The many invertebrates and reptiles of the shifting-sand deserts, and their rare mammals, live in and on the sand. Their conservation requires a thorough understanding of the physiological mechanisms that not only enable them to function, but that may buffer them against future change. The almost-spherical shape of the aeolian sand grains of shifting-sand deserts enables gas diffusion from the surface to maintain a normoxic environment deep in the sand, with sufficient volume to sustain sand-swimming animals, even the mammals. In the resource-poor desert landscapes, cellulose is the dominant source of metabolic energy for herbivores, much of it contained in detritus that can originate hundreds of kilometres from the desert. Carnivores and omnivores consume mainly detritivores, with termites often contributing substantially to their diets. Many animals of shifting-sand deserts manage their body water by the acquisition of metabolic and preformed water, balanced against limited loss of body water via integument, excreta and respiration. Those that do drink usually use droplets from fog or dew. Some small invertebrates extract water vapour from unsaturated air. The animals have evolved mechanisms to counteract osmotic shock when they consume pure water that can constitute more than 40% of body mass in a single drink. In a companion review, we discuss how the animals move and how they cope with potentially lethal desert heat and cold in and on the sand. We need to conserve their fortresses in the sand, and, anomalously, it is food, not water, that requires attention.

Climate change is increasing human exposure to novel environments and generating serious practical challenges in human health, while the role of past climates in shaping hominin evolution remains a fascination. Models of human heat exchange vary from simple environmental indices to extremely detailed physiological calculations. Physiologically explicit models vary in their physical explicitness, and significant trade-offs exist between model realism and computational speed. We presently lack agile models that are not so over-parameterised that they are slow, that maintain generality across diverse and complex physical environments, and that have sufficient biological realism to capture human (and, more generally, hominin) diversity in physiology, body size and proportions, activity level, clothing, hair, and skin. Here we present a model (HomoTherm) to fill this gap, built upon the general endotherm model from the NicheMapR package for biophysical modelling in R. This steady-state heat budget model is unique among existing open-source models in that it solves for the metabolic rate, evaporation rate, skin temperature, and clothing temperature of a multi-part (head, torso, and limbs) human of variable shape, size, or colour and at variable levels of physical activity. The model can handle complex microclimates, including environments that differ in downwelling and upwelling radiation, and connects directly with the microclimate modelling capabilities of NicheMapR. Because of its general nature, it can easily be adapted to model other hominins by changing proportions, fur properties, and any other known physiological trait. We illustrate the model's capabilities through comparisons with actual human responses in published empirical studies of indoor and outdoor exposures to heat and cold, and compare HomoTherm's performance with that of other models. Vignettes and an open-source Shiny app facilitate the use of HomoTherm in addressing fundamental and applied problems in human thermal biology.

The conservation of the animals of the shifting-sand deserts requires a thorough understanding of the physiological mechanisms that not only enable them to function in the desert, but also may buffer them against future change. The animals there contend with little water and poor food resources. The animals of shifting-sand deserts also must move on and in loose sand and cannot construct unsupported burrows. Most are invertebrates, but the deserts also are home to many vertebrate species, mainly lizards. We describe the biomechanics of their locomotion on and in the sand. Some are quiescent buriers that remain near the surface, but remarkably, small desert mammals can swim to depths of more than 500 mm. Diurnally active animals retreat into the sand beneath their feet to avoid overheating in summer and freezing in winter, but burial and emergence are not determined by selection of the best thermal or hygric environment. We address the conundrum of why so many species that are active in the sun are black. The sand provides a fortress that resists invasion, but from which the residents cannot escape. The animals are under threat from climate change and from human use and abuse of the shifting-sand deserts, even though the sand is unsuitable for concrete. We argue that efforts to combat desertification do not absolve us from a need to conserve shifting-sand deserts. In a companion review, we discuss how the physiology of breathing, the acquisition of metabolic energy and the management of body water impacts the conservation of the animals of shifting-sand deserts.

Most scenarios that seek to predict the responses of terrestrial mammals to climate change focus on the direct thermal effects of higher ambient temperatures. Measurements from free-living mammals reveal that the physiological challenge for many terrestrial mammals facing climate change will arise from the compound effects of higher heat loads, reduced water, and reduced energy intake. Deaths from climate change, particularly for large mammals, are more likely to result from starvation than from heat stroke. The extent of heterothermy exhibited by a mammal, which results from the relaxation of temperature regulation in response to demands from competing homeostatic systems, provides an index of its physiological welfare and, therefore, a tool to assess sensitivity and responses to climate change. Studies of responses to heat in laboratory or captive individuals can identify what mammals can achieve physiologically, but they do not necessarily reveal what an animal will actually do in its natural habitat.

Most experts agree that the dominant mechanism through which body temperature is regulated, under a thermal challenge, environmental or metabolic, is negative feedback control. However, some consider negative feedback to be too sluggish to account for the rapid speed of response. The impression of sluggishness is based on an assumption that the body temperature that is regulated is a core temperature, whereas we concur with those who have concluded that what is regulated is an integrated temperature compiled from inputs from multiple body parts, including the skin. Negative feedback control is supplemented, though, by feedforward control, which is initiated by cues about the predicted magnitude and timing of the thermal challenge. Feedforward control is anticipatory because it can excite thermo-effectors in advance of the thermal challenge impacting on body temperature. Feedforward control is improved by learning but always is supervised by feedback control. There is disagreement about whether the pro-active excitation of thermo-effectors by temperature receptors in the skin occurs by fast feedback control or by feedforward control. We show that skin temperature receptors can provide physiologically meaningful negative feedback within seconds. Both the feedback and the feedforward regulation of integrated body temperature can be modulated by regulation of the temperature of body parts that have special thermoregulatory needs, notably the scrotum.

Onymacris plana (Coleoptera: Tenebrionidae) is a black beetle that runs at high speed for a pedestrian insect in direct solar radiation in the Namib Desert, a behaviour expected to impose potentially lethal body temperature within minutes. We measured the body temperature of beetles active in their natural habitat using fine thermocouples inserted into the prothorax. The measurements revealed that when beetles sprinted in conditions of low wind, high radiation and moderate ambient temperature, their body temperature dropped rather than rose. The effect depended on convective cooling and efficient locomotion, i.e., sprinting with low energy expenditure. We confirmed the convection effect in the laboratory by exposing beetles to combinations of radiation, air temperature and wind speed comparable to those found in the Namib Desert and simulating the forced convection of running in a headwind. Under these simulated conditions, peak radiation caused the temperature of stationary male beetles to rise at about 6°C min−1 and females at almost 4°C min−1. However, in wind-calm conditions at peak radiation, the convection of simulated running dropped the equilibrium body temperature of live beetles by about 13°C. We believe that ours is the first report of exercise-induced cooling in a pedestrian animal and that O. plana's diurnal lifestyle depends on that exercise-induced cooling.

The thermoregulatory system of homeothermic endotherms operates to attain thermal equilibrium, that is no net loss or gain of heat, where possible, under a thermal challenge, and not to attain a set-point or any other target body temperature. The concept of a set-point in homeothermic temperature regulation has been widely misinterpreted, resulting in such confusion that some thermoregulation specialists have recommended that it be abandoned. But the set-point concept has enjoyed a resurgence in a different domain, lizard microclimate selection. We review the principles of thermoregulation in homeotherms, endorse a negative feedback system with independent set-points for individual thermo-effectors as its core mechanism, and address the misconceptions about homeothermic set-point. We also explore the concept of set-point range in lizard microclimate selection and conclude that there is substantial convergence between that concept and the set-points of homeothermic thermo-effectors, as thresholds. In neither homeothermic nor lizard thermoregulation is the concept of a unitary set-point appropriate. We review the problems of measuring the set-points for lizard microclimate selection. We do not believe that the set-point concept in thermoregulation should be abandoned just because it has been misinterpreted by some users. It is a valid concept, identifying the threshold body temperatures at which regulatory thermo-effectors will be activated, to aid in attaining thermal equilibrium.

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.

ABSTRACT As the world warms, it will be tempting to relate the biological responses of terrestrial animals to air temperature. But air temperature typically plays a lesser role in the heat exchange of those animals than does radiant heat. Under radiant load, animals can gain heat even when body surface temperature exceeds air temperature. However, animals can buffer the impacts of radiant heat exposure: burrows and other refuges may block solar radiant heat fully, but trees and agricultural shelters provide only partial relief. For animals that can do so effectively, evaporative cooling will be used to dissipate body heat. Evaporative cooling is dependent directly on the water vapour pressure difference between the body surface and immediate surroundings, but only indirectly on relative humidity. High relative humidity at high air temperature implies a high water vapour pressure, but evaporation into air with 100% relative humidity is not impossible. Evaporation is enhanced by wind, but the wind speed reported by meteorological services is not that experienced by animals; instead, the wind, air temperature, humidity and radiation experienced is that of the animal's microclimate. In this Commentary, we discuss how microclimate should be quantified to ensure accurate assessment of an animal's thermal environment. We propose that the microclimate metric of dry heat load to which the biological responses of animals should be related is black-globe temperature measured on or near the animal, and not air temperature. Finally, when analysing those responses, the metric of humidity should be water vapour pressure, not relative humidity.

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.

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.

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.