Contribution of shivering and nonshivering thermogenesis to thermogenic capacity for the deer mouse (Peromyscus maniculatus)

Small mammals that are active all year must develop ways to survive the cold winters. Endotherms that experience prolonged cold exposure often increase their thermogenic capacity. Thermogenic capacity incorporates basal metabolic rate (BMR), nonshivering thermogenesis (NST), and shivering thermogenesis (ST). Increasing the capacity of any of these components will result in increased thermogenic capacity. It is often thought that NST should be the most plastic component of thermogenic capacity and as such is the most likely to increase with cold acclimation. We used deer mice to test this hypothesis by acclimating 27 animals to one of two temperatures (5 degrees or 22 degrees C) for 8 wk. We then measured and compared values for thermogenic capacity--BMR, ST, and NST--between the two groups. Thermogenic capacity and NST increased by 21% and 42%, respectively, after cold acclimation. Neither BMR nor ST showed any change after acclimation. Therefore, it appears that deer mice raise their thermogenic capacity in response to prolonged cold by altering NST only.     

Maximal thermogenic capacity and non-shivering thermogenesis in the South American subterranean rodent Ctenomys talarum

Subterranean rodents inhabit closed tunnel systems that are hypoxic and hypercapnic and buffer aboveground ambient temperature. In contrast to other strictly subterranean rodents, Ctenomys talarum exhibits activity on the surface during foraging and dispersion and hence, is exposed also to the aboveground environment. In this context, this species is a valuable model to explore how the interplay between underground and aboveground use affects the relationship among basal metabolic rate (BMR), cold-induced maximum metabolic rate (MMR), shivering (ST), and non-shivering thermogenesis (NST). In this work, we provide the first evidence of the presence of NST, including the expression of uncoupling proteins in brown adipose tissue (BAT), and shivering thermogenesis in Ctenomys talarum, a species belonging to the most numerous subterranean genus, endemic to South America. Our results show no differences in BMR, cold-induced MMR, and NST between cold- (15?°C) and warm- (25?°C) acclimated individuals. Furthermore, thermal acclimation had no effect on the expression of mitochondrial uncoupling protein 1 (UCP1) in BAT. Only cytochrome c oxidase (COX) content and activity increased during cold acclimation. When interscapular BAT was removed, NST decreased more than 30?%, whereas cold-induced MMR remained unchanged. All together, these data suggest that cold-induced MMR reaches a maximum in warm-acclimated individuals and so a probable ceiling in NST and UCP1 expression in BAT. Possible thermogenic mechanisms explaining the increase in the oxidative capacity, mediated by COX in BAT of cold-acclimated individuals and the role of ST in subterranean life habits are proposed.     

Cold adaptive thermogenesis in small mammals from different geographical zones of China

The mechanisms of thermogenesis and thermoregulation were studied in the tree shrew (Tupaia belangeri) and greater vole (Eothenomys miletus) of the subtropical region, and Brandt's vole (Microtus brandti), Mongolian gerbil (Meriones unguiculatus), Daurian ground squirrel (Spermophilus dauricus) and plateau pika (Ochotona curzoniae) of the northern temperate zone. Resting metabolic rate (RMR) and non-shivering thermogenesis (NST) increased significantly in T. belangeri, E. miletus, M. brandti and M. unguiculatus after cold acclimation (4 degrees C) for 4 weeks. In T. belangeri, the increase in RMR and thermogenesis at liver cellular level were responsible for enhancing the capacity of enduring cold stress, and homeothermia was simultaneously extended. Stable body temperature in M. brandti, E. miletus, M. unguiculatus and O. curzoniae was maintained mainly through increase in NST, brown adipose tissue (BAT) mass and its mitochondrial protein content, and the upregulation of uncoupling protein (UCP1) mRNA, as well as enhancement of the activity of cytochrome C oxidase, alpha-glycerophosphate oxidase and T(4) 5'-deiodinase in BAT mitochondria. The RMR in O. curzoniae and euthermic S. dauricus was not changed, while NST significantly increased during cold exposure; the former maintained their stable body temperature and mass, while body temperature in the latter declined by 4.8 degrees C. The serum T(3) concentration or ratio of T(3)/T(4) in all the species was enhanced after cold acclimation. Results indicated that: (1) the adaptive mechanisms of T. belangeri residing in the subtropical region to cold are primarily by increasing RMR and secondly by increasing NST, and the mechanisms of thermogenesis are similar to those in tropical mammals; (2) in small mammals residing in northern regions, the adaptation to cold is chiefly to increase NST; (3) the mechanism of cold-induced thermogenesis in E. miletus residing in subtropical and high mountain regions is similar to that in the north; (4) a low RMR in warm environments and peak RMR and NST in cold environments enabled M. unguiculatus to tolerate a semi-desert climate; (5) O. curzoniae has unusually high RMR and high NST, acting mainly via increasing NST to adapt to extreme cold of the Qinghai-Tibet Plateau; (6) the adaptation of euthermic S. dauricus to cold is due to an increase in NST and a relaxed homeothermia; and lastly (7) the thyroid hormone is involved in the regulation of cold adaptive thermogenesis in all the species studied.     

Quantitative genetics of bioenergetics and growth-related traits in the wild mammal, Phyllotis darwini

We studied the potential for response to selection in typical physiological-thermoregulatory traits of mammals such as maximum metabolic rate (MMR), nonshivering thermogenesis (NST) and basal metabolic rate (BMR) on cold-acclimated animals. We used an animal model approach to estimate both narrow-sense heritabilities (h2) and genetic correlations between physiological and growth-related traits. Univariate analyses showed that MMR presented high, significant heritability (h2 = 0.69 +/- 0.35, asymptotic standard error), suggesting the potential for microevolution in this variable. However, NST and BMR presented low, nonsignificant h2, and NST showed large maternal/common environmental/nonadditive effects (c2 = 0.34 +/- 0.17). Heritabilities were large and significant (h2 > 0.5) for all growth-related traits (birth mass, growth rate, weaning mass). The only significant genetic correlations we found between a physiological trait and a growth-related trait was between NST and birth mass (r = -0.74; P < 0.05). Overall, these results suggest that additive genetic variance is present in several bioenergetic traits, and that genetic correlations could be present between those different kinds of traits.     

Thermogenic capabilities of the opossum Monodelphis domestica when warm and cold acclimated: similarities between American and Australian marsupials

1. Monodelphis domestica is a small marsupial mammal from South America. Its thermogenic abilities in the cold were determined when the opossums were both warm (WA) and cold (CA) acclimated. Maximum heat production of M. domestica was obtained at low temperatures in helium-oxygen. 2. Basal metabolic rate (BMR) in the WA animals was 3.2 W/kg and mean body temperature was 32.6 degrees C at 30 degrees C. These values were lower than those generally reported for marsupials. Nevertheless, these M. domestica showed considerable metabolic expansibility in response to cold. Sustained (summit) metabolism was 8-9 times BMR, while peak metabolism was 11-13 times BMR. These maximum values were equal to, or above, those expected in small placentals. 3. Cold acclimation altered the thermal responses of M. domestica, particularly in warm TaS. However, summit metabolism was not significantly increased; nor did M. domestica show a significant thermogenic response to noradrenaline, which in many small placentals elicits non-shivering thermogenesis. The thermoregulatory responses of this American marsupial were, in most aspects, similar to those of Australian marsupials. This suggests that the considerable thermoregulatory abilities of marsupials are of some antiquity.     

Heritability of energetics in a wild mammal,the leaf-eared mouse (Phyllotis darwini)

Abstract.— As a first examination of the additive genetic variance of thermoregulatory traits in a natural population of endotherms, we studied the quantitative genetics of key physiological ecology traits in the leaf-eared mouse, Phyllotis darwini. We measured basal metabolic rate (BMR), nonshivering thermogenesis (NST), maximum metabolic rate for thermoregulation (MMR), thermal conductance (C_T), body temperature (T_b), and factorial aerobic scope (FAS) in individuals acclimated to cold and warm conditions. For comparability with previous studies, we included the following morphological traits: foot length (F_L), total length (T_L), body mass (mb, at birth, sexual maturity, 6 months, and 8 months). Variance components were obtained from two different procedures: the expected variance component in an ANOVA Type III sum of squares and an animal model approach using restricted maximum likelihood. Results suggest the presence of additive genetic variance in F_L (h²= 0.47, P = 0.045), C_T of cold-acclimated animals (h²= 0.66, P = 0.041), and night body temperature, measured in cold-acclimated animals (h²= 0.68, P = 0.080). Heritabilities of m_b were near zero at all ages, but maternal effects and common environment effects were high and significant. We found no evidence of additive genetic variance in BMR, NST, MMR, or FAS (i.e., estimates were not significantly different from zero for all tests). Our results are in general agreement with previous studies of mammals that reported low heritability for: (1) BMR and MMR; (2) daytime body temperature; and (3) body mass for wild, but not laboratory or domestic, populations.     

Can non-shivering thermogenesis in brown adipose tissue following NA injection be quantified by changes in overlying surface temperatures using infrared thermography?

We aimed to investigate whether infra red thermography (IRT) can be used to measure and quantify non-shivering thermogenesis (NST) in the short-tailed field vole Microtus agrestis, by directly comparing it with a standard method, i.e. metabolic response following Noradrenaline injection (NA). Mean skin surface temperature overlying Brown adipose tissue (BAT) depot was 0.82 degrees C higher than mean surface temperature that did not overly BAT. The difference in temperature increased by 1.26 degrees C after NA was administered. Mean skin surface temperature overlying BAT increased by 0.32 degrees C after NA was administered; however, surface temperature decreased by 1.32 degrees C after saline was administered. Mean skin surface temperature overlying BAT did not change significantly between warm and cold acclimated voles; in contrast metabolic peak following NA injection significantly increased in cold acclimated voles. There was no significant correlation between change in surface temperature after NA injection and metabolic peak following NA injection. The results of this study suggest that IRT is not a sensitive enough method to measure changes in NST capacity in BAT following NA injection, or to detect changes in NST capacity induced by cold acclimation. However, IRT can distinguish between skin surfaces overlying BAT and skin surfaces that do not.     

大绒鼠冷驯化和脱冷驯化能量代谢特征的变化

通过测定冷驯化(5℃)到脱冷驯化(30℃)条件下,大绒鼠(Eothenomys miletus)的体重、摄入能、静止代谢率(RMR)、非颤抖性产热(NST)和血清瘦素含量等参数,探讨了血清瘦素浓度与能量收支的关系。结果表明,冷驯化可致大绒鼠体重下降,RMR、NST、摄入能升高,血清瘦素浓度降低;脱冷驯化后大绒鼠体重增加,RMR、NST、摄入能降低,血清瘦素浓度增加。血清瘦素含量与体重呈正相关,与RMR、NST、摄入能呈负相关。表明大绒鼠的体重、摄入能和产热能力具有较强的可塑性,且瘦素可能参与了大绒鼠适应冷驯化及恢复过程中的能量平衡和体重的调节。     

The molecular and biochemical basis of nonshivering thermogenesis in an African endemic mammal, Elephantulus myurus

Uncoupling protein 1 (UCP1) mediated nonshivering thermogenesis (NST) in brown adipose tissue (BAT) is an important avenue of thermoregulatory heat production in many mammalian species. Until recently, UCP1 was thought to occur exclusively in eutherians. In the light of the recent finding that UCP1 is already present in fish, it is of interest to investigate when UCP1 gained a thermogenic function in the vertebrate lineage. We elucidated the basis of NST in the rock elephant shrew, Elephantulus myurus (Afrotheria: Macroscelidea). We sequenced Ucp1 and detected Ucp1 mRNA and protein restricted to brown fat deposits. We found that cytochrome c oxidase activity was highest in these deposits when compared with liver and skeletal muscle. Consistent with a thermogenic function of UCP1 isolated BAT mitochondria showed increased state 4 respiration in the cold, as well as palmitate-induced, GDP-sensitive proton conductance, which was absent in liver mitochondria. On the whole animal level, evidence of thermogenic function was further corroborated by an increased metabolic response to norepinephrine (NE) injection. Cold acclimation (18 degrees C) led to an increased basal metabolic rate relative to warm acclimation (28 degrees C) in E. myurus, but there was no evidence of additional recruitment of NE-induced NST capacity in response to cold acclimation. In summary, we showed that BAT and functional UCP1 are already present in a member of the Afrotheria, but the seasonal regulation and adaptive value of NST in Afrotherians remain to be elucidated.     

Seasonal adjustments in body mass and thermogenesis in Mongolian gerbils (<Emphasis Type="Italic">Meriones unguiculatus</Emphasis>): the roles of short photoperiod and cold

Seasonal adjustments in body mass and thermogenesis are important for the survival of small mammals during acclimatization in the temperate zone. To determine the contributions of short photoperiod and cold temperatures to seasonal changes in thermogenesis and body mass in Mongolian gerbils (Meriones unguiculatus), body mass, basal metabolic rate (BMR), nonshivering thermogenesis (NST), energy intake and energy digestibility were determined in seasonally acclimatized and laboratory acclimated animals. Body mass showed significant seasonal changes and decreased to a minimum in winter. Both BMR and NST increased in winter, and these changes were mimicked by exposing animals to short photoperiod or cold temperatures in the animal house. Digestible energy intake also increased significantly in winter, and also during exposure of housed animals to both short photoperiod and cold. These results suggest that Mongolian gerbils overcome winter thermoregulatory challenges by increasing energy intake and thermogenesis, and decreasing body mass to reduce total energy requirements. Short photoperiod and cold can serve as effective environmental cues during seasonal acclimatization.     

Effects of consumption of juniper (Juniperus monosperma) on cost of thermoregulation in the woodrats Neotoma albigula and Neotoma stephensi at different acclimation temperatures

A study was done to test whether toxic plants that occur naturally in the diet affect thermoregulation in mammalian herbivores. The woodrats Neotoma albigula and Neotoma stephensi both consume juniper (Juniperus monosperma), a plant with high levels of toxic compounds. Body temperature (Tb), basal metabolic rate (BMR), and the minimum cost of thermogenesis (Cmin) were measured for both species on control and juniper diets following warm (25 degrees C) and cold (18 degrees C) acclimation. In N. albigula, diet had no uniform effect on Tb, BMR, or Cmin, but dietxacclimation-temperature interaction effects on Tb and Cmin were highly significant (P<0.005). For thermoregulation at 15 degrees C, juniper consumption increased the metabolic cost for warm-acclimated N. albigula by 50% but decreased the metabolic cost in cold-acclimated N. albigula by 24%. In N. stephensi, diet significantly affected Tb and Cmin (P<0.05), but there were no significant dietxacclimation-temperature interaction effects. For thermoregulation at 15 degrees C, juniper consumption increased the metabolic cost for warm-acclimated N. stephensi by 33% but had no significant effect on metabolic cost in cold-acclimated N. stephensi.     

Does thermal history affect metabolic plasticity?: a study in three Phyllotis species along an altitudinal gradient

(1) The aim of this study was to understand the effects of thermal history in metabolic features such as maximum (MMR) and basal (BMR) metabolic rates, as well as in metabolic plasticity, considered as the total variation of MMR and BMR during the acclimation period. (2) We studied three species of the genus Phyllotis, from different thermal environments, in an altitudinal gradient from sea level to 3800m.a.s.l. Animals were acclimated to contrasting temperatures of 5 and 30 degrees C. To determine the metabolic flexibility, MMR was measured at intervals of 6 days during the acclimation period, while BMR values were obtained at the end of acclimations. Aerobic scope and the rates of change of MMR were estimated in all populations. (3) High- and low-altitude rodents did not show differences in BMR. However, both upper and lower limits of MMR, as well as aerobic scope, were significantly different between high- and low-altitude species, indicating similar ranges of metabolic plasticity. On the other hand, the rates of change of MMR were similar in all populations. (4) Our results indicate that thermal history has a profound effect on the individuals' thermogenic capacity, probably in both phylogenetic and ontogenetic levels. Low-altitude species could not increase MMR to the same levels as high-altitude species, while the later were unable to decrease MMR to achieve the values of the low-altitude species.     

被毛在黑线仓鼠能量代谢、产热和体温调节中的作用

为阐明小型哺乳动物被毛的季节性变化及其在能量代谢和体温调节中的作用,测定了季节性驯化,以及不同光周期和温度驯化的黑线仓鼠的被毛重量、体温和能量收支。结果显示: (1)冬季黑线仓鼠的被毛重量和产热显著高于夏季;(2)短光照(8L∶16D)和低温(5℃ ) 对被毛生长的影响不显著; (3)与室温组(21℃ )相比,低温组(5℃ )摄入能、基础代谢率(BMR)、非颤抖性产热(NST) 、褐色脂肪组织细胞色素c 氧化酶活性和热传导率显著增加,而暖温组(30℃ )显著降低; (4)室温和低温下,剃毛导致摄入能、BMR、NST 和热传导率显著增加;结果表明:被毛的作用与环境温度有关,低温下被毛降低了能量需求,增强了动物应对低温环境的适应能力;被毛的适应性变化是独居的野生小型哺乳动物应对环境温度季节性波动的主要适应策略之一,在其能量代谢和体温调节中具有重要作用和意义。     

Solar Radiation during Rewarming from Torpor in Elephant Shrews: Supplementation or Substitution of Endogenous Heat Production?

Many small mammals bask in the sun during rewarming from heterothermy, but the implications of this behaviour for their energy balance remain little understood. Specifically, it remains unclear whether solar radiation supplements endogenous metabolic thermogenesis (i.e., rewarming occurs through the additive effects of internally-produced and external heat), or whether solar radiation reduces the energy required to rewarm by substituting (i.e, replacing) metabolic heat production. To address this question, we examined patterns of torpor and rewarming rates in eastern rock elephant shrews (Elephantulus myurus) housed in outdoor cages with access to either natural levels of solar radiation or levels that were experimentally reduced by means of shade cloth. We also tested whether acclimation to solar radiation availability was manifested via phenotypic flexibility in basal metabolic rate (BMR), non-shivering thermogenesis (NST) capacity and/or summit metabolism (M_sum). Rewarming rates varied significantly among treatments, with elephant shrews experiencing natural solar radiation levels rewarming faster than conspecifics experiencing solar radiation levels equivalent to approximately 20% or 40% of natural levels. BMR differed significantly between individuals experiencing natural levels of solar radiation and conspecifics experiencing approximately 20% of natural levels, but no between-treatment difference was evident for NST capacity or M_sum. The positive relationship between solar radiation availability and rewarming rate, together with the absence of acclimation in maximum non-shivering and total heat production capacities, suggests that under the conditions of this study solar radiation supplemented rather than substituted metabolic thermogenesis as a source of heat during rewarming from heterothermy.     

Effects of cold acclimation on body mass,serum leptin level,energy metabolism and thermognesis in Eothenomys miletus in Hengduan Mountains region

Eothenomys miletus is an important species inhabiting Hengduan mountains region. In order to study adaptive strategy and the role of serum leptin level in response to a 49 d cold exposure, body mass, energy intake, basal metabolic rate (BMR), nonshivering thermogenesis (NST) in E. miletus were measured. During cold exposure (5±1 ^oC), body mass decreased; serum leptin levels decreased significantly and were positively correlated with body mass and fat mass; energy intake, BMR and NST were higher at 5 °C than that of controls. These results suggest that E. miletus enhanced thermogenic capacity and increased maintenance cost during cold acclimation, resulting in increased energy intake. Serum leptin participated in the regulation of energy balance and body mass in E. miletus.     

光周期和高脂食物对布氏田鼠能量代谢和产热的影响

为了研究光周期和高脂食物对小型哺乳动物能量代谢和产热的影响,将成年雌性布氏田鼠(Lasiopodomys brandtii)分别驯化于长光照低脂、高脂食物和短光照低脂、高脂食物,7周后测定动物的体重、能量摄入、产热、血清瘦素浓度以及褐色脂肪组织解偶联蛋白1(BAT-UCP1)含量等参数.结果发现:1)短光照抑制体重增长、降低体脂重量和血清瘦素水平,增加非颤抖性产热(NST)和UCP1含量;2)高脂食物使摄入能减少和消化率提高,但未显著影响体重、基础代谢率、NST、UCP1含量和血清瘦素;3)血清瘦素与摄入能不相关,但与体脂含量正相关.结果暗示:短光照下瘦素作用敏感性增加和产热能力增强,可能介导了抵抗高脂食物诱导的肥胖.在野外条件下草食性的布氏田鼠能通过能量代谢和产热的适应性调节避免体重的过度增长,有利于降低捕食风险,增强生存能力.同时布氏田鼠是研究食物诱导肥胖机理的一个好模型     

Adaptive thermogenesis in Brandt's vole (Lasiopodomys brandti) during cold and warm acclimation

Brandt's voles (Lasiopodomys brandti) exposed to cold (5±1 °C) or warm (23±1 °C) showed some physiological and biochemical variations which might be important in adaptation to their environments. Cold acclimation induced increases in resting metabolic rate (RMR) and the serum triiodothyronine (T₃) level, the state-4 respiration of liver and muscle mitochondria were activated after 7 days when animals exposed to cold, and the activity of cytochrome c oxidase (COX) of liver and muscle mitochondria tended to rise with cold exposure. RMR and T₃ level decreased during warm acclimation. The state-4 respiration of liver mitochondria declined after 3 days and muscle after 7 days when animals exposed to warm, and the activities of COX of liver and muscle mitochondria tended to decrease with warm acclimation. The cold activation of liver and muscle mitochondrial respiration (regulated by T₃) was one of the cytological mechanisms of elevating RMR. Both state-4 respiration and COX activity of brown adipose tissue (BAT) mitochondria increased significantly during cold acclimation and decreased markedly after acclimated to warm. The uncoupling protein 1 (UCP1) contents in BAT increased after exposure to cold and decreased after warm acclimation. Nonshivering thermogenesis (NST) plays an important role in the process of thermoregulation under cold acclimation for Brandt's voles. Changes in thermogenesis is a important way to cold adaptation for Brandt's voles in natural environments.     

哺乳动物最大代谢率的研究进展

最大代谢率（Maximum metabolic rate,MMR)是动物代谢的生理有限。本阐述了其有关概念,测定方法,发育过程中的变化,两种不同方法导致的MMR及其得验性,制约因子,影响因素（包括个体大小,系统发生,环境温度,海拔梯度,寄生虫与病毒）,与非颤抖性产热（Nn-shivering thermogenesis,NST)和基础代谢率（Basal metabolic rate,BMR)的关系,以及其未来的研究方向。其中NMR和BMR的关系涉及到了内温性进化的呼吸能力模型,介绍了相关的生化基础及验证。     

Nonshivering thermogenesis in a marsupial (the tasmanian bettong Bettongia gaimardi) is not attributable to brown adipose tissue

The Tasmanian bettong (Bettongia gaimardi, a marsupial) is a rat-kangaroo that increases nonshivering thermogenesis (NST) in response to norepinephrine (NE). This study attempted to assess whether brown adipose tissue (BAT), a specialized thermogenic effector, is involved in NST in the bettong. Regulatory NST, indicated by resting oxygen consumption (Vo2) of the whole body, was measured under conscious conditions at 20 degrees C with various stimuli: cold (4 degrees -5 degrees C) or warm (25 degrees C) acclimation, NE injection, and the beta3-adrenoceptor agonist (BRL) 37344. In line with the functional studies in vivo, the presence of BAT was evaluated by examining the expression of the uncoupling protein 1 (UCP1) with both rat cDNA and oligonucleotide probes. Both NE and BRL 37344 significantly stimulated NST in the bettong. After cold acclimation of the animals (at 4 degrees -5 degrees C for 2 wk), the resting Vo2 was increased by 15% and the thermogenic effect of NE was enhanced; warm-acclimated animals showed a slightly depressed response. However, no expression of UCP1 was detected in bettongs either before or after cold exposure (2 wk). These data suggest that the observed NST in the marsupial bettong is not attributable to BAT.     

The influence of foraging mode and arid adaptation on the basal metabolic rates of burrowing mammals

Two competing but nonexclusive hypotheses to explain the reduced basal metabolic rate (BMR) of mammals that live and forage underground (fossorial species) are examined by comparing this group with burrowing mammals that forage on the surface (semifossorial species). These hypotheses suggest that the low BMR of fossorial species either compensates for the enormous energetic demands of subterranean foraging (the cost-of-burrowing hypothesis) or prevents overheating in closed burrow systems (the thermal-stress hypothesis). Because phylogentically informed allometric analysis showed that arid burrowing mammals have a significantly lower BMR than mesic ones, fossorial and semifossorial species were compared within these groups. The BMRs of mesic fossorial and semifossorial mammals could not be reliably distinguished, nor could the BMRs of large (>77 g) arid fossorial and semifossorial mammals. This finding favours the thermal-stress hypothesis, because the groups appear to have similar BMRs despite differences in foraging costs. However, in support of the cost-of-burrowing hypothesis, small (<77 g) arid fossorial mammals were found to have a significantly lower BMR than semifossorial mammals of the similar size. Given the high mass-specific metabolic rates of small animals, they are expected to be under severe energy and water stress in arid environments. Under such conditions, the greatly reduced BMR of small fossorial species may compensate for the enormous energetic demands of subterranean foraging.