A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation.

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise.

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements.

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 1: Foundational principles and theories of regulation.

This contribution is the first of a four-part, historical series encompassing foundational principles, mechanistic hypotheses and supported facts concerning human thermoregulation during athletic and occupational pursuits, as understood 100 years ago and now. Herein, the emphasis is upon the physical and physiological principles underlying thermoregulation, the goal of which is thermal homeostasis (homeothermy). As one of many homeostatic processes affected by exercise, thermoregulation shares, and competes for, physiological resources. The impact of that sharing is revealed through the physiological measurements that we take (Part 2), in the physiological responses to the thermal stresses to which we are exposed (Part 3) and in the adaptations that increase our tolerance to those stresses (Part 4). Exercising muscles impose our most-powerful heat stress, and the physiological avenues for redistributing heat, and for balancing heat exchange with the environment, must adhere to the laws of physics. The first principles of internal and external heat exchange were established before 1900, yet their full significance is not always recognised. Those physiological processes are governed by a thermoregulatory centre, which employs feedback and feedforward control, and which functions as far more than a thermostat with a set-point, as once was thought. The hypothalamus, today established firmly as the neural seat of thermoregulation, does not regulate deep-body temperature alone, but an integrated temperature to which thermoreceptors from all over the body contribute, including the skin and probably the muscles. No work factor needs to be invoked to explain how body temperature is stabilised during exercise.

Reliability of methods to determine cutaneous evaporative water loss rate in furred and fleeced mammals.

We used a high-precision weighing system and flow-through respirometry to quantify cutaneous evaporative water loss rates in woolly sheep (wool thickness, ca. 6.5 cm) and haired goats (coat thickness, ca. 2.5 cm), while simultaneously recording parallel data obtained from (1) a flow-through ventilated capsule, (2) a closed hand-held electronic evaporimeter chamber, and (3) a closed colorimetric paper disc chamber. In comparison to the weighing system and respirometry, used here as a "gold standard" measure of cutaneous evaporative water loss rate, we found relatively good agreement with data obtained from the flow-through ventilated capsules. However, we found poor agreement with data obtained from the closed electronic evaporimeter chambers (underestimated by 60%, on average) and the closed colorimetric paper disc chambers (overestimated by 52%, on average). This deviation was likely associated with a requirement for shaved skin in the closed chamber methods. Our results therefore cast doubt on the validity of the closed chamber methods for measurement of cutaneous evaporative water loss rates in furred and fleeced mammals, and instead show that more accurate values can be obtained using flow-through ventilated capsules.

Why It Is Important to Consider the Effects of Analgesics on Sleep: A Critical Review.

We review the known physiological mechanisms underpinning all of pain processing, sleep regulation, and pharmacology of analgesics prescribed for chronic pain. In particular, we describe how commonly prescribed analgesics act in sleep-wake neural pathways, with potential unintended impact on sleep and/or wake function. Sleep disruption, whether pain- or drug-induced, negatively impacts quality of life, mental and physical health. In the context of chronic pain, poor sleep quality heightens pain sensitivity and may affect analgesic function, potentially resulting in further analgesic need. Clinicians already have to consider factors including efficacy, abuse potential, and likely side effects when making analgesic prescribing choices. We propose that analgesic-related sleep disruption should also be considered. The neurochemical mechanisms underlying the reciprocal relationship between pain and sleep are poorly understood, and studies investigating sleep in those with specific chronic pain conditions (including those with comorbidities) are lacking. We emphasize the importance of further work to clarify the effects (intended and unintended) of each analgesic class to inform personalized treatment decisions in patients with chronic pain. © 2021 American Physiological Society. Compr Physiol 11:1-31, 2021.

How dryland mammals will respond to climate change: the effects of body size, heat load and a lack of food and water.

Mammals in drylands are facing not only increasing heat loads but also reduced water and food availability as a result of climate change. Insufficient water results in suppression of evaporative cooling and therefore increases in body core temperature on hot days, while lack of food reduces the capacity to maintain body core temperature on cold nights. Both food and water shortage will narrow the prescriptive zone, the ambient temperature range over which body core temperature is held relatively constant, which will lead to increased risk of physiological malfunction and death. Behavioural modifications, such as shifting activity between night and day or seeking thermally buffered microclimates, may allow individuals to remain within the prescriptive zone, but can incur costs, such as reduced foraging or increased competition or predation, with consequences for fitness. Body size will play a major role in predicting response patterns, but identifying all the factors that will contribute to how well dryland mammals facing water and food shortage will cope with increasing heat loads requires a better understanding of the sensitivities and responses of mammals exposed to the direct and indirect effects of climate change.

Increased Diurnal Activity Is Indicative of Energy Deficit in a Nocturnal Mammal, the Aardvark.

Shifting activity to cooler times of day buffers animals from increased heat and aridity under climate change. Conversely, when resources are limited, some nocturnal species become more diurnal, reducing energetic costs of keeping warm at night. Aardvarks (Orycteropus afer) are nocturnal, obligate ant- and termite-eating mammals which may be threatened directly by increasing heat and aridity, or indirectly by the effects of climate change on their prey. We hypothesised that the minimum 24-h body temperature of aardvarks would decline during energy scarcity, and that aardvarks would extend their active phases to compensate for reduced resource availability, possibly resulting in increased diurnal activity when aardvarks were energetically compromised. To measure their thermoregulatory patterns and foraging activity, we implanted abdominal temperature and activity data loggers into 12 adult aardvarks and observed them for varying durations over 3 years in the Kalahari. Under non-drought conditions, aardvarks tightly controlled their 24-h body temperature rhythm (mean amplitude of the 24-h body temperature rhythm was 1.8 ± 0.3°C during summer and 2.1 ± 0.1°C during winter) and usually were nocturnal. During a summer drought, aardvarks relaxed the precision of body temperature regulation (mean 24-h amplitude 2.3 ± 0.4°C) and those that subsequently died shifted their activity to progressively earlier times of day in the weeks before their deaths. Throughout the subsequent winter, the aardvarks' minimum 24-h body temperatures declined, causing exaggerated heterothermy (4.7 ± 1.3°C; absolute range 24.7 to 38.8°C), with one individual's body temperature varying by 11.7°C within 8 h. When body temperatures were low, aardvarks often emerged from burrows during daytime, and occasionally returned before sunset, resulting in completely diurnal activity. Aardvarks also shortened their active periods by 25% during food scarcity, likely to avoid energetic costs incurred by foraging. Despite their physiological and behavioural flexibility, aardvarks were unable to compensate for reduced food availability. Seven study aardvarks and several others died, presumably from starvation. Our results do not bode well for aardvarks facing climate change, and for the many animal species dependent on aardvark burrows for refuge.

Almost 1 in 5 South African adults have chronic pain: a prevalence study conducted in a large nationally representative sample.

Limited information on the prevalence and risk factors for chronic pain is available for developing countries. Therefore, we investigated the prevalence of chronic pain and the association between this pain and various personal and sociodemographic factors by including questions in the South Africa Demographic and Household Survey 2016. The survey was conducted by face-to-face interviews with a nationally representative sample of the adult population (ages 15 and older, n = 10,336). Chronic pain was defined as pain or discomfort that had been experienced all the time or on and off for 3 months or more. The prevalence of chronic pain was 18.3% (95% confidence interval [CI]: 17.0-19.7). Women were more likely than were men to have chronic pain (men = 15.8% [95% CI: 13.9-17.8]; woman = 20.1% [95% CI: 18.4-21.8]), and the prevalence of chronic pain increased from 11.3% (95% CI: 9.6-13.3) for the age range 15 to 24 years to 34.4% (95% CI: 30.6-38.4) for the age range over 65 years. The body sites affected most frequently were the limbs (43.6% [95% CI: 40.4-46.9]), followed by the back (30.5% [95% CI: 27.7-33.6]). This article presents the prevalence of chronic pain in the general population of a middle-income African country. These data give much needed insights into the burden of, and risk factors for, chronic pain in low-resource settings, and identify priority groups for intervention.

Pain in Clients Attending a South African Voluntary Counseling and Testing Center Was Frequent and Extensive But Did Not Depend on HIV Status.

BACKGROUND: The frequency of pain is reported to be high in people living with HIV, but valid comparisons between people living with HIV and HIV-negative cohorts are rare. We investigated whether HIV infection influenced frequency and characteristics of pain in adults undergoing voluntary testing for HIV. SETTING: Participants were recruited from an HIV voluntary counseling and testing center at the Chris Hani Baragwanath Academic Hospital, Soweto, South Africa. METHODS: Pain was assessed using the Wisconsin Brief Pain Questionnaire. Depressive and anxiety symptomatology was determined using the Hopkins Symptom checklist-25. We then stratified by HIV status. RESULTS: Data from 535 black South Africans were analyzed: HIV-infected n = 70, HIV-uninfected n = 465. Overall, frequency of any current pain was high with 59% [95% confidence interval (CI): 55 to 63, n: 316/535] of participants reporting pain, with no difference related to HIV status: HIV-infected 50% (95% CI: 37 to 61, n: 35/70), HIV-uninfected 60% (95% CI: 56 to 65, n: 281/465). Pain intensity and number of pain sites were similar between the groups as were symptoms of anxiety and depression: mean Hopkins Symptom Checklist-25 1.72 (95% CI: 1.57 to 1.87) HIV-infected participants and 1.68 (95% CI: 1.63 to 1.73) HIV-uninfected participants. Univariate analysis showed female sex and greater depressive and anxiety symptomatology associated with pain. In a multivariable modeling, only depressive and anxiety symptomatology was retained in the model. CONCLUSION: The high frequency of pain found in both HIV-infected and HIV-uninfected individuals presenting at a voluntary counseling and testing center was more likely to be associated with depression and anxiety, than with the presence or absence of HIV.

Ultradian oscillations in brain temperature in sheep: implications for thermoregulatory control?

We compared body temperature patterns and selective brain cooling (SBC) in eight adult female sheep in an indoor (22-25 °C) and outdoor (mean ~ 21 °C) environment, by measuring brain, carotid arterial, and jugular venous blood temperatures at 5-min intervals using implanted data loggers. To investigate whether ultradian oscillations in brain temperature had thermoregulatory consequences for the sheep, we determined the cranial arterio-venous (AV) temperature difference as an indicator of respiratory evaporative heat loss (REHL). The 24-h pattern of SBC was similar in both environments, despite carotid blood temperature fluctuating 0.4 °C more outdoors compared to indoors. The sheep employed SBC more often during the night than during the day, but SBC was abolished at intervals of 1-3 h throughout the 24-h period. The suppression of SBC appeared to be associated with events that increased sympathetic nervous system activity, including shifts between stages of sleep. Short-term changes (over 5-min) in brain temperature were positively correlated with changes in the AV temperature difference 5 min later, and negatively correlated with changes in carotid temperature 10 min later. These data support the idea that increases in brain temperature modulate thermoregulation by increasing REHL, which leads to a decrease in carotid blood temperature. Ultradian oscillations in core temperature of sheep, therefore, appear to arise as a consequence of frequent brain temperature changes invoked by non-thermal inputs, in animals housed both in indoor and outdoor environments.

Keeping cool in the heat: Behavioral thermoregulation and body temperature patterns in wild vervet monkeys.

OBJECTIVES: Climate change is having a significant impact on biodiversity and increasing attention is therefore being devoted to identifying the behavioral strategies that a species uses to cope with climatic stress. We explore how wild vervet monkeys (Chlorocebus pygerythrus) respond to heat stress, and how behavioral adaptations are used to regulate body temperature. MATERIALS AND METHODS: We implanted wild vervet monkeys with temperature-sensitive data loggers and related the body temperature rhythms of these animals to their use of thermoregulatory behaviors. RESULTS: Environmental temperature had a positive effect on the mean, minima and maxima of daily body temperatures. Environmental temperature had a positive effect on the amount of time that vervet monkeys spent in the shade, and animals that spent more time in the shade had lower body temperature maxima. Drinking water did not have a proximate effect on body temperature, most likely a consequence of their regular access to drinking water. Body temperatures were observed to decrease after swimming events, but tended to return to pre-swim temperatures within 1 hr, suggesting a limited thermal benefit of this behavior. CONCLUSIONS: Our data support the view that vervet monkeys cope well in the heat, and use behavior as a means to aid thermoregulation. The ability of primates to be flexible in their use of thermoregulatory behaviors can contribute positively to their capacity to cope with environmental variability. However, given its broad effect on plant productivity and habitat loss, climate change is a major threat to species' biogeographical distribution and survival.

Scaling of cardiac morphology is interrupted by birth in the developing sheep Ovis aries.

Scaling of the heart across development can reveal the degree to which variation in cardiac morphology depends on body mass. In this study, we assessed the scaling of heart mass, left and right ventricular masses, and ventricular mass ratio, as a function of eviscerated body mass across fetal and postnatal development in Horro sheep Ovis aries (~50-fold body mass range; N = 21). Whole hearts were extracted from carcasses, cleaned, dissected into chambers and weighed. We found a biphasic relationship when heart mass was scaled against body mass, with a conspicuous 'breakpoint' around the time of birth, manifest not by a change in the scaling exponent (slope), but rather a jump in the elevation. Fetal heart mass (g) increased with eviscerated body mass (Mb , kg) according to the power equation 4.90 Mb0.88 ± 0.26 (± 95%CI) , whereas postnatal heart mass increased according to 10.0 Mb0.88 ± 0.10 . While the fetal and postnatal scaling exponents are identical (0.88) and reveal a clear dependence of heart mass on body mass, only the postnatal exponent is significantly less than 1.0, indicating the postnatal heart becomes a smaller component of body mass as the body grows, which is a pattern found frequently with postnatal cardiac development among mammals. The rapid doubling in heart mass around the time of birth is independent of any increase in body mass and is consistent with the normalization of wall stress in response to abrupt changes in volume loading and pressure loading at parturition. We discuss variation in scaling patterns of heart mass across development among mammals, and suggest that the variation results from a complex interplay between hard-wired genetics and epigenetic influences.

Biologging subcutaneous temperatures to detect orientation to solar radiation remotely in savanna antelope.

Observations of animal thermoregulatory behavior are labor-intensive, and human presence may disturb the normal behavior of the animal. Therefore, we investigated whether a remote biologging technique could be used to detect orientation to solar radiation in savanna antelope. We predicted that when a mammal was orientated perpendicular to solar radiation, the subcutaneous temperature on the side of the body facing the sun would be greater than that on the opposite side, whereas when the mammal was orientated parallel to solar radiation, subcutaneous temperatures on both sides would be similar. A pilot study showed that the difference between left- and right-side temperatures under a pelt reflected orientation to solar radiation if a pelt-covered cylinder had been orientated for 15 min or longer. In addition, the rate of change in temperature difference could detect orientation that had changed within the previous 5 min. We implanted temperature-sensitive data loggers subcutaneously into the flanks of eight black (Connochaetes gnu) and eight blue (Connochaetes taurinus) wildebeest. By incorporating both the rate of change and subcutaneous temperature differences and excluding times when wildebeest were lying down, our predictions correctly matched behavioral observations of wildebeest orientation to solar radiation 71% of the time. Our technique tended to fail when wildebeest were lying down, wind speeds were high and the sun was overhead. But those are conditions in which the benefits of manipulating orientation to solar radiation is of diminishing importance to a free-living mammal. Therefore, subcutaneous temperatures provide physiologically relevant information on the importance of solar radiation to mammals.

Mothers' perspectives on the lived experience of children with intellectual disability and challenging behaviour.

Children with intellectual disability and behavioural needs (challenging behaviour) are vulnerable to exclusion from services and communities. The situation is exacerbated by difficulties in accessing appropriate support and services to effectively meet the needs of children and carers. Family perspectives on the 'lived experience' of children can provide insight into how behavioural needs can affect their ability to access everyday experiences. Semi-structured interviews were conducted with mothers of children with intellectual disabilities and challenging behaviours. Phenomenological thematic analysis provided four key themes: finding our way; square services, round needs; behaviour touches everything and belonging. Experience of inclusion and exclusion was a central tenet of the lived experience. Recommendations call for timely proactive and bespoke interventions to identify and support children at risk of exclusion from communities. Early intervention and effective local provision will avoid increased burdens placed on families and services, in supporting children whose needs are currently unmet within child-centred provision.

Body temperature, activity patterns and hunting in free-living cheetah: biologging reveals new insights.

As one of the few felids that is predominantly diurnal, cheetahs (Acinonyx jubatus) can be exposed to high heat loads in their natural habitat. Little is known about long-term patterns of body temperature and activity (including hunting) in cheetahs because long-term concurrent measurements of body temperature and activity have never been reported for cheetahs, or, indeed, for any free-living felid. We report here body temperature and locomotor activity measured with implanted data loggers over 7 months in 5 free-living cheetahs in Namibia. Air temperature ranged from a maximum of 39 °C in summer to -2 °C in winter. Cheetahs had higher (∼0.4 °C) maximum 24-h body temperatures, later acrophase (∼1 h), with larger fluctuations in the range of the 24-h body temperature rhythm (approximately 0.4 °C) during a hot-dry period than during a cool-dry period, but maintained homeothermy irrespective of the climatic conditions. As ambient temperatures increased, the cheetahs shifted from a diurnal to a crepuscular activity pattern, with reduced activity between 900 and 1500 hours and increased nocturnal activity. The timing of hunts followed the general pattern of activity; the cheetahs hunted when they were on the move. Cheetahs hunted if an opportunity presented itself; on occasion they hunted in the midday heat or in total darkness (new moon). Biologging revealed insights into cheetah biology that are not accessible by traditional observer-based techniques.

Ameliorating the adverse cardiorespiratory effects of chemical immobilization by inducing general anaesthesia in sheep and goats: implications for physiological studies of large wild mammals.

Chemical immobilization is necessary for the physiological study of large wild animals. However, the immobilizing drugs can adversely affect the cardiovascular and respiratory systems, yielding data that do not accurately represent the normal, resting state. We hypothesize that these adverse effects can be ameliorated by reversing the immobilizing agent while holding the animal under general anaesthesia. We used habituated sheep Ovis aries (N = 5, 46.9 ± 5.3 kg body mass, mean ± SEM) and goats Capra hircus (N = 4, 27.7 ± 2.8 kg) as ungulate models for large wild animals, and measured their cardiorespiratory function under three conditions: (1) mild sedation (midazolam), as a proxy for the normal resting state, (2) immobilization (etorphine and azaperone), and (3) general anaesthesia (propofol) followed by etorphine antagonism (naltrexone). Cardiac output for both sheep and goats remained unchanged across the three conditions (overall means of 6.2 ± 0.9 and 3.3 ± 0.3 L min-1, respectively). For both sheep and goats, systemic and pulmonary mean arterial pressures were significantly altered from initial midazolam levels when administered etorphine + azaperone, but those arterial pressures were restored upon transition to propofol anaesthesia and antagonism of the etorphine. Under etorphine + azaperone, minute ventilation decreased in the sheep, though this decrease was corrected under propofol, while the minute ventilation in the goats remained unchanged throughout. Under etorphine + azaperone, both sheep and goats displayed arterial blood hypoxia and hypercapnia (relative to midazolam levels), which failed to completely recover under propofol, indicating that more time might be needed for the blood gases to be adequately restored. Nonetheless, many of the confounding cardiorespiratory effects of etorphine were ameliorated when it was antagonized with naltrexone while the animal was held under propofol, indicating that this procedure can largely restore the cardiovascular and respiratory systems closer to a normal, resting state.

Scaling of morphology and ultrastructure of hearts among wild African antelope.

The hearts of smaller mammals tend to operate at higher mass-specific mechanical work rates than those of larger mammals. The ultrastructural characteristics of the heart that allow for such variation in work rate are still largely unknown. We have used perfusion-fixation, transmission electron microscopy and stereology to assess the morphology and anatomical aerobic power density of the heart as a function of body mass across six species of wild African antelope differing by approximately 20-fold in body mass. The survival of wild antelope, as prey animals, depends on competent cardiovascular performance. We found that relative heart mass (g kg-1 body mass) decreases with body mass according to a power equation with an exponent of -0.12±0.07 (±95% confidence interval). Likewise, capillary length density (km cm-3 of cardiomyocyte), mitochondrial volume density (fraction of cardiomyocyte) and mitochondrial inner membrane surface density (m2 cm-3 of mitochondria) also decrease with body mass with exponents of -0.17±0.16, -0.06±0.05 and -0.07±0.05, respectively, trends likely to be associated with the greater mass-specific mechanical work rate of the heart in smaller antelope. Finally, we found proportionality between quantitative characteristics of a structure responsible for the delivery of oxygen (total capillary length) and those of a structure that ultimately uses that oxygen (total mitochondrial inner membrane surface area), which provides support for the economic principle of symmorphosis at the cellular level of the oxygen cascade in an aerobic organ.