Search

The cold and hypoxic conditions at high altitude necessitate high metabolic O2 demands to support thermogenesis while hypoxia reduces O2 availability. Skeletal muscles play key roles in thermogenesis, but our appreciation of muscle plasticity and adaptation at high altitude has been hindered by past emphasis on only a small number of muscles. We examined this issue in deer mice (Peromyscus maniculatus). Mice derived from both high-altitude and low-altitude populations were born and raised in captivity and then acclimated as adults to normoxia or hypobaric hypoxia (12 kPa O2 for 6–8 wk). Maximal activities of citrate synthase (CS), cytochrome c oxidase (COX), β-hydroxyacyl-CoA dehydrogenase (HOAD), hexokinase (HK), pyruvate kinase (PK), and lactate dehydrogenase (LDH) were measured in 20 muscles involved in shivering, locomotion, body posture, ventilation, and mastication. Principal components analysis revealed an overall difference in muscle phenotype between populations but no effect of hypoxia acclimation. High-altitude mice had greater activities of mitochondrial enzymes and/or lower activities of PK or LDH across many (but not all) respiratory, limb, core and mastication muscles compared with low-altitude mice. In contrast, chronic hypoxia had very few effects across muscles. Further examination of CS in the gastrocnemius showed that population differences in enzyme activity stemmed from differences in protein abundance and mRNA expression but not from population differences in CS amino acid sequence. Overall, our results suggest that evolved increases in oxidative capacity across many skeletal muscles, at least partially driven by differences in transcriptional regulation, may contribute to high-altitude adaptation in deer mice. NEW & NOTEWORTHY: Most previous studies of muscle plasticity and adaptation in high-altitude environments have focused on a very limited number of skeletal muscles. Comparing high-altitude versus low-altitude populations of deer mice, we show that a large number of muscles involved in shivering, locomotion, body posture, ventilation, and mastication exhibit greater mitochondrial enzyme activities in the high-altitude population. Therefore, evolved increases in mitochondrial oxidative capacity across skeletal muscles contribute to high-altitude adaptation. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

Simple Summary: Although ruminant newborns are a precocial species, they are susceptible to hypothermia and neonatal mortality during the first hours of life due to intrinsic and extrinsic factors that challenge their thermostability. Several mechanisms are activated when the organism perceives a temperature decrease, such as vasoconstriction, shivering, and non-shivering thermogenesis. These have been widely studied in lambs, goats, and cattle. However, for water buffalo, a relevant, productive species, the investigation of thermoregulation in the neonate immediately after calving, and of the tools to assess their thermal state, is limited. This review aims to analyze behavioral, morphological, and physiological strategies in newborn water buffaloes facing thermal stressors and discuss the role of infrared thermography in monitoring hypothermic states. Hypothermia is one of the principal causes of perinatal mortality in water buffaloes and can range from 3% to 17.9%. In ruminants, factors affecting hypothermia in newborns may be of intrinsic (e.g., level of neurodevelopment, birth weight, vitality score, amount of brown fat, skin features) or extrinsic origin (e.g., maternal care, environmental conditions, colostrum consumption). When newborn buffaloes are exposed to cold stress, thermoregulatory mechanisms such as peripheral vasoconstriction and shivering and non-shivering thermogenesis are activated to prevent hypothermia. Due to the properties of infrared thermography (IRT), as a technique that detects vasomotor changes triggered by a reduction in body temperature, evaluating the central and peripheral regions in newborn buffaloes is possible. This review aims to analyze behavioral, physiological, and morphological strategies and colostrum consumption as thermal compensation mechanisms in newborn water buffalo to cope with environmental changes affecting thermoneutrality. In addition, the importance of monitoring by IRT to identify hypothermia states will be highlighted. Going deeper into these topics related to the water buffalo is essential because, in recent years, this species has become more popular and is being bred in more geographic areas. [ABSTRACT FROM AUTHOR]

Hypothermia is one of the principal causes of perinatal mortality in water buffaloes and can range from 3% to 17.9%. In ruminants, factors affecting hypothermia in newborns may be of intrinsic (e.g., level of neurodevelopment, birth weight, vitality score, amount of brown fat, skin features) or extrinsic origin (e.g., maternal care, environmental conditions, colostrum consumption). When newborn buffaloes are exposed to cold stress, thermoregulatory mechanisms such as peripheral vasoconstriction and shivering and non-shivering thermogenesis are activated to prevent hypothermia. Due to the properties of infrared thermography (IRT), as a technique that detects vasomotor changes triggered by a reduction in body temperature, evaluating the central and peripheral regions in newborn buffaloes is possible. This review aims to analyze behavioral, physiological, and morphological strategies and colostrum consumption as thermal compensation mechanisms in newborn water buffalo to cope with environmental changes affecting thermoneutrality. In addition, the importance of monitoring by IRT to identify hypothermia states will be highlighted. Going deeper into these topics related to the water buffalo is essential because, in recent years, this species has become more popular and is being bred in more geographic areas.

Daily compensable cold exposure in humans reduces shivering by ~20% without changing total heat production, partly by increasing brown adipose tissue thermogenic capacity and activity. Although acclimation and acclimatization studies have long suggested that daily reductions in core temperature are essential to elicit significant metabolic changes in response to repeated cold exposure, this has never directly been demonstrated. The aim of the present study is to determine whether daily cold-water immersion, resulting in a significant fall in core temperature, can further reduce shivering intensity during mild acute cold exposure. Seven men underwent 1 h of daily cold-water immersion (14°C) for seven consecutive days. Immediately before and following the acclimation protocol, participants underwent a mild cold exposure using a novel skin temperature clamping cold exposure protocol to elicit the same thermogenic rate between trials. Metabolic heat production, shivering intensity, muscle recruitment pattern, and thermal sensation were measured throughout these experimental sessions. Uncompensable cold acclimation reduced total shivering intensity by 36% (P = 0.003), without affecting whole body heat production, double what was previously shown from a 4-wk mild acclimation. This implies that nonshivering thermogenesis increased to supplement the reduction in the thermogenic contribution of shivering. As fuel selection did not change following the 7-day cold acclimation, we suggest that the nonshivering mechanism recruited must rely on a similar fuel mixture to produce this heat. The more significant reductions in shivering intensity compared with a longer mild cold acclimation suggest important differential metabolic responses, resulting from an uncompensable compared with compensable cold acclimation. NEW & NOTEWORTHY Several decades of research have been dedicated to reducing the presence of shivering during cold exposure. The present study aims to determine whether as little as seven consecutive days of cold-water immersion is sufficient to reduce shivering and increase nonshivering thermogenesis. We provide evidence that whole body nonshivering thermogenesis can be increased to offset a reduction in shivering activity to maintain endogenous heat production. This demonstrates that short, but intense cold stimulation can elicit rapid metabolic changes in humans, thereby improving our comfort and ability to perform various motor tasks in the cold. Further research is required to determine the nonshivering processes that are upregulated within this short time period. [ABSTRACT FROM AUTHOR]

Divers are at enhanced risk of hypothermia, due to the independent action of the inspired inert gases on thermoregulation. Thus, narcosis induced by acute (≤2 h) exposure to either hyperbaric nitrogen or normobaric nitrous oxide (N 2 O) impairs shivering thermogenesis and accelerates body core cooling. Animal-based studies, however, have indicated that repeated and sustained N 2 O administration may prevent N 2 O-evoked hypometabolism. We, therefore, examined the effects of prolonged intermittent exposure to 30% N 2 O on human thermoeffector plasticity in response to moderate cold. Fourteen men participated in two ∼12-h sessions, during which they performed sequentially three 120-min cold-water immersions (CWIs) in 20°C water, separated by 120-min rewarming. During CWIs, subjects were breathing either normal air or a normoxic gas mixture containing 30% N 2 O. Rectal and skin temperatures, metabolic heat production (via indirect calorimetry), finger and forearm cutaneous vascular conductance (CVC; laser-Doppler fluxmetry/mean arterial pressure), and thermal sensation and comfort were monitored. N 2 O aggravated the drop in rectal temperature ( P = 0.01), especially during the first (by ∼0.3°C) and third (by ∼0.4°C) CWIs. N 2 O invariably blunted the cold-induced elevation of metabolic heat production by ∼22%-25% ( P < 0.001). During the initial ∼30 min of the first and second CWIs, N 2 O attenuated the cold-induced drop in finger ( P ≤ 0.001), but not in forearm CVC. N 2 O alleviated the sensation of coldness and thermal discomfort throughout ( P < 0.001). Thus, the present results demonstrate that, regardless of the cumulative duration of gas exposure, a subanesthetic dose of N 2 O depresses human thermoregulatory functions and precipitates the development of hypothermia. NEW & NOTEWORTHY Human thermoeffector plasticity was evaluated in response to prolonged iterative exposure to 30% N 2 O and moderate cold stress. Regardless of the duration of gas exposure, N 2 O-induced narcosis impaired in a persistent manner shivering thermogenesis and thermoperception.

Abstract: Peroxisomes are essential for intermediary lipid metabolism, but the role of these organelles has been primarily studied in the liver. We recently generated aP2–Pex5 conditional knockout mice that due to the nonselectivity of the aP2 promoter, not only had dysfunctional peroxisomes in the adipose tissue but also in the central and peripheral nervous system, besides some other tissues. Peroxisomes were however intact in the liver, heart, pancreas and muscle. Surprisingly, these mice not only showed dysfunctional white adipose tissue with increased fat mass and reduced lipolysis but also the skeletal muscle was affected including impaired shivering thermogenesis, reduced motor performance and increased insulin resistance. Non-shivering thermogenesis by brown adipose tissue was not altered. Strongly reduced levels of plasma adrenaline and to a lesser extent noradrenaline, impaired expression of catecholamine synthesizing enzymes in the adrenal medulla and reversal of all pathologies after administration of the β-agonist isoproterenol indicated that β-adrenergic signaling was reduced. Based on normal white adipose and muscle function in Nestin–Pex5 and Wnt–Pex5 knockout mice respectively, it is unlikely that peroxisome absence from the central and peripheral nervous system caused the phenotype. We conclude that peroxisomal metabolism is necessary to maintain the adrenergic tone in mice, which in turn determines metabolic homeostasis. [Copyright &y& Elsevier]

The article discusses research which was conducted to investigate whether ingesting glucose in combination with fructose (GLU + FRU) after 60 minutes of cold exposure could increase the rate of exogenous carbohydrate (CHO) oxidation and reduce the reliance on endogenous CHO reserves compared with ingesting a glucose drink (GLU). Researchers evaluated six healthy non-cold-acclimatized men who were exposed to low-intensity shivering. They found that the peak exogenous CHO oxidation rate was 30% greater in GLU + FRU compared with GLU and that the utilization of endogenous CHO sources was the same between experimental conditions but was nearly 2.5 times lower than values previously reported by their laboratory when only water was ingested.

Objective: To explore the effect of aspirin on the thermoregulatory responses of men during exposure to 12°C air (acute cold exposure) for a period of 120 minutes and recovery in 25°C air (rewarming) for 120 minutes. Methods: Seven male subjects (26.1 ± 2.4 yr) underwent pre-experimental testing to determine peak VO2 and body composition. Participants underwent 3 trials in which they ingested the following for 1 week prior to each experimental trial: a capsule filled with cellulose (placebo), 81 mg · day−1 of aspirin (low-dose aspirin), or 650 mg · day−1 of aspirin (high-dose aspirin). Each trial consisted of a 30-minute baseline period, 120 minutes of exposure to 12°C air, and 120 minutes of recovery in 25°C air. Mean skin temperature and rectal temperature (Tre) were measured, and heat production was calculated. Results: During both acute cold exposure and rewarming, analysis of variance revealed a main effect for time (P < .05) with respect to Tre, mean skin temperature, and heat production. However, there were no significant differences between the treatments or treatment by time interactions. Conclusions: These data demonstrate that aspirin had no significant effect on the thermal and metabolic responses during acute cold exposure and rewarming. [Copyright &y& Elsevier]

Differences in core temperature and body heat content, generally observed between the luteal and follicular phase of the menstrual cycle, have been reported to modulate the thermogenic activity of cold-exposed women. However, it is unclear how this change in whole body shivering activity will influence fuel selection. The goal of this study was to quantify the effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during low-intensity shivering. Electromyographic activity of eight large muscles was monitored while carbohydrate, lipid, and protein utilization was simultaneously quantified in the follicular and luteal phases of the menstrual cycle in nonacclimatized women shivering at a low intensity. The onset (~25 min), intensity (~ 15% of maximal voluntary contraction), and pattern (~6 shivering bursts/min) of the shivering response did not differ between menstrual cycle phases, regardless of differences in core temperature and hormone levels. This resulted in lipids remaining the predominant substrate, contributing 75% of total heat production, independent of menstrual phase. We conclude that hormone fluctuations inherent in the menstrual cycle do not affect mechanisms of substrate utilization in the cold. Whether the large contribution of lipids to total heat production in fuel selection confers a survival advantage remains to be established. [ABSTRACT FROM AUTHOR]

Abstract: This study evaluated the thermogenic capacity of three species of fruit-eating phyllostomid bats (Carollia perspicillata, Sturnira lilium and Artibeus lituratus) during the dry-cool and wet-warm seasons, by measuring changes in body mass, basal metabolic rate (BMR), maximal metabolic rate (MMR), nonshivering thermogenesis and shivering thermogenesis. Body mass was lower, on average, during the dry-cool season and all species of fruit-eating bats showed an increase in oxygen consumption after noradrenaline injection and after exposure to a He–O2 atmosphere. However, the magnitude of this increase was similar in both seasons. BMR also did not vary between seasons. Although, our results showed for the first time that all three species studied were able to increase thermogenesis by both nonshivering and shivering thermogenesis, we did not find significant differences in any thermoregulatory variable measured when comparing data from the two different seasons. Probably the difference in the mean and variance of the temperature profile between seasons were not strong enough to alter the thermogenic capacity of these species. Furthermore, the use of alternative physiological (torpor) or behavioral (huddling) strategies might have alleviated the need to trigger energetic-costly thermogenic responses. [Copyright &y& Elsevier]

One of the factor limiting the oxidation of exogenous glucose during cold exposure may be the delay in establishing a shivering steady state (∼60 min), reducing glucose uptake into skeletal muscle. Therefore, using indirect calorimetry and isotopic methodologies in non-cold-acclimatized men, the main purpose of this study was to determine whether ingesting glucose at a moment coinciding with the maximal shivering intensity could increase the utilization rate of the ingested glucose. 13C-enriched glucose was ingested (800 mg/min) from the onset (GO) or after 60 min (G60) of cold exposure when the thermogenic rate was stabilized to low-intensity shivering (∼2.5 times resting metabolic rate). For the same quantity of glucose ingested, the oxidation rate of exogenous glucose was 35% higher in G60 (159 ± 17 vs. 118 ± 1 mg/min in GO) between minutes 60 and 90. By the end of cold exposure, exogenous glucose oxidation was significantly greater in GO, reaching 231 ± 14 mg/min, ∼15% higher than the only rate previously reported. This considerably reduced the utilization of endogenous reserves over time and compared with the G60 condition. This study also demonstrates a fall in muscle glycogen utilization, when glucose was ingested from the onset of cold exposure (from ∼150 to ∼75 mg/min). Together, these findings indicate the importance of ingesting glucose immediately on exposure to a cold condition, relying on shivering thermogenesis and sustaining that consumption for as long as possible. This substrate not only provides an auxiliary fuel source for shivering thermogenesis, but, more importantly, preserves the limited endogenous glucose reserves. [ABSTRACT FROM AUTHOR]

Although the importance of food consumption to survive in the cold is well established, most shivering studies have focused on fuel selection in fasting subjects. Therefore, the aim of the present study was to provide the first estimates of exogenous glucose as well as liver and muscle glycogen oxidation rates of non-cold acclimatized men (n = 6) ingesting glucose in trace amounts (Control; C), and at rates of 400 mg min(-1) (Low Glucose; LG), and 800 mg min(-1) (High Glucose; HG) during moderate-intensity shivering (~3 times resting metabolic rate or ~20% VO(2max)) using indirect calorimetry and stable isotope methodologies. Exogenous glucose oxidation peaked at ~200 mg min(-1) at the lowest glucose ingestion rate (~400 mg min(-1)). In addition, glucose ingestion increased the contribution of plasma glucose to total heat production by ~50% but did not change the role played by muscle glycogen (~27% of heat production for control condition and ~23-28% for LG and HG). Instead, the contribution of liver-derived glucose to total heat production was reduced by 40-60% in LG and HG, respectively. In conclusion, glucose ingestion even at low rates contributes a significant proportion of total heat production during moderate intensity shivering and reduces the utilization of liver-derived glucose but not muscle glycogen. [ABSTRACT FROM AUTHOR]

For long migrations, birds must rely on high flux capacities at all steps of lipid metabolism, from the mobilization of adipose reserves to fatty acid oxidation in flight muscle mitochondria. Substrate kinetics and indirect calorimetry were used to investigate key parameters of lipid metabolism in a highly aerobic shorebird: the ruff sandpiper Philomachus pugnax. In this study, we have quantified the effects of cold exposure because such measurements are presently impossible during flight. Lipolytic rate was monitored by continuous infusion of 2-[³H]-glycerol and lipid oxidation by respirometry. Plasma lipid concentrations (non-esterified fatty acids, neutral lipids and phospholipids) and their fatty acid composition were also measured to assess whether cold exposure causes selective metabolism of specific lipids. Results show that shivering leads to a 47% increase in metabolic rate (44.4±3.8 ml O2 kg-1 min-1 to 65.2±8.1 ml O2 kg-1 min-1), almost solely by stimulating lipid oxidation (33.3± 3.3 ml O2 kg-1 min-1 to 48.2±6.8 ml O2 kg-1 min-1) because carbohydrate oxidation remains close to 11.5± 0.5 ml O2 kg-1 min-1. Sandpipers support an unusually high lipolytic rate of 55-60 µmol glycerol kg-1 min-1. Its stimulation above thermoneutral rates is unnecessary during shivering when the birds are still able to re-esterify 50% of released fatty acids. No changes in plasma lipid composition were observed, suggesting that cold exposure does not lead to selective metabolism of particular fatty acids. This study provides the first measurements of lipolytic rate in migrant birds and shows that their capacity for lipid mobilization reaches the highest values measured to date in vertebrates. Extending the limits of conventional lipid metabolism has clearly been necessary to achieve long-distance migrations. [ABSTRACT FROM AUTHOR]

In cold-exposed adult humans, significant or lethal decreases in body temperature are delayed by reducing heat loss via peripheral vasoconstriction and by increasing rates of heat production via shivering thermogenesis. This brief review focuses on the mechanisms of fuel selection responsible for sustaining long-term shivering thermogenesis. It provides evidence to explain large discrepancies in fuel selection measurements among shivering studies, and it proposes links between choices in fuel selection mechanism and human survival in the cold. Over the last decades, a number of studies have quantified the contributions of carbohydrate (CHO) and lipid to total heat generation. However, the exact contributions of these fuels still remain unclear because of large differences in fuel selection measurements even at the same metabolic rate. Recent advances on the mechanisms of fuel selection during shivering provide some plausible explanations for these discrepancies between shivering studies. This new evidence indicates that muscles can sustain shivering over several hours using a variety of fuel mixtures achieved by modifying diet (changing the size of CHO reserves) or by changing muscle fiber recruitment (increasing or decreasing the recruitment of type II fibers). From a practical perspective, how does the choice of fuel selection mechanism affect human survival in the cold? Based on a glycogen-depletion model, estimates of shivering endurance show that, whereas the oxidation of widely different fuel mixtures does not improve survival time, the selective recruitment of fuel-specific muscle fibers provides a substantial advantage for cold survival. By combining fundamental research on fuel metabolism and applied strategies to improve shivering endurance, future research in this area promises to yield important new information on what limits human survival in the cold. [ABSTRACT FROM AUTHOR]

Heat exchange has been thoroughly studied in cold-exposed humans, but the metabolic substrates used for thermogenesis have received less attention. This review deals with oxidative fuel selection in shivering humans. Lipids provide most of the heat during low-intensity shivering, whereas carbohydrates become dominant under more extreme cold conditions. The contribution from plasma glucose always remains minor, but muscle glycogen plays an important role during intense shivering. Whether the size of muscle glycogen stores influences endurance in the cold remains to be demonstrated. The fuel selection patterns of shivering and exercise are different, but the mechanisms underlying this difference have not been investigated. The simultaneous measurement of metabolic substrate oxidation and muscle fibre recruitment has allowed to characterize two different mechanisms of fuel selection in shivering humans: the recruitment of different pathways within the same fibres and of different fuel-specific fibres within the same muscles. This suggests that muscle fibre composition of each individual may affect survival. Future research promises to provide a combination of theoretical advances on fundamental principles of fuel selection and applied strategies to manipulate fibre composition (through training) or fuel metabolism (through diet) to prolong human survival in cold environments. [ABSTRACT FROM AUTHOR]

Abstract: To be able to match ATP supply with demand, animals must ensure adequate delivery of metabolic fuels and oxygen to tissue mitochondria. Therefore, the mixture of fuels provided and their individual flux must be tightly orchestrated to cope with changing physiological needs. In exercising mammals, metabolic rate—expressed relatively to the aerobic maximum: %VO2 max—determines what mixture of oxidative fuels is being used. This simple model of fuel selection accurately predicts the relative contributions of lipids and carbohydrates to total metabolism, and it applies widely across body sizes, aerobic capacities, and even to exercise in hypoxic environments. However, it is also becoming obvious that significant exceptions to this pattern exist in other vertebrates that rely more heavily on lipids (e.g., migrating birds) or proteins (e.g., migrating salmonids), or for stresses other than exercise (e.g., cold exposure in mammals). Instantaneous fuel use is determined by multiple interacting mechanisms involving fuel availability, storage location, muscle recruitment, fiber recruitment within each muscle, and metabolic pathway selection within each fiber. These various mechanisms are being characterized in more detail to try designing a general model of fuel selection applicable to a wider range of animals and physiological stresses. [Copyright &y& Elsevier]

Muscular changes accompanying and/or promoting the rapid postnatal improvement of the thermogenic efficiency of shivering were investigated in piglets. Animals were obtained at birth or killed after 5 days at thermoneutrality (34–30 °C) or in the cold (24–15 °C), to stimulate intense shivering thermogenesis. Fast-twitch-glycolytic (longissimus lumborum) and slow-twitch-oxidative (rhomboid) muscles were prepared for electron microscopic examination and chemical measurements. Muscle-specific changes in energy stores and metabolism were observed after birth, including the switch from glycogen to lipids and variation of the lactate/pyruvate ratio corresponding to the progressive acquisition of the metabolic type of the mature muscles. There was major age-related and/or cold-induced development of the structures involved in excitation-contraction coupling (triadic profiles, +80% in the cold), oxidative metabolism (number of lipid droplets, +81% with age in the cold; number of mitochondria, +29% with age or cold; surface of mitochondrial inner membranes, +18% with age and +32% in the cold) and contraction potential (myofibril volume, +62% with age). In contrast, neither age nor cold affected capillary volume density and capillary-to-fibre ratio. The observed changes reflect the immaturity and remarkable plasticity of piglet skeletal muscle and are likely to underlie its enhanced capacity for shivering thermogenesis after birth. [ABSTRACT FROM AUTHOR]

The rapid maturation of thermoregulatory mechanisms may be of critical importance for optimising chick growth and survival and parental energy investment under harsh climatic conditions. The ontogeny of thermoregulatory mechanisms was studied in growing king penguin chicks from hatching to the full emancipation observed at 1 month of age in the sub-Antarctic area (Crozet Archipelago). Newly hatched chicks showed small, but significant regulatory thermogenesis (21% rise in heat production assessed by indirect calorimetry), but rapidly became hypothermic. Within a few days, both resting (+32%) and peak (+52%) metabolic rates increased. The first week of life was characterised by a two-fold rise in thermogenic capacity in the cold, while thermal insulation was not improved. During the second and third weeks of age, thermal insulation markedly rose (two-fold drop in thermal conductance) in relation to down growth, while resting heat production was slightly reduced (−13%). Shivering (assessed by electromyography) was visible right after hatching, although its efficiency was limited. Thermogenic efficiency of shivering increased five-fold with age during the first weeks of life, but there was no sign of non-shivering thermogenesis. We conclude that thermal emancipation of king penguin chicks may be primarily determined by improvement of thermal insulation after thermogenic processes have become sufficiently matured. Both insulative and metabolic adaptations are required for the rapid ontogeny of thermoregulation and thermal emancipation in growing king penguin chicks. [Copyright &y& Elsevier]

Hypothalamic temperature ( Thypo) and metabolic heat production (M) were measured in seven conscious rabbits injected intravenously with either saline or with Staphylococcus aureus, (8 · 107 cell walls · kg−1) while being subjected to a 3-h period of ramp-like total body cooling using a chronically implanted intravascular heat exchanger. In pyrogen-injected animals cooling started (1) at the time of injection or (2) 70 min after injection. In (1) the fall in Thypo induced by heat extraction was similar (1.0 °C) in afebrile and febrile animals. In (2) there was a transient increase in Thypo of about 0.5 °C at a time corresponding to the start of fever resulting in a significantly smaller fall in Thypo at the end of the 3-h cooling period (0.5 °C vs 0.9 °C, P < 0.05, n = 5). At this time in both (1) and (2) M was lower than theoretically expected from the increase in shivering threshold during fever. However, most of this effect can be explained when available data showing a decrease in thermosensitivity during S. aureus-induced fever are taken into account. After cessation of cooling in both groups of febrile animals Thypo rose to about 1 °C higher than the precooling level, which is comparable to the fever level in a separate series of experiments with S. aureus injection without cooling (1.2 °C). [ABSTRACT FROM AUTHOR]