The limits on maximum sustained energy expenditure are unclear but are of interest because they constrain reproduction, thermoregulation, and physical activity. Here, we show that sustained expenditure in humans, measured as maximum sustained metabolic scope (SusMS), is a function of event duration. We compiled measurements of total energy expenditure (TEE) and basal metabolic rate (BMR) from human endurance events and added new data from adults running ~250 km/week for 20 weeks in a transcontinental race. For events lasting 0.5 to 250+ days, SusMS decreases curvilinearly with event duration, plateauing below 3× BMR. This relationship differs from that of shorter events (e.g., marathons). Incorporating data from overfeeding studies, we find evidence for an alimentary energy supply limit in humans of ~2.5× BMR; greater expenditure requires drawing down the body’s energy stores. Transcontinental race data suggest that humans can partially reduce TEE during long events to extend endurance.

Femoral shape changes during the course of human growth, transitioning from a subcircular tube to a teardrop-shaped diaphysis with a posterior pilaster. Differences between immature and mature bipedalism and body shape may generate different loads, which, in turn, may influence femoral modeling and remodeling during the course of the human lifespan. This study uses two different approaches to evaluate the hypotheses that differences in gait between young and mature walkers result in differences in ground reaction forces (GRFs) and that the differences in loading regimes between young children and adults will be reflected in the geometric structure of the midshaft femur. The results of this analysis indicate that GRFs differ between young walkers and adults in that normalized mediolateral (ML) forces are significantly higher in younger age groups. In addition, these differences between children and adults in the relative level of ML bending force are reflected in changes in femoral geometry during growth. During the earlier stages of human development, immature femoral diaphyses are heavily reinforced in approximately ML plane. The differences in gait between mature and immature walkers, and hence the differences in femoral shape, are likely partially a product of a minimal bicondylar angle and relatively broad body in young children.

Energy is the fundamental currency of life-needed for growth, repair, and reproduction-but little is known about the metabolic physiology and evolved energy use strategies of the great apes, our closest evolutionary relatives. Here we report daily energy use in free-living orangutans (Pongo spp.) and test whether observed differences in energy expenditure among orangutans, humans, and other mammals reflect known differences in life history. Using the doubly labeled water method, we measured daily energy expenditure (kCal/d) in orangutans living in a large indoor/outdoor habitat at the Great Ape Trust. Despite activity levels similar to orangutans in the wild, Great Ape Trust orangutans used less energy, relative to body mass, than nearly any eutherian mammal ever measured, including sedentary humans. Such an extremely low rate of energy use has not been observed previously in primates, but is consistent with the slow growth and low rate of reproduction in orangutans, and may be an evolutionary response to severe food shortages in their native Southeast Asian rainforests. These results hold important implications for the management of orangutan populations in captivity and in the wild, and underscore the flexibility and interdependence of physiological, behavioral, and life history strategies in the evolution of apes and humans.daily energy expenditure | energetics | life history | doubly labeled water A ll animals require energy to grow, maintain homeostasis, and reproduce. Much of the variation in energy use among species is related to body mass; large animals generally require more energy each day than small animals (1). There are also significant differences among taxonomic groups, with birds using more energy per day than other vertebrates, eutherian mammals using more energy per day than marsupials, and reptiles using far less energy than birds or mammals (1). Still, after accounting for the effects of body mass and taxonomic class, a sixfold range of variation in daily energy expenditure, (DEE; in kCal/d) remains among vertebrate species in the wild (N = 229 species, ref. 1).There is growing evidence that the variation in DEE among species reflects evolved energy-use strategies to maximize the probability of survival and reproduction in a given habitat. Both within (2) and among (3) species, increased energy throughput (i.e., calories consumed and expended per day) is associated with increased reproductive output (i.e., grams of offspring produced per year). In habitats in which food resources are abundant, organisms may benefit from adopting higher energy throughput, increasing their food requirements but providing more energy for reproduction (4-6). Conversely, if food availability is highly variable or if foraging incurs the risk of predation, it may be advantageous to decrease DEE, even at the cost of decreased reproductive rates, to avoid starvation or predation (5, 6). Indeed, several comparative studies have suggested a continuum of energyuse strategies among mammals, from high-energy throughput to ...

Objective: Elevated resting metabolic rates (RMR, kcal/day) are a wellestablished mechanism for maintaining core body temperature among cold climate populations. A high degree of interindividual variation has recently been noted among circumpolar populations. To further examine RMR variability, we investigated anthropometric and metabolic differences among reindeer herders from subarctic Finland. Methods: Resting metabolic rates, body mass, body composition, height, age, and sex were measured among 20 reindeer herders (5 females, 15 males, 20-64 years) from seven herding districts surrounding the Arctic Circle of Finland in January of 2019. Results: Females had a mean RMR of 1798 ± 216 kcal/day and males 1753 ± 503 kcal/day. When controlling for body mass and fat-free mass, females had significantly higher RMRs relative to males (P < .01). Contrary to previous cold climate population studies, measured RMR among males was not significantly different from predictive equation estimates (P > .05). However, predictive equations significantly underestimated female RMR by a mean of 25.2% ± 5.9% (P < .01). Conclusion: These results mirror earlier findings, though in ways previously unseen. In this population: (a) There is a high degree of RMR interindividual variability, but only among males, and (b) there is evidence for elevated RMR, but only among females. Though the sample size is small, preliminary results suggest the presence of sex-based differences in metabolic adaptations to cold climates within this population subset. Potential reasons for this sex-based difference are discussed including a presentation of a hypothesis about the dual role of thyroid hormone in both reproductive and metabolic processes.