Ultra-endurance athletic performance suggests that energetics drive human morphological thermal adaptation

Ecological rules for global species distribution also predict performance variation in Ironman triathletes

Bergmann's and Allen's rules predict changes in body size and appendage length across temperature gradients for species with broad geographic distributions. Larger bodies and longer limbs facilitate cooling whereas smaller bodies and compact limbs limit heat loss. Although these patterns are highly repeatable (hence "rules" of ecology) the patterns and underlying mechanisms are less-well understood in humans. Here I show that variation in running performance among human male triathletes is consistent with both Bergmann's and Allen's rules. Males (but not females) with relatively larger body size and longer limbs performed better at hot compared to cold race venues and vice-versa. Consistent with results in other taxa, sex-specificity may reflect selection for sexual dimorphism. Results suggest that ecological patterns detected over large-spatial scales may arise from fine-scale variation in locomotor performance.

Bergmann’s rule is a “just-so” story of human body size

Carl Bergmann was an astute naturalist and physiologist. His ideas about animal size and shape were important advances in the pre-Darwinian nineteenth century. Bergmann’s rule claims that that in cold climates, large body mass increases the ratio of volume-to-surface area and provides for maximum metabolic heat retention in mammals and birds. Conversely, in warmer temperatures, smaller body mass increases surface area relative to volume and allows for greater heat loss. For humans, we now know that body size and shape are regulated more by social-economic-political-emotional (SEPE) factors as well as nutrition-infection interactions. Temperature has virtually no effect. Bergmann’s rule is a “just-so” story and should be relegated to teaching and scholarship about the history of science. That “rule” is no longer acceptable science and has nothing to tell us about physiological anthropology.

Comparing walking and running in persistence hunting

It has been proposed that humans' exceptional locomotor endurance evolved partly with foraging in hot open habitats and subsequently about 2 million years ago with persistence hunting, for which endurance running was instrumental. However, persistence hunting by walking, if successful, could select for locomotor endurance even before the emergence of any running-related traits in human evolution. Using a heat exchange model validated here in 73 humans and 55 ungulates, we simulated persistence hunts for prey of three sizes (100, 250, and 400 kg) and three sweating capacities (nonsweating, low, high) at 6237 combinations of hunter's velocity (1-5 m s^-1, intermittent), air temperature (25-45 °C), relative humidity (30-90%), and start time (8:00-16:00). Our simulations predicted that walking would be successful in persistence hunting of low- and nonsweating prey, especially under hot and humid conditions. However, simulated persistence hunts by walking yielded a 30-74% lower success rate than hunts by running or intermittent running. In addition, despite requiring 10-30% less energy, successful simulated persistence hunts by walking were twice as long and resulted in greater exhaustion of the hunter than hunts by running and intermittent running. These shortcomings of pursuit by walking compared to running identified in our simulations could explain why there is only a single direct description of persistence hunting by walking among modern hunter-gatherers. Nevertheless, walking down prey could be a viable option for hominins who did not possess the endurance-running phenotype of the proposed first persistence hunter, Homo erectus. Our simulation results suggest that persistence hunting could select for both long-distance walking and endurance running and contribute to the evolution of locomotor endurance seen in modern humans.

Variability and the form-function framework in evolutionary biomechanics and human locomotion

The form-function conceptual framework, which assumes a strong relationship between the structure of a particular trait and its function, has been crucial for understanding morphological variation and locomotion among extant and fossil species across many disciplines. In biological anthropology, it is the lens through which many important questions and hypotheses have been tackled with respect to relationships between morphology and locomotor kinematics, energetics and performance. However, it is becoming increasingly evident that the morphologies of fossil hominins, apes and humans can confer considerable locomotor diversity and flexibility, and can do so with a range of kinematics depending on soft tissue plasticity and environmental and cultural factors. This complexity is not built into traditional biomechanical or mathematical models of relationships between structure and kinematics or energetics, limiting our interpretation of what bone structure is telling us about behaviour in the past. The nine papers presented in this Special Collection together address some of the challenges that variation in the relationship between form and function pose in evolutionary biomechanics, to better characterise the complexity linking structure and function and to provide tools through which we may begin to incorporate some of this complexity into our functional interpretations.

Alternative Metabolic Strategies are Employed by Endurance Runners of Different Body Sizes; Implications for Human Evolution

Objective

A suite of adaptations facilitating endurance running (ER) evolved within the hominin lineage. This may have improved our ability to reach scavenging sites before competitors, or to hunt prey over long distances. Running economy (RE) is a key determinant of endurance running performance, and depends largely on the magnitude of force required to support body mass. However, numerous environmental factors influence body mass, thereby significantly affecting RE. This study tested the hypothesis that alternative metabolic strategies may have emerged to enable ER in individuals with larger body mass and poor RE.

Methods

A cohort of male (n = 25) and female (n = 19) ultra-endurance runners completed submaximal and exhaustive treadmill protocols to determine RE, and V̇O2Max.

Results

Body mass was positively associated with sub-maximal oxygen consumption at both LT1 (male r=0.66, p<0.001; female LT1 r=0.23, p=0.177) and LT2 (male r=0.59, p=0.001; female r=0.23, p=0.183) and also with V̇O2Max (male r=0.60, p=0.001; female r=0.41, p=0.046). Additionally, sub-maximal oxygen consumption varied positively with V̇O2Max in both male (LT1 r=0.54, p=0.003; LT2 r=0.77, p<0.001) and female athletes (LT1 r=0.88, p<0.001; LT2 r=0.92, p<0.001).

Conclusions

The results suggest that, while individuals with low mass and good RE can glide economically as they run, larger individuals can compensate for the negative effects their mass has on RE by increasing their capacity to consume oxygen. The elevated energy expenditure of this low-economy high-energy turnover approach to ER may bring costs associated with energy diversion away from other physiological processes, however.

Cortisol, Temperament and Serotonin in Karate Combats: An Evolutionary Psychobiological Perspective

Objectives

There is evidence suggesting that in martial arts competitions athletes characterized by higher anxiety and harm avoidance may be more likely to lose a fight. This psychological profile has been hypothesized to explain in part the observation that cortisol is higher in losers before and in response to a competition. An important research target that needs further exploration is the identification of phenotypic traits that can be helpful in predicting athletes’ performance. Here we present a brief description of the theoretical bases that drives our research in the evolutionary psychobiology of sports and illustrate preliminary data on the relationship between the 5HTTLPR genotype, salivary cortisol, temperament and competition.

Methods

Sixty-five healthy male non-professional athletes provided saliva samples 10 min before and after a kumite session and filled out the Tridimensional Personality Questionnaire.

Results

Salivary cortisol levels 10 min before the competition were higher in losers and in athletes with the S allele. Temperament was associated with competition outcome and cortisol: losers were characterized by higher scores of harm avoidance and harm avoidance was positively correlated with cortisol levels.

Conclusions

The results confirm previous findings linking temperamental traits, pre-and post- competition physiological stress response with competition outcome in kumite fight. Moreover, they indicate an association between the 5HTTLPR polymorphism and pre-competition salivary cortisol, thus providing a preliminary but non-conclusive evidence on the role played by the 5HTTLPR genotype as a vulnerability factor in sport competition.

Relationship between Biological, Training, and Physical Fitness Variables in the Expression of Performance in Non-Professional Runners

Sports performance is a multifactorial trait that can be associated with individual and environmental characteristics. In this study, the sample comprised 35 male runners, enrolled in the "InTrack" project. Information regarding variables related to runners' training was obtained via an online questionnaire, while anthropometric and body composition variables, as well as physical fitness components (muscular power, isometric strength, local muscular endurance, agility, and aerobic capacity) were measured, and a global physical fitness score (based on physical fitness components measured) was computed. The Weltman test (3200 m) was used to estimate runners' pace and their stride frequency. Linear regression was used, taking the running pace as dependent variable. The final model, comprising biological, physical fitness, spatiotemporal, and training variables, explained 86% of the running performance variance. Muscular power (β = -1.02; 95% CI = (-1.69)-(-0.35)), abdominal muscle endurance (β = -4.81; 95% CI = (-7.52)-(-2.10)), isometric strength (β = -422.95; 95% CI = (-689.65)-(-156.25)), global physical fitness (β = 27.14; 95% CI = 9.52-45.03), and stride frequency (β = -2.99; 95% CI = (-4.29)-(-1.69)) were significantly associated with performance, meaning that better results in tests and increasing the stride frequency leads to better performance. Individual characteristics and physical fitness components were demonstrated to be significant predictors for running performance.

Energetics as a driver of human morphological thermal adaptation; evidence from female ultra-endurance athletes

Functional benefits of the morphologies described by Bergmann's and Allen's rules in human males have recently been reported. However, the functional implications of ecogeographical patterning in females remain poorly understood. Here, we report the findings of preliminary work analysing the association between body shape and performance in female ultramarathon runners (n = 36) competing in hot and cold environments. The body shapes differed between finishers of hot and cold races, and also between hot race finishers and non-finishers. Variability in race performance across different settings supports the notion that human phenotype is adapted to different thermal environments as ecogeographical patterns have reported previously. This report provides support for the recent hypothesis that the heightened thermal strain associated with prolonged physical activity in hot/cold environments may have driven the emergence of thermally adaptive phenotypes in our evolutionary past. These results also tentatively suggest that the relationship between morphology and performance may be stronger in female vs. male athletes. This potential sex difference is discussed with reference to the evolved unique energetic context of human female reproduction. Further work, with a larger sample size, is required to investigate the observed potential sex differences in the strength of the relationship between phenotype and performance.

Fluctuating asymmetry, a marker of poor growth quality, is associated with adult male metabolic rate

OBJECTIVES

Life history theory, a branch of evolutionary theory, predicts the existence of trade-offs in energetic allocation between competing physiological functions. The core metabolic cost of self-maintenance, measured by resting metabolic rate (RMR), represents a large component of human daily energy expenditure. Despite strong selective pressures for energetic frugality and high observed interindividual variation in RMR, the link between RMR and energetic allocation to life-history traits remains understudied in humans.

MATERIALS

In a sample of 105 (m = 57, f = 48), we investigated the relationship between adult RMR and investment in growth quality, as measured by fluctuating asymmetry (FA).

RESULTS

Measurement of RMR and FA in university rowers revealed a significant positive correlation amongst males (n = 57, r = 0.344, p = 0.005, 1-tailed; standardized 95% CI, 0.090 to 0.598). Convincing evidence for a correlation among females was not found (n = 48, r = 0.142, p = 0.169, 1-tailed, standardized 95% CI, -0.152 to 0.435).

DISCUSSION

The data suggest that low-quality asymmetrical growth is associated with later-life metabolic inefficiencies in males. Energetic investment in processes (likely concerning the stress-response) unrelated to growth during childhood may thereby trade-off against adult metabolic efficiency. We suggest that the presence of a relationship between RMR and FA in males but not females may be explained by the additional metabolic strain associated with larger body size and increased male muscularity, which may amplify the inefficiencies arising from low-quality growth.

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.