Endurance running and digit ratio (2D:4D): Implications for fetal testosterone effects on running speed and vascular health

The associations between digit ratio (2D:4D and right - left 2D:4D), maximal oxygen consumption and ventilatory thresholds in professional male football players

INTRODUCTION

Digit ratio (2D:4D: the relative length of the 2nd and 4th digit) is thought to be a negative correlate of prenatal testosterone. The 2D:4D is related to oxygen metabolism, but the precise nature of this relationship is unclear. The purpose of the present study was to consider associations between digit ratios (right 2D:4D, left 2D:4D, right-left 2D:4D [Dr-l]) and VO2max and ventilatory thresholds (VT1 and VT2).

METHODS

One hundred and thirty-three Caucasian (n = 133) professional football players competing in Cyprus participated in the study. Players underwent anthropometric measurements, and digit lengths were measured from hand scans. They also completed an incremental cardiopulmonary test to exhaustion on a treadmill.

RESULTS

There were negative correlations between digit ratios and VO2max (right 2D:4D, r = -.65; left 2D:4D r = -.37, both p < .0001; Dr-l r = -.30, p = .0005). There were no relationships between digit ratios and VT1. For VT2, there were negative relationships with digit ratios (right 2D:4D, r = -.43, p < .0001; left 2D:4D, r = -.21 and Dr-l, r = -.21, both p = .02). Digit ratios are negatively related to VO2max with large (right 2D:4D) and medium (left 2D:4D, Dr-l) effect sizes. For VT2, there were also negative correlations, which were medium (right 2D:4D) and small (left 2D:4D, Dr-l).

CONCLUSION

Our findings may help clarify the relationships between digit ratios and high-intensity actions for extended periods, which are dependent on efficient oxygen metabolism.

Is digit ratio (2D:4D) a biomarker for lactate? Evidence from a cardiopulmonary test on professional male footballers

BACKGROUND

Lactate accumulation is associated with vigorous exercise, cardiovascular disease and a number of cancers. Digit ratio (2D:4D) has also been linked to oxygen metabolism, myocardial infarction and various cancers. Such similarities suggest the possibility that 2D:4D is a biomarker of lactate. Here, we consider the relationship between 2D:4D and lactate during an incremental cardiopulmonary exercise test.

METHOD

The participants were male professional football players. The treadmill test began at a speed of 8 km/h when the first lactate measurement was taken. The speed was increased by 2 km/h every 3.15 min, with measurements at 10, 12, 14 and 16 km/h.

RESULTS

There were 72 Caucasian and 7 Black participants, results are reported for the most numerous group. Lactate levels increased with running speed and were not correlated with age, body size or body composition. Median splits of digit ratios (right, left and right-left 2D:4D [Dr-l]) were calculated. In comparison to the Low ratio group, the High ratio group showed higher lactate levels across speeds. Effect sizes varied from very large to huge (right 2D:4D), large (left 2D:4D) and medium (Dr-l). At the individual level, positive correlations between digit ratios and lactate at the five different speeds varied from large (right 2D:4D), medium (left 2D:4D) and small (Dr-l).

CONCLUSION

There were large positive associations between right 2D:4D and lactate at all running speeds. We discuss our findings in relation to oxygen metabolism and suggest that 2D:4D may be a biomarker for lactate in the wider context of the latter's importance in health and disease.

Prenatal androgenization (2D:4D) predictions of tennis match-play success in junior players: A search for physiological explanations

AIM

This study aims to investigate the possible association between digit ratio (2D:4D) and match-play success (MPS) in junior tennis players. In addition, we consider the possible explanatory pathways of these associations in relation to psychological, strength, power, and hormonal parameters.

METHODS

We performed a cross-sectional study, with a sample comprised of 64 male junior tennis players (11-18 years old). Digit ratio was calculated from direct finger measurements. In addition, we measured the ratio of wins by number of matches played in 5 years of official competition (MPS), handgrip strength (HGS), standing long jump (SLJ), training (in weekly hours), and expertise (number of years in official competition). Salivary testosterone and cortisol levels were measured before and after physical "challenge" tests.

RESULTS

The 2D:4D correlated negatively with HGS and SLJ. MPS was also negatively associated with 2D:4D, but was positively correlated to HGS, expertise, training, and self-confidence (SC). Multiple linear regression showed 2D:4D and expertise were associated with MPS (43%-54%). None of the physical, or hormonal variables tested explained the links between 2D:4D and MPS.

CONCLUSION

Therefore, the specific fitness components influenced by prenatal androgenization that moderate sports success remain unknown. Future studies should explore the interaction of 2D:4D, with tennis exercises as a challenge to induce hormonal change, the effect of pubertal stage, and the influence of aerobic endurance in determining MPS.

Digit ratio and the femoral circumflex artery perforator: A sex-dependent association between the vascular system of the thigh and prenatal sex hormones?

BACKGROUND

The anterolateral thigh flap (ALTF) is a type of free flap which is commonly used in reconstructive surgery procedures and is based on the perforator from femoral circumflex artery. Some authors found that the location of the perforator differs between sexes.

OBJECTIVE

To verify the influence of prenatal sex hormones (as measured by 2D:4D) on the morphology of the perforator of the descending branch of the femoral circumflex artery.

METHODS

88 participants (43 women) of a mean age of 28.5 years (SD 8.1 years) had the following measurements performed: height and weight; right and left-hand 2nd and 4th fingers' lengths; and distance from anterior superior iliac spine to the perforator from descending branch of lateral femoral circumflex artery related to the thigh length (right and left) (AP ratio).

RESULTS

Right 2D:4D and Dr-l (right minus left 2D:4D) were dimorphic, similarly to AP ratio (right and left), which in women was significantly higher than in men. A significant correlation was found between right and left 2D:4D and location of the perforator of the left thigh in men. The higher right and left 2D:4D (more feminine) the higher the left AP ratio (more feminine).

CONCLUSION

Our findings suggest that the location of perforator of femoral circumflex artery may be influenced by prenatal sex steroids such that low prenatal testosterone and high prenatal oestrogen are correlated to high AP (lower branching of the artery).

Patterns of energy allocation during energetic scarcity; evolutionary insights from ultra-endurance events

Exercise physiologists and evolutionary biologists share a research interest in determining patterns of energy allocation during times of acute or chronic energetic scarcity.. Within sport and exercise science, this information has important implications for athlete health and performance. For evolutionary biologists, this would shed new light on our adaptive capabilities as a phenotypically plastic species. In recent years, evolutionary biologists have begun recruiting athletes as study participants and using contemporary sports as a model for studying evolution. This approach, known as human athletic palaeobiology, has identified ultra-endurance events as a valuable experimental model to investigate patterns of energy allocation during conditions of elevated energy demand, which are generally accompanied by an energy deficit. This energetic stress provokes detectable functional trade-offs in energy allocation between physiological processes. Early results from this modelsuggest thatlimited resources are preferentially allocated to processes which could be considered to confer the greatest immediate survival advantage (including immune and cognitive function). This aligns with evolutionary perspectives regarding energetic trade-offs during periods of acute and chronic energetic scarcity. Here, we discuss energy allocation patterns during periods of energetic stress as an area of shared interest between exercise physiology and evolutionary biology. We propose that, by addressing the ultimate "why" questions, namely why certain traits were selected for during the human evolutionary journey, an evolutionary perspective can complement the exercise physiology literature and provide a deeper insight of the reasons underpinning the body's physiological response to conditions of energetic stress.

Anthropometrics, Performance, and Psychological Outcomes in Mixed Martial Arts Athletes

The digit ratio (2D:4D) is related to prenatal testosterone (T) and sports performance. Few investigations have explored 2D:4D in determining the potential performance of individuals in power-based sports, specifically combat sports. This study compared 2D:4D between mixed martial arts (MMA) athletes and non-athletes and investigated the association between (1) handgrip strength (HGS) and lean body mass (LBM) with 2D:4D in MMA athletes and (2) psychometric variables and 2D:4D in MMA athletes and non-athletes. In total, 122 men participated in this study (53 non-athletes, 45 professionals, and 24 amateur fighters). The 2D:4D was measured using a caliper, HGS was assessed with a dynamometer, and psychometric variables were evaluated using questionnaires. Athletes displayed significantly (p < 0.05) lower median values of right (0.95 ± 0.04) and left (0.96 ± 0.03) 2D:4D in comparison with non-athletes, (0.97 ± 0.03) and (0.99 ± 0.03), respectively. We observed that left hand 2D:4D was negatively correlated with HGS (r = −0.43; p < 0.05) and lean body mass (r = −0.49; p < 0.05) in professional athletes. Professional athletes also displayed significant differences (* p < 0.05; ** p < 0.001) in psychometric variables compared to non-athletes for (1) agreeableness **, median values (interquartile range) = −0.11 (−0.19−0.07) and 0.2 (−0.09−0.33), respectively; (2) anger **, mean ± standard error = 2.40 ± 0.12 and 2.89 ± 0.89, respectively; and (3) openness *, mean ± standard error = 0.17 ± 0.04 and −0.006 ± 0.04, respectively. However, we did not observe an association between psychometric variables or wins with 2D:4D. In conjunction with other measures, 2D:4D is valuable in determining the potential athleticism of an MMA athlete.

Exploring Finger Digit Ratios (2D:4D) in Surgeons, Professional Rugby Players, and Political Journalists to Form a Directional Hypothesis: Could Finger Length Predict Attention and Focus?

In this short report we explore the predictive nature of finger digit ratio (i.e., second/index finger length divided by fourth/ring finger length; 2D:4D) and achievement. This research, with niche and specialized populations, was intended to support and grow on knowledge obtained from other large population 2D:4D studies and help form a directional hypothesis for future work exploring finger digit ratio and “success.” Twenty-nine professional rugby players aged 25.1 ± 4.2 years, height 185.2 ± 6.3 cm and weight 101.9 ± 11.8 kg; n = 16 orthopedic surgeons aged 55.3 ± 9.3 years with height 183.8 ± 10.2 cm and weight 90.8 ± 14.0 kg; and n = 18 political journalists with age, height and weight of 38.8 ± 7.3 years, 182.8 ± 7.8 cm, and 84.4 ± 11.4 kg, respectively, were recruited. Three experiments were conducted where we (1) explored relationships for 2D:4D with testosterone and cortisol responsiveness to low stress exercise, (2) explored relationships for 2D:4D with pupil constriction and pupil constriction latency (pupillometry measures related to testosterone and cortisol responsiveness and to attentiveness), and (3) compared 2D:4D between rugby players, surgeons, and journalists. Our results revealed 2D:4D was not predictive of testosterone and cortisol responsiveness to low-level exercise stress. However, relationships exist for 2D:4D and pupillometry measures (p < 0.05). Journalists right minus left 2D:4D difference was significantly different to rugby players’ and surgeons (p < 0.05). We argue 2D:4D is likely predictive of testosterone sensitivity and associated ability to focus attention; a skill important to high achievement in various contexts.

Associations between bio-motor ability, endocrine markers and hand-specific anthropometrics in elite female futsal players: a pilot study

BACKGROUND

The second-to-fourth digit ratio (2D:4D) has been calculated for individual athletes and sports, but it has not been investigated in futsal performance. Therefore, the aim of this study was to investigate any relationships between 2D:4D of the dominant and non-dominant hands and physical capacity performances, selected functional variables and hormone concentrations in elite female futsal players.

METHODS

Twenty-four elite female futsal players were measured for 2D:4D in the dominant (2D:4D_D) and non-dominant (2D:4D_ND) hand. The futsal specific performance test (FSPT), hand-grip strength (HGS) and aerobic power were also assessed. In addition, selected circulatory hormones were measured (estradiol, cortisol, growth hormone and insulin like growth factor-1). Pearson's correlation test was used to identify correlational relationships.

RESULTS

Total test-time and performance time (total time + penalty time) for the FSPT showed a significant correlation with 2D:4D_D (r = 0.53, p = 0.005 and r = 0.55, p = 0.003, respectively). HGS_ND also displayed a significant correlation with the 2D:4D_D (r = 0.59, p = 0.002). Aerobic power and time spent running on the treadmill also showed a significant relationship with 2D:4D_ND (both, r = 0.54, p = 0.006). Cortisol showed a significant correlation with 2D:4D_D (- 0.58, p = 0.003) and 2D:4D_ND(- 0.52, p = 0.008).

CONCLUSIONS

The measurement of 2D:4D ratio could be an important factor in determining potential performance attributes and talent identification of elite female futsal players. Further studies are needed in this area to further examine the results presented here.

Handgrip strength and 2D : 4D in women: homogeneous samples challenge the (apparent) gender paradox

The length ratio between the second and the fourth digit (2D : 4D) is a retrospective, non-invasive biomarker for prenatal androgen exposure. It was found to be negatively correlated with handgrip strength (HGS) in men, but the evidence for women is mixed. Such studies in women call for increased detection sensitivity. The present study was designed to reduce potential confounding factors, especially age and ethnicity variation. We measured the digit ratios and HGS of 125 healthy women between 19 and 31 years of age from a remote region in Austria. 2D : 4D of both hands was significantly and negatively correlated with HGS (n = 125, right hand: r = -0.255, p = 0.002, left hand: r = -0.206, p = 0.011). Size, direction and significance of correlation coefficients remained stable when statistically controlling for age, body weight, body height, body mass index or hours of exercise per week. This yields theory-consistent evidence that HGS and 2D : 4D are clearly associated in women-when sufficiently reducing genetic variation (confounding 2D : 4D), the ontogenetic environment and age ranges (confounding HGS) in the study population. This finding implies similar organizing effects of prenatal androgens as in men, pointing to a more parsimonious developmental mechanism and a new look into its proximate and ultimate causes.

Influence of 2D:4D ratio on fitness parameters and accumulated training load in elite youth soccer players

BACKGROUND

Digit ratio (2D:4D) characterized by the length of the second digit (2D) divided by the length of the fourth digit (4D), is a powerful marker of athletic performance. Some studies showed a negative correlation between 2D:4D ratio and sports performances.

OBJECTIVES

The purpose of the present study was three-fold: (1) to analyze the influence of anthropometric and 2D:4D ratio on variations of accumulated training load (ATL) and fitness parameters: maximal oxygen uptake (V̇O_2max), countermovement jump (CMJ), isometric muscular strength of the knee extensor for hamstring (ISH) and flexor for quadriceps (ISQ) muscles; along three stages of evaluation of soccer players based on playing positions; (2) to analyze the correlations between 2D:4D ratio and aforementioned parameters; and (3) to investigate if variance in fitness levels and ATL can explain the 2D:4D ratio.

METHODS

Twenty-four elite players under 17 years were daily monitored for their rating perceived exertion and ATL across 24 weeks over the season. Soccer players have also measured in three stages for anthropometric traits and fitness parameters.

RESULTS

Significant differences were observed between playing positions for body mass, goalkeepers had higher body mass compared to centre-midfielder and winger players. Moreover, there were significant differences in ATL between early-season to mid-season in goalkeepers (P = 0.032). The 2D:4D ratio (left and right) shown largely and negatively association with muscular strength (ISQ: r =  - 0.80; r =  - 0.78, P ≤ 0.001, ISH: r =  - 0.63; r =  - 0.62, P = 0.001, respectively) and VO_2max changes (r =  - 0.55, P = 0.005; r =  - 0.50, P = 0.013, respectively); lastly, both 2D:4D ratio significantly predicted changes in muscular strength and VO_2max in young soccer players.

CONCLUSIONS

Goalkeepers tended to have higher body mass compared to centre-midfielder and winger players; and 2D:4D ratio revealed a mighty predictor of physical fitness changes in soccer players. Evidence should be helpful to professionals to highlight the usefulness of the 2D:4D into the identification of talent, but also to optimize young players' performance.

Digit ratio (2D:4D) and body mass index in the BBC Internet Study: prenatal sex steroids and a Trivers-Willard effect on body composition

Digit ratio - a putative measure of prenatal sex steroids - may be related to body mass index (BMI). However, reports of correlations between 2D:4D and BMI have yielded mixed results with some studies showing no relationship while others have reported positive associations in men or women only. This study considers associations between self-reported 2D:4D and BMI in a large online survey (i.e. the BBC Internet Study). At the individual level, there was a weak positive association between 2D:4D and BMI in both sexes with greater effect sizes in women. Body mass index was positively related to age and negatively related to parental income; however, the relationship between 2D:4D and BMI was independent of both variables. At the national level, mean 2D:4D per country showed positive associations with mean national BMI but those correlations were restricted to females. It is concluded that BMI is positively related to low prenatal testosterone and high prenatal oestrogen. Parental income inequality may influence both prenatal sex steroids (through a 'Trivers-Willard' effect) and BMI such that increases in inequality result in reductions in prenatal testosterone and increases in BMI at the individual and national level.

The relationship between digit ratio (2D:4D) and muscular fitness: A systematic review and meta-analysis

BACKGROUND

Digit ratio (2D:4D), a marker of prenatal testosterone exposure, is a weak negative correlate of sports/athletic/fitness performance. While numerous studies have examined the relationship between 2D:4D and physical fitness, there has never been a comprehensive study that has synthesized studies examining relationships between 2D:4D and muscular fitness.

OBJECTIVES

To systematically review and meta-analyze the relationship between 2D:4D and muscular fitness measured as handgrip strength (HGS).

METHODS

We systematically searched five electronic databases, reference lists, topical systematic reviews/meta-analyses, and personal libraries in November 2020. Peer-reviewed, cross-sectional studies that reported Pearson's correlation coefficients between objectively measured 2D:4D and HGS were included. We used random-effects meta-analysis to estimate the pooled correlation and the 95% confidence interval (95%CI), and moderator analyses to estimate the influence of sex and age.

RESULTS

Data from 22 studies, representing 5271 individuals from 11 countries ranging in (mean) age from 10.4 to 58.0 years, were included. Overall, there was a weak negative correlation between 2D:4D and HGS (r = -0.15, 95%CI = -0.20 to -0.09), indicating that individuals with low 2D:4Ds had high HGS. We found substantial heterogeneity between studies (Q = 123.4, p < .0001; I²  = 74%), but neither sex (Q = 0.003, p = .96) nor age (Q = 0.46, p = .50) significantly moderated the pooled correlation.

CONCLUSIONS

We found a weak negative relationship between 2D:4D and HGS, which showed substantial heterogeneity between studies, but was neither moderated by sex nor age. Our finding probably reflects both the long-term (organizational) and short-term (activational) benefits of testosterone.

Dissociation of disease onset, progression and sex differences from androgen receptor levels in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder caused by loss of motor neurons. ALS incidence is skewed towards males with typically earlier age of onset and limb site of onset. The androgen receptor (AR) is the major mediator of androgen effects in the body and is present extensively throughout the central nervous system, including motor neurons. Mutations in the AR gene lead to selective lower motor neuron degeneration in male spinal bulbar muscular atrophy (SBMA) patients, emphasising the importance of AR in maintaining motor neuron health and survival. To evaluate a potential role of AR in onset and progression of ALS, we generated SOD1G93A mice with either neural AR deletion or global human AR overexpression. Using a Cre-LoxP conditional gene knockout strategy, we report that neural deletion of AR has minimal impact on the disease course in SOD1G93A male mice. This outcome was potentially confounded by the metabolically disrupted Nestin-Cre phenotype, which likely conferred the profound lifespan extension observed in the SOD1G93A double transgenic male mice. In addition, overexpression of human AR produced no benefit to disease onset and progression in SOD1G93A mice. In conclusion, the disease course of SOD1G93A mice is independent of AR expression levels, implicating other mechanisms involved in mediating the sex differences in ALS. Our findings using Nestin-Cre mice, which show an inherent metabolic phenotype, led us to hypothesise that targeting hypermetabolism associated with ALS may be a more potent modulator of disease, than AR in this mouse model.

No credible evidence for links between 2D:4D and COVID-19 outcomes: A probabilistic perspective on digit ratio, ACE variants, and national case fatalities

Research into COVID-19 susceptibility and outcomes are critical, but claims must be carefully evaluated to inform policy decisions. In a recent series of articles, Manning and Fink [1-3] use national-level data to describe associations between case-fatality ratios and male and female finger ratios (2D:4D), a suggested measure of prenatal androgen exposure, as well as angiotensin-converting enzyme (ACE) allele and genotype frequencies. The authors suggest that 2D:4D is linked with ACE variant prevalence, and that higher male 2D:4D is associated with higher case fatality ratios, and point to 2D:4D as a useful prognostic measure for COVID-19 susceptibility. A critical review and robust Bayesian analysis of the hypothesis is described here, finding no conclusive evidence of COVID-19 mortality and 2D:4D, nor associations between 2D:4D and ACE1 allele or ACE2 genotype frequency. This absence of evidence is present for data taken from the second wave of COVID-19 in October 2020. Problematic theoretical grounding, individual-level conclusions drawn from national-level data, and issues with statistical inference in the original articles are discussed. Taken together, the current data offer no clear utility of 2D:4D in determining COVID-19 outcomes.

Understanding COVID-19: A hypothesis regarding digit ratio (2D:4D), ACE I/D polymorphism, oxygen metabolism and national case fatality rates

OBJECTIVE

Male digit ratio (2D:4D) correlates positively with the national case fatality rate (CFR) for COVID-19. The severity of COVID-19 may be influenced by a counterbalance between the angiotensin-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2). SARS-CoV2 cleaves with ACE2 and enters cells leaving an unopposed effect of ACE in the lungs. Both 2D:4D and the ACE I/D polymorphism are covariates of oxygen metabolism. COVID-19 leads to lung damage and a reduction in oxygen saturation of the blood. Here, we examine the interrelationships between 2D:4D, ACE polymorphism, and COVID-19 CFR.

METHODS

National frequencies/rates were obtained for 2D:4D from the BBC Internet study (n = 41), published values of ACE I/II (n = 39), and COVID-19 CFR from three World Health Organization situation reports (n = 41).

RESULTS

2D:4D was negatively associated with national ACE I/II frequencies. However, there was a positive relationship between male 2D:4D and CFR (right and left 2D:4D, two, and three situation reports respectively). The relationships between ACE I/II and CFR were non-significant. Relationships between male 2D:4D and CFR's were independent of female 2D:4D and ACE I/II.

CONCLUSIONS

The ACE I/D polymorphism may influence 2D:4D such that ACE II individuals have lower 2D:4D than ACE DD individuals. Low 2D:4D and ACE II individuals show efficient oxygen metabolism. Therefore, low 2D:4D and ACE II together may protect against COVID-19 severity. The sex-dependent positive correlation between male 2D:4D and CFR is independent of ACE I/II, suggesting that the sex-dependent variation in the ACE2 gene may also influence the 2D:4D phenotype.

The associations between digit ratio, aerobic fitness, physical skills, and overall physical fitness of elite youth distance runners

OBJECTIVE

Lower digit ratios between the lengths of fingers 2 (2D) and 4 (4D) (2D:4D) are associated with superior distance running and athletic performance. We examined relationships between 2D:4D, aerobic fitness, physical skills, and overall physical fitness of elite adolescent boy and girl distance runners.

METHODS

Subjects were top five finishers for their sex and age in 10 or more races of 10 km or longer in Michigan in 1981. We calculated 2D:4D of 15 girls and 11 boys from radiographs. Subject peak O₂ consumption (VO_2Peak ), ventilatory threshold (VT), and point of equivalent change (PEC) were collected during intermittent treadmill protocol tests. Performances on physical skills tests (flex-arm hang, broad jump, vertical jump, figure-8-run, sit ups, and sit-and-reach test) were collected in the laboratory. We examined the interrelationships between 2D:4D, subject sex, aerobic fitness, physical skills test performance, and overall physical fitness, a composite of aerobic and physical skills performance with correlation, linear regression, t tests, and principle component analyses.

RESULTS

Girls had significantly larger right hand (R) 2D:4D than boys. Boys had greater VO_2Peak by mass than girls. Boys with lower R2D:4D had significantly greater VO_2Peak and PEC. Girls with lower R2D:4D had significantly greater VT. Factors associated with aerobic fitness explained most of the variation in composite physical fitness scores. Composite aerobic fitness, physical skills, and overall physical fitness scores of boys were negatively correlated with R2D:4D.

CONCLUSIONS

These data suggest that R2D:4D may help predict distance running performance in girls and boys and overall physical fitness in boys and provide additional insights into the innate factors influencing youth physical fitness.

Human athletic paleobiology; using sport as a model to investigate human evolutionary adaptation

The use of sport as a conceptual framework offers unprecedented opportunities to improve our understanding of what the body does, shedding new light on our evolutionary trajectory, our capacity for adaptation, and the underlying biological mechanisms. This approach has gained traction over recent years. To date, sport has facilitated exploration not only of the evolutionary history of our species as a whole, but also of human variation and adaptation at the interindividual and intraindividual levels. At the species level, analysis of lower and upper limb biomechanics and energetics with respect to walking, running and throwing have led to significant advances in the understanding of human adaptations relative to other hominins. From an interindividual perspective, investigation of physical activity patterns and endurance running performance is affording greater understanding of evolved constraints of energy expenditure, thermoregulatory energetics, signaling theory, and morphological variation. Furthermore, ultra-endurance challenges provoke functional trade-offs, allowing new ground to be broken in the study of life history trade-offs and human adaptability. Human athletic paleobiology-the recruitment of athletes as study participants and the use of contemporary sports as a model for studying evolutionary theory-has great potential. Here, we draw from examples in the literature to provide a review of how the use of athletes as a model system is enhancing understanding of human evolutionary adaptation.

No Association between 2D:4D Ratio and Hunting Success among Hadza Hunters

The ratio of index- and ring-finger lengths (2D:4D ratio) is thought to be related to prenatal androgen exposure, and in many, though not all, populations, men have a lower average digit ratio than do women. In many studies an inverse relationship has been observed, among both men and women, between 2D:4D ratio and measures of athletic ability. It has been further suggested that, in hunter-gatherer populations, 2D:4D ratio might also be negatively correlated with hunting ability, itself assumed to be contingent on athleticism. This hypothesis has been tested using endurance running performance among runners from a Western, educated, and industrialized population as a proximate measure of hunting ability. However, it has not previously been tested among actual hunter-gatherers using more ecologically valid measures of hunting ability and success. The current study addresses this question among Tanzanian Hadza hunter-gatherers. I employ a novel method of assessing hunting reputation that, unlike previous methods, allows granular distinctions to be made between hunters at all levels of perceived ability. I find no statistically significant relationship between digit ratio and either hunting reputation or two important hunting skills. I confirm that Hadza men have higher mean 2D:4D ratios than men in many Western populations. I discuss the notion that 2D:4D ratio may be the consequence of an allometric scaling relationship between relative and absolute finger lengths. Although it is difficult to draw clear conclusions from these results, the current study provides no support for the theorized relationship between 2D:4D ratio and hunting skill.

Anogenital distance as a measure of male competitive ability in Rwenzori Angolan colobus

Anogenital distance (AGD) is positively correlated to fetal androgen exposure and developmental masculinization in mammals. Independent of overall body size, AGD shows a strong positive correlation with male fertility and in rodents, AGD is a good indicator of male competitive ability and is associated with female choice. We hypothesized that AGD will also predict male competitive ability in non-human primates. To test this, we measured AGD noninvasively with a parallel laser in a wild population of Angolan colobus monkeys (Colobus angolensis ruwenzorii) in Uganda and correlated to it to their social structure. C. angolensis ruwenzorii form a multilevel society with both one-male/multifemale units (OMUs) and multimale/multifemale units (MMUs). We compared AGD in males from five OMUs and six MMUs and related it to their fecal androgen metabolite concentrations, dominance rank and body size, and to the number of females in their unit. Males in OMUs had greater access to females, so were predicted to have longer AGDs, but this was not found. AGD also did not correlate overall with mean fecal androgen metabolites in MMUs. However, AGD was correlated with dominance rank in MMUs, demonstrating that higher-ranking males in these multimale units had longer AGDs. Body size did not show the same relationship with dominance rank, suggesting that male rank was not just a reflection of absolute male size. Our findings indicate that AGD predicts male competitive ability in this species and that it may be a useful correlate throughout the non-human primates. These results also support the idea that prenatal androgen exposure increases the likelihood of the expression of behaviors that maintain high dominance rank.

Digit Ratio (2D:4D) and Physical Performance in Female Olympic Athletes

Background: The second to fourth digit ratio (2D:4D ratio) is suggested to be a negative correlate of prenatal testosterone. Little is known about the role of the 2D:4D ratio in relation to serum and urinary androgens for physical performance in female athletes. We aimed to compare the 2D:4D ratio in female Olympic athletes with sedentary controls, and to investigate the 2D:4D ratio in relation to serum and urinary androgens and physical performance in the athletes. Methods: This cross-sectional study included 104 Swedish female Olympic athletes participating in power, endurance and technical sports and 117 sedentary controls. The 2D:4D ratio was calculated using direct digit measurements. Serum androgens and urinary androgen metabolites were analyzed by liquid chromatography-tandem mass spectrometry. The athletes performed standardized physical performance tests and body composition was established by dual-energy X-ray absorptiometry. Results: The 2D:4D ratio was significantly lower in the athletes compared with controls although serum testosterone levels were comparable between groups and within normal reference values. The 2D:4D ratio correlated negatively with urinary levels of testosterone glucuronide and 5α- and 5βAdiol-17G, whereas there were no correlations to serum androgen levels. Furthermore, the 2D:4D ratio correlated negatively with strength tests and positively with 3,000-meter running in the athletes. Conclusion: Female Olympic athletes had a lower 2D:4D ratio, possibly reflecting a higher prenatal androgen exposure, than sedentary controls. Furthermore, the 2D:4D ratio was related to urinary levels of androgen metabolites and physical performance in the athletes but not to serum androgen levels. It is suggested that the 2D:4D ratio could reflect androgen metabolism and may be of importance for sporting success in female athletes.

Population-neuroscience study of the Tokyo TEEN Cohort (pn-TTC): Cohort longitudinal study to explore the neurobiological substrates of adolescent psychological and behavioral development

AIM

Adolescence is a crucial stage of psychological development and is critically vulnerable to the onset of psychopathology. Our understanding of how the maturation of endocrine, epigenetics, and brain circuit may underlie psychological development in adolescence, however, has not been integrated. Here, we introduce our research project, the population-neuroscience study of the Tokyo TEEN Cohort (pn-TTC), a longitudinal study to explore the neurobiological substrates of development during adolescence.

METHODS

Participants in the first wave of the pn-TTC (pn-TTC-1) study were recruited from those of the TTC study, a large-scale epidemiological survey in which 3171 parent-adolescent pairs were recruited from the general population. Participants underwent psychological, cognitive, sociological, and physical assessment. Moreover, adolescents and their parents underwent magnetic resonance imaging (MRI; structural MRI, resting-state functional MRI, and magnetic resonance spectroscopy), and adolescents provided saliva samples for hormone analysis and for DNA analysis including epigenetics. Furthermore, the second wave (pn-TTC-2) followed similar methods as in the first wave.

RESULTS

A total of 301 parent-adolescent pairs participated in the pn-TTC-1 study. Moreover, 281 adolescents participated in the pn-TTC-2 study, 238 of whom were recruited from the pn-TTC-1 sample. The instruction for data request is available at: http://value.umin.jp/data-resource.html.

CONCLUSION

The pn-TTC project is a large-scale and population-neuroscience-based survey with a plan of longitudinal biennial follow up. Through this approach we seek to elucidate adolescent developmental mechanisms according to biopsychosocial models. This current biomarker research project, using minimally biased samples recruited from the general population, has the potential to expand the new research field of population neuroscience.

The digit ratio (2D:4D) relationship with testosterone is moderated by physical training: Evidence of prenatal organizational influences on activational patterns of adult testosterone in physically-active women

BACKGROUND

The 2nd to 4th digit ratio (2D:4D) is regarded as a sexually dimorphic trait governed by prenatal testosterone (T) and estradiol exposure. However, relationships between the 2D:4D and adult sex hormone concentrations are inconsistent in females. Environmental contingencies (e.g., physical training) may provide a stronger basis for establishing these linkages, particularly if the relationships are plastic and not fixed.

AIMS

To investigate associations between 2D:4D, training hours and salivary T (sal-T) measures in physically-active women.

STUDY DESIGN

Descriptive and quasi-experimental monitoring across the follicular (day 7), ovulatory (day 14) and luteal (day 21) phases of three menstrual cycles.

SUBJECTS

35 naturally-cycling women training regularly for sport (range 3-12 h a week).

OUTCOME MEASURES

Morning sal-T concentrations and changes in sal-T to a physical and psychological stimulus (~10 min) were measured.

RESULTS

Both 2D:4D and training hours were related to morning sal-T concentrations in all three menstrual phases (p < 0.01) and their interaction was significant during ovulation. Follow-up testing revealed a negative 2D:4D and sal-T association in women reporting high, but not low, training hours each week. Participant 2D:4D was also related to sal-T changes under physical and/or psychological challenge across each menstrual phase (p < 0.01), whereas training hours and its interaction with 2D:4D were not significant predictors of these outcomes.

CONCLUSIONS

2D:4D linkages with basal and challenge-induced sal-T changes were identified in women undertaking regular physical training. Training hours also moderated the 2D:4D link to morning sal-T, but with some menstrual-phase dependency. These complexities and environmental relationships may help reconcile conflicting results.

Differential Placing of Flexion Creases Contributes to Sex Differences in the Second-to-Fourth Digit Ratio (2D:4D)

The present research examined whether differential placing of the basal flexion creases contributes to the occurrence of sex differences in digit ratio (2D:4D) derived from palmar digit lengths. The ratio of palmar-to-dorsal digit length, a measure of the placing of the basal flexion crease in the finger, was derived for the digits 2 and 4 of the right hand in two independent samples (Study I: N = 100; Study II: N = 200), in accordance with discovery-replication sample approach. The results show that men have lower palmar-to-dorsal digit ratios than women, and this effect is significantly stronger for digit 2 than for digit 4. Thus, the present study supports the likelihood that differential placing of flexion creases in the digits contributes to the occurrence of sex differences in palmar 2D:4D. In addition (Study II), the measurement procedure assessing the placing of flexion creases was validated. In conclusion, this evidence highlights potential conceptual shortcomings and technical limitations in the measurement conventions and methods currently employed in the field of 2D:4D research.

Relationships between the digit ratio (2D:4D) and game-related statistics in professional and semi-professional male basketball players

OBJECTIVES

The primary aim of this study was to examine relationships between digit ratio (2D:4D) and game-related statistics in professional and semi-professional male basketball players. The secondary aim was to quantify differences in mean 2D:4Ds between starting and reserve players.

METHODS

Using a cross-sectional design, 93 male basketball players from the professional Australian National Basketball League and the semi-professional South Australian Premier League were measured in-season for height, mass, and 2D:4D, with game-related statistics collected end-season. Linear relationships between right and left 2D:4Ds and game-related statistics were quantified using nonparametric partial correlations, and differences in mean 2D:4Ds between starting and reserve players were quantified using analysis of covariance (ANCOVA). All partial correlations and ANCOVAs were adjusted for playing experience, body size, and competitive standard.

RESULTS

2D:4D was a weak to moderate negative correlate of points scored and assists-to-turnovers ratio, indicating that males with lower 2D:4Ds were generally better offensively as they recorded more points and assists relative to turnovers. The difference in mean 2D:4D between starting and reserve players was negligible.

CONCLUSIONS

2D:4D was favorably correlated with open-skill sports performance, as evidenced by the better offensive statistics of male basketball players with lower 2D:4Ds. These results probably reflect the organizational benefits of prenatal testosterone and indicate that 2D:4D may be a useful complement to traditional physical, physiological, skill, and behavioral predictors of basketball success.

College-aged women in the United States that play overhand throwing sports have masculine digit ratios

Athletic prowess in both males and females is negatively correlated with the ratio between the lengths of the second and fourth fingers (2D:4D), a correlate of prenatal testosterone exposure. Because multiple lines of evidence suggest that prenatal testosterone exposure is associated with sports interest, motivation, and athletic performance we measured the digit ratios of 77 non-athletes, 103 varsity athletes, and 78 club sport athletes to test 8 hypotheses about the relationship between digit ratio and the athletic behavior of college-age women in the USA. Using independent samples t-tests, we found no significant differences between the digit ratios of women that (1) were athletes and non-athletes, (2) were varsity or club sport athletes, (3) had played or were currently playing individual or team sports, (4) played contact and non-contact sports, (5) played sports involving a ball and those that do not, (6) played sports where the outcome was determined by a score or the outcome of direct physical competition or subjectively by judges, or (7) were starters or reserves on their teams. However, women that played overhand throwing sports softball and water polo had significantly smaller digit ratios than did women that played other sports. These differences were not due to scaling effects. The independent samples t-test results were supported by subsequent Monte Carlo bootstrap, Bayesian, Random Forest, and multiple linear regression analyses. We suggest that the organizational consequences of prenatal testosterone exposure may influence the anatomy and physiology of women that leads to success playing overhand throwing sports.

The Second to Fourth Digit Ratio in Elite and Non-Elite Greco-Roman Wrestlers

A low second-to-fourth digit ratio (2D:4D) has been reported to correlate with high performance and athletic potential of an individual in sport. It has been suggested that 2D:4D is a relatively weak predictor of strength and a stronger predictor of efficiency in aerobic exercise. Comparing extreme groups on a continuum of sports performance requiring high power (physical strength) output would be helpful to resolve this issue. Therefore, the purpose of the present study was to compare the 2D:4D ratio of world-class elite Greco-Roman wrestlers (n = 10) taking part in Olympic fitness camps in 2013 with the 2D:4D ratio of non-elite collegiate wrestlers (n = 20), and age-matched sedentary males (n = 40). The 2D:4D ratios of elite wrestlers were lower compared to non-elite athletes (p < 0.01, right hand d = 1.70, left hand d = 1.67) and the control group (p < 0.0001, right hand d = 3.16, left hand d = 2.00). No significant differences were noted among the groups for right - left 2D:4D. We concluded that 2D:4D may discriminate between non-elite and world-class wrestlers. We also suggest that a low 2D:4D ratio could be linked to performance potential in wrestlers. As such, 2D:4D may provide additional information, which is valuable in determining the potential athleticism of an individual, when it is used in conjunction with other measures.

Does the index-to-ring finger length ratio (2D:4D) differ in amyotrophic lateral sclerosis (ALS)? Results from an international online case-control study

OBJECTIVES

The ratio of the length of the index finger (2D) to the ring finger (4D) (2D:4D) has been reported to be lower (ie, 2D<4D) in people with amyotrophic lateral sclerosis (ALS) than non-ALS controls. This has led to suggestions that exposure to increased prenatal testosterone, which also lowers this ratio, could be a risk factor for ALS. In an attempt to test this hypothesis, we examined 2D:4Ds from large numbers of patients with ALS and controls.

SETTING

An online multilingual questionnaire enabling respondents to measure their own index and ring finger lengths.

PARTICIPANTS

Of the initial 949 respondents, 572 remained for analysis after elimination for inability to straighten fingers, not answering the question, statistical outliers and aged <40 years. Respondents remaining for analysis were 202 patients with ALS (125 males, 77 females) and 370 non-ALS controls (112 males, 258 females).

RESULTS

Unpaired t-tests with 95% CIs were used to assess differences in mean 2D:4Ds. Males had significantly lower mean 2D:4Ds than females, in both ALS and control groups, for both left and right hands. No significant differences were found in 2D:4Ds between ALS and control groups, in either males or females, for either left or right hands. Receiver operating characteristic curves showed no power for 2D:4Ds to predict ALS status in either males or females.

CONCLUSIONS

2D:4Ds did not differ between patients with ALS and controls in this study. This was despite the dataset being large enough to confirm the established finding of lower 2D:4Ds in males compared with females. These findings do not support the hypothesis that exposure to increased prenatal testosterone is a risk factor for ALS. A putative lower 2D:4D has been proposed to explain the link between ALS and exercise, but our results indicate that other exercise-related factors are more likely to explain this association.

Phenotypic integration mediated by hormones: associations among digit ratios, body size and testosterone during tadpole development

BACKGROUND

Developmental associations often explain phenotypic integration. The intersected hormonal regulation of ontogenetic processes fosters predictions of steroid-mediated phenotypic integration among sexually dimorphic traits, a statement defied by associations between classical dimorphism predictors (e.g. body size) and traits that apparently lack sex-specific functions (e.g. ratios between the lengths of Digits II and IV - 2D:4D). Developmental bases of female-biased 2D:4D have been identified, but these remain unclear for taxa presenting male-biased 2D:4D (e.g. anura). Here we propose two alternative hypotheses to investigate evolution of male-biased 2D:4D associated with sexually dimorphic body size using Leptodactylus frogs: I)'hypothesis of sex-specific digit responses' - Digit IV would be reactive to testosterone but exhibit responses in the opposite direction of those observed in female-biased 2D:4D lineages, so that Digit IV turns shorter in males; II) 'hypothesis of identity of the dimorphic digit'- Digit II would be the dimorphic digit.

RESULTS

We compiled the following databases using Leptodactylus frogs: 1) adults of two species from natural populations and 2) testosterone-treated L. fuscus at post-metamorphic stage. Studied traits seem monomorphic in L. fuscus; L. podicipinus exhibits male-biased 2D:4D. When present, 2D:4D dimorphism was male-biased and associated with dimorphic body size; sex differences resided on Digit II instead of IV, corroborating our 'hypothesis of identity of the dimorphic digit'. Developmental steroid roles were validated: testosterone-treated L. fuscus frogs were smaller and exhibited masculinized 2D:4D, and Digit II was the digit that responded to testosterone.

CONCLUSION

We propose a model where evolution of sexual dimorphism in 2D:4D first originates from the advent, in a given digit, of increased tissue sensitivity to steroids. Phenotypic integration with other sexually dimorphic traits would then occur through multi-trait hormonal effects during development. Such process of phenotypic integration seems fitness-independent in its origin and might explain several cases of steroid-mediated integration among sexually dimorphic traits.

Why is digit ratio correlated to sports performance?

Second to fourth digit ratio is the ratio of second to fourth digit length. It has been known that digit ratio is sexually dimorphic in humans, such that males tend to have lower digit ratio (longer fourth digits relative to second digits) than females. Digit ratio is thought to be a biomarker of the balance between fetal testosterone (FT) and fetal estrogen (FE) in a relatively narrow developmental window at the end of the first trimester of pregnancy. On the contrary, the relationships between digit ratio and levels of sex steroids in adults are not clear. Most correlational studies between digit ratio and adult sex steroids have revealed that this association is statistically not significant. However, for many years, a lot of researches showed negative relationships between digit ratio and sports performance such as rugby, surfing, rowing, sprinting, endurance, and hand grip strength. Here, we discuss possible mechanisms about the relationships between digit ratio and sports performance.

Putative Androgen Exposure and Sexual Orientation: Cross-Cultural Evidence Suggesting a Modified Neurohormonal Theory

INTRODUCTION

According to neurohormonal theory, prenatal androgens are key determinants of sexual orientation. As a reputed marker for prenatal androgens, the 2D:4D finger length ratio has been used in more than a dozen studies to test the hypothesis that prenatal androgens influence sexual orientation. Findings have been very inconsistent.

AIM

The present study sought to retest the hypothesis that 2D:4D and sexual orientation are related is a manner consistent with neurohormonal theory.

METHODS

A 2D:4D measure (of the right hand) along with four additional somatic markers of androgen exposure (height, physical strength, muscularity, and athletic ability) with samples of college students in Malaysia (N = 2,058) and the United States (N = 2,511). The five androgen measures were factor analyzed, resulting in a two-factor solution: Factor 1 consisted of strength, muscularity, and athletic ability (the muscular coordination factor), and Factor 2 was comprised of the r2D:4D and adult height (the bone growth factor).

MAIN OUTCOME MEASURES

Sexual orientation was measured by asking each respondent the extent to which they were sexually attracted to males and the extent to which they were sexually attracted to females, both on 11-point scales.

RESULTS

When the countries and sexes were analyzed separately, neither the r2D:4D measure nor Factor 2 correlated with sexual orientation to significant degrees. Instead, it was the muscular coordination factor that correlated the best. Support was found for the hypothesis that prenatal androgens influence sexual orientation, but the nature of these influences was more complex than neurohormonal theory predicted.

CONCLUSION

A modified theory is needed and presented to accommodate the results from this study.

Studies of human sex ratios at birth may lead to the understanding of several forms of pathology

This article deals with the problem of the causes of the variation of sex ratio (proportion male) at birth. This problem is common to a number of areas in biology and medicine, for example, obstetrics, neurology/psychiatry, parasitology, virology, oncology, and teratology. It is established that there are signifi cantly biased, but unexplained, sex ratios in each of these fields. Yet workers in them (with the possible exception of virology) have regarded the problem as a minor loose end, irrelevant to the field's major problems. However, as far as I know, no one has previously noted that unexplained biased sex ratios occur, and thus pose (perhaps similar) problems, in all these fields. Here it is suggested that similar sorts of solutions apply in each. Further research is proposed for testing each solution. If the argument here is substantially correct across this range of topics, it may lead to an improved understanding not only of sex ratio but also of some of the pathologies in these specialties.

The Influence of Facial Characteristics on the Relation between Male 2D:4D and Dominance

Although relations between 2D:4D and dominance rank in both baboons and rhesus macaques have been observed, evidence in humans is mixed. Whereas behavioral patterns in humans have been discovered that are consistent with these animal findings, the evidence for a relation between dominance and 2D:4D is weak or inconsistent. The present study provides experimental evidence that male 2D:4D is related to dominance after (fictitious) male-male interaction when the other man has a dominant, but not a submissive or neutral face. This finding provides evidence that the relationship between 2D:4D and dominance emerges in particular, predictable situations and that merely dominant facial characteristics of another person are enough to activate supposed relationships between 2D:4D and dominance.

The 2D:4D ratio, a proxy for prenatal androgen levels, differs in men with and without MS

OBJECTIVE

To determine whether the 2D:4D ratio (ratio of the second and fourth digit lengths), a proxy for lower prenatal androgen to estrogen ratio, differs in men with and without multiple sclerosis (MS) using a case-control study design.

METHODS

We obtained 2 digital scans of the right hand for men with MS presenting to a scheduled clinic visit at a large MS referral center, and for men without autoimmune or endocrine diseases. All individuals were aged 18 to 65 years, right-handed, and reported no prior digit trauma. We calculated a mean 2D:4D ratio using digital calipers. In participants with MS, we assessed age at first MS symptoms, MS type, and the MS Severity Score; 51 had provided a testosterone level within 10 years of symptom onset. Our primary analysis was a cross-sectional comparison of the 2D:4D ratio between men with and without MS, using a 2-sample t test for independent samples assuming unequal variance.

RESULTS

In total, we scanned 137 men with MS and 145 men without MS. A statistically significant association between 2D:4D ratio and MS status was observed in the univariate logistic regression model (p<0.05). These differences were not associated with age or race, which differed between the 2 groups. In participants with MS, the 2D:4D ratio was not correlated with MS type, age at first symptoms, or MS Severity Score (p>0.15 for each), and it was not correlated with adult testosterone levels (r=0.06, p=0.68, n=51).

CONCLUSIONS

During the prenatal period, low androgens could represent a risk factor for MS.

Digit ratio (2D:4D) and physical fitness (Eurofit test battery) in school children

BACKGROUND

The relative lengths of the index finger to the ring finger (2D:4D) is sexually dimorphic and is thought to be a correlate of prenatal sex steroids (low 2D:4D=high prenatal testosterone and low prenatal oestrogen). In adults there have been reports that low 2D:4D is consistently associated with high sports performance.

AIMS

To investigate correlations between 2D:4D and fitness levels in children.

STUDY DESIGN

Right 2D:4D and body size were measured, in addition to flexibility, speed, endurance and strength (Eurofit tests).

SUBJECTS

922 boys and 835 girls (mean ages (years): 10.8 ± 1.01 and 10.07 ± 1.00 respectively).

OUTCOME MEASURES

height, mass, BMI, triceps and subscapular skinfolds, 20 m shuttle run, sit and reach, standing broad jump, hand grip strength, 10 × 5m sprint, and the sit and reach test.

RESULTS

Boys significantly outperformed the girls in the 10 × 5m sprints, the 20 m shuttle run, standing broad jump, and hand grip strength. In boys but not girls, 2D:4D was significantly negatively correlated with scores in all these tests except the standing broad jump. In girls but not boys, 2D:4D was significantly positively correlated to stature, mass, BMI and waist circumference.

CONCLUSIONS

It is suggested that high prenatal testosterone and low prenatal oestrogen (low 2D:4D) is implicated in high sprinting speed, endurance and hand grip strength in boys. In girls low prenatal testosterone and high prenatal oestrogen is associated with large body size.

Can persistence hunting signal male quality? A test considering digit ratio in endurance athletes

Various theories have been posed to explain the fitness payoffs of hunting success among hunter-gatherers. ‘Having’ theories refer to the acquisition of resources, and include the direct provisioning hypothesis. In contrast, ‘getting’ theories concern the signalling of male resourcefulness and other desirable traits, such as athleticism and intelligence, via hunting prowess. We investigated the association between androgenisation and endurance running ability as a potential signalling mechanism, whereby running prowess, vital for persistence hunting, might be used as a reliable signal of male reproductive fitness by females. Digit ratio (2D:4D) was used as a proxy for prenatal androgenisation in 439 males and 103 females, while a half marathon race (21km), representing a distance/duration comparable with that of persistence hunting, was used to assess running ability. Digit ratio was significantly and positively correlated with half-marathon time in males (right hand: r = 0.45, p<0.001; left hand: r = 0.42, p<0.001) and females (right hand: r = 0.26, p<0.01; left hand: r = 0.23, p = 0.02). Sex-interaction analysis showed that this correlation was significantly stronger in males than females, suggesting that androgenisation may have experienced stronger selective pressure from endurance running in males. As digit ratio has previously been shown to predict reproductive success, our results are consistent with the hypothesis that endurance running ability may signal reproductive potential in males, through its association with prenatal androgen exposure. However, further work is required to establish whether and how females respond to this signalling for fitness.

Scientometric analyses of studies on the role of innate variation in athletic performance

Historical events have produced an ideologically charged atmosphere in the USA surrounding the potential influences of innate variation on athletic performance. We tested the hypothesis that scientific studies of the role of innate variation in athletic performance were less likely to have authors with USA addresses than addresses elsewhere because of this cultural milieu. Using scientometric data collected from 290 scientific papers published in peer-reviewed journals from 2000–2012, we compared the proportions of authors with USA addresses with those that listed addresses elsewhere that studied the relationships between athletic performance and (a) prenatal exposure to androgens, as indicated by the ratio between digits 2 and 4, and (b) the genotypes for angiotensin converting enzyme, α-actinin-3, and myostatin; traits often associated with athletic performance. Authors with USA addresses were disproportionately underrepresented on papers about the role of innate variation in athletic performance. We searched NIH and NSF databases for grant proposals solicited or funded from 2000–2012 to determine if the proportion of authors that listed USA addresses was associated with funding patterns. NIH did not solicit grant proposals designed to examine these factors in the context of athletic performance and neither NIH nor NSF funded grants designed to study these topics. We think the combined effects of a lack of government funding and the avoidance of studying controversial or non-fundable topics by USA based scientists are responsible for the observation that authors with USA addresses were underrepresented on scientific papers examining the relationships between athletic performance and innate variation.