually peaking during the breeding season, but occurrence of such cycles in male mysticete whales has been difficult to confirm. Baleen, a keratinized filter-feeding apparatus of mysticetes, incorporates hormones as it grows, such that a single baleen plate can record years of endocrine history with sufficient temporal resolution to discern seasonal patterns. We analyzed patterns of T every 2 cm across the full length of baleen plates from nine male bowhead whales (Balaena mysticetus) to investigate occurrence and regularity of T cycles and potential inferences about timing of breeding season, sexual maturation, and reproductive senescence. Baleen specimens ranged from 181–330 cm in length, representing an estimated 11 years (smallest whale) to 22 years (largest whale) of continuous baleen growth, as indicated by annual cycles in stable isotopes. All baleen specimens contained regularly spaced areas of high T content (T peaks) confirmed by time series analysis to be cyclic, with periods matching annual stable isotope cycles of the same individuals. In 8 of the 9 whales, T peaks preceded putative summer isotope peaks by a mean of 2.8 months, suggesting a mating season in late winter / early spring. The only exception to this pattern was the smallest and youngest male, which had T peaks synchronous with isotope peaks. This smallest, youngest whale also did not have T peaks in the first half of the plate, suggesting initiation of T cycling during the period of baleen growth. Linear mixed effect models suggest that whale age influences T concentrations, with the two largest and oldest males exhibiting a dramatic decline in T peak concentration across the period of baleen growth. Overall, these patterns are consistent with onset of sexual maturity in younger males and possible reproductive senescence in older males. We conclude that adult male bowheads undergo annual T cycles, and that analyses of T in baleen may enable investigation of reproductive seasonality, timing of the breeding season, and life history of male whales.

Testing the role of testosterone versus estrogens in mediating reproductive transitions in female rhesus macaques

In male vertebrates, testosterone is generally known to coordinate reproductive trade-offs, in part by promoting the transition to the next reproduction at the expense of current parental care. The role of testosterone in reproductive transitions has been little tested in female vertebrates, especially in mammals. The present study sought to fill this gap, by first undertaking an experimental study, in which we identified DHT, androstenediol, and in particular etiocholanolone, as fecal androgen metabolites which reflect serum testosterone concentration in female rhesus macaques (Macaca mulatta). Using concentrations of fecal etiocholanolone as proxy for circulating testosterone, we then conducted a field study on 46 free-ranging rhesus macaques of Cayo Santiago, Puerto Rico, to test if testosterone mediates the trade-off between reproductive transition (a higher chance of reproducing in the next year) and current reproduction (providing more care to current offspring). While the evidence for testosterone was weak, the testing of fecal immunoreactive estrogen metabolites suggested a potential role of estrogen in reproductive trade-offs. We found large individual differences in fecal etiocholanolone concentrations during the early postpartum period that were unexplained even after accounting for sociodemographic factors such as age and dominance rank. Further investigation is needed to understand this variation. Our study suggests that the actions of testosterone in females may not have evolved to fulfil the same role in primate reproductive transitions as it does in males, and we encourage more studies to consider the function of testosterone in reproductive behaviors and life history transitions in females of mammalian taxa.

Behavioural and physiological plasticity in social hierarchies

Individuals occupying dominant and subordinate positions in social hierarchies exhibit divergent behaviours, physiology and neural functioning. Dominant animals express higher levels of dominance behaviours such as aggression, territorial defence and mate-guarding. Dominants also signal their status via auditory, visual or chemical cues. Moreover, dominant animals typically increase reproductive behaviours and show enhanced spatial and social cognition as well as elevated arousal. These biobehavioural changes increase energetic demands that are met via shifting both energy intake and metabolism and are supported by coordinated changes in physiological systems including the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes as well as altered gene expression and sensitivity of neural circuits that regulate these behaviours. Conversely, subordinate animals inhibit dominance and often reproductive behaviours and exhibit physiological changes adapted to socially stressful contexts. Phenotypic changes in both dominant and subordinate individuals may be beneficial in the short-term but lead to long-term challenges to health. Further, rapid changes in social ranks occur as dominant animals socially ascend or descend and are associated with dynamic modulations in the brain and periphery. In this paper, we provide a broad overview of how behavioural and phenotypic changes associated with social dominance and subordination are expressed in neural and physiological plasticity. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Hormonal correlates of male dominance rank, age, and genital colouration in vervet monkey (Chlorocebus pygerythrus)

Primates are the most colourful members of the Mammalian clade. In vervet monkeys (Chlorocebus pygerythrus), males are characterized by their red penis and blue scrotum. Such colour signals are often used in conspecific communication, and thus could be used to convey signaller condition. We quantified scrotal and penile colour characteristics using digital photographs between May-June 2016 from males in two neighboring groups along the shores of Lake Nabugabo, Uganda. We examined the relationship between fecal hormones, male dominance rank, age (adult vs. immature), and colour. Adult males were higher ranking than immatures, but there were no rank or age differences in fecal hormone levels. Glucocorticoids and androgens were positively correlated in immature, but not adult males. All scrotal characteristics were predicted by age, with adult males having more teal (i.e., less blue, more green) and more luminant scrota. Within adult males, those with higher androgens levels had more saturated blue scrotal colouration and higher-ranking males were more luminant. Penile colouration was also associated with age and rank. High-ranking males had a more saturated red penis, and adult male penile colour was more luminant and bluer than in immature males. Our findings are consistent with previous reports that scrotal colouration advertises sexual or reproductive maturity (i.e., age), but we also find that within adult males, colour also advertises dominance rank and may be mediated by androgen levels. Penile colouration also appears to signal information about male age and dominance rank but does not appear to be mediated by hormones.

Blood testosterone levels in sickness and in health: Male chimpanzee testosterone levels decrease in face of an immune challenge

As an integral part of the immune response, testosterone secretion is inhibited when an individual is confronted with an immune challenge. Testosterone-mediated physiological, morphological, and behavioral traits are compromised at times of impaired health. Nevertheless, males of some species seem to maintain high levels of testosterone when confronted with an immune challenge, upholding competitive strength but compromising their immune response. It has been argued that this phenomenon will occur only in species living in social systems with high degrees of male-male competition over mating opportunities. Male chimpanzees contest over access to fertile females and dominants sire the majority of offspring. This male mating pattern makes chimpanzees a candidate species where we could expect males to maintain high testosterone levels, compromising their immune response, to ensure immediate reproductive success. We measured blood testosterone levels in male and female chimpanzees, who expressed clinical symptoms (symptomatic) or showed no evidence of clinical disease on assessment (asymptomatic). For females, we expected to find lower testosterone levels in symptomatic individuals than in asymptomatic subjects. In males, we would predict lower testosterone levels in symptomatic individuals than in asymptomatic males, if the immune response leads to a decrease in testosterone secretion. Alternatively, males could have equal levels of testosterone when symptomatic and asymptomatic, upholding competitive strength. Our results show that male chimpanzees exhibit lower levels of testosterone when confronted with an immune challenge than when being asymptomatic. This suggests that male testosterone secretion is suppressed as part of the immune response, which potentially increases survival and lifetime reproductive success. It will, however, negatively impact momentary competitive ability. Also, males may employ different mating strategies, some of which are less testosterone-driven (e.g., affiliative strategies). Consequently, in some individuals, the costs of maintaining high testosterone levels may not outweigh the potential gain in reproductive success.

Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior

Ultraviolet (UV) light affects endocrinological and behavioral aspects of sexuality via an unknown mechanism. Here we discover that ultraviolet B (UVB) exposure enhances the levels of sex-steroid hormones and sexual behavior, which are mediated by the skin. In female mice, UVB exposure increases hypothalamus-pituitary-gonadal axis hormone levels, resulting in larger ovaries; extends estrus days; and increases anti-Mullerian hormone (AMH) expression. UVB exposure also enhances the sexual responsiveness and attractiveness of females and male-female interactions. Conditional knockout of p53 specifically in skin keratinocytes abolishes the effects of UVB. Thus, UVB triggers a skin-brain-gonadal axis through skin p53 activation. In humans, solar exposure enhances romantic passion in both genders and aggressiveness in men, as seen in analysis of individual questionaries, and positively correlates with testosterone level. Our findings suggest opportunities for treatment of sex-steroid-related dysfunctions.

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UVB exposure increases circulating sex-steroid levels in mice and humans

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UVB exposure enhances female attractiveness and receptiveness toward males

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UVB exposure increases females’ estrus phase, HPG axis hormones, and follicle growth

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Skin p53 regulates UVB-induced sexual behavior and ovarian physiological changes

Age-related cognitive impairment is associated with low serum concentrations of testosterone and CSF levels of amyloid beta 42 in male cynomolgus monkeys (Macaca fascicularis)

Aged memory-impaired cynomolgus monkeys had significantly lower levels of cerebrospinal amyloid (Aβ₄₂ ) and serum testosterone compared with young animals and non-memory-impaired controls. Our findings confirm similar findings in the human and substantiate the usefulness of the cynomolgus monkey as a spontaneous model for aging-associated senile dementia of the Alzheimer type.

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.