Warm spells in winter affect the equilibrium between winter phenotypes.

Each phenotype is a product of the interaction of the genes and the environment. Although winter phenotype in seasonal mammals is heritable, its development may be modified by external conditions. In today's world, global climate change and increasing frequency of unpredictable weather events may affect the dynamic equilibrium between phenotypes. We tested the effect of changes in ambient temperature during acclimation to short photoperiod on the development of winter phenotypes in three generations of Siberian hamsters (Phodopus sungorus). Based on seasonal changes in fur colour, body mass, and expression of daily torpor we distinguished three different winter phenotypes: responding, non-responding, and partially-responding to short photoperiod. We found that warm spells in winter can increase the proportion of non-responding individuals in the population, while stable winter conditions can increase photoresponsiveness among the offspring of non-responders. We conclude that the polymorphism of winter phenotype is an inherent characteristic of the Siberian hamster population but the development of winter phenotype is not fixed but rather a plastic response to the environmental conditions.

Siberian hamsters nonresponding to short photoperiod use fasting-induced torpor.

Nonresponding Siberian hamsters (Phodopus sungorus) do not develop the winter phenotype of white fur, low body mass (Mb) and spontaneous torpor in response to short photoperiod. However, their thermoregulatory response to fasting remains unknown. We measured body temperature and Mb of 12 nonresponders acclimated to short photoperiod and then to cold and fasted four times for 24 h. Four individuals used torpor, and in total, we recorded 19 torpor bouts, which were shallow, short and occurred at night. Moreover, fasting increased the heterothermy index in all hamsters. Low Mb was not a prerequisite for torpor use and Mb loss did not correlate with either heterothermy index or torpor use. This is the first evidence that individuals which do not develop the winter phenotype can use torpor or increase body temperature variability to face unpredictable, adverse environmental conditions. Despite the lack of seasonal changes, thermoregulatory adjustments may increase the probability of winter survival in nonresponders.

Is torpor a water conservation strategy? Heterothermic responses to acute water and food deprivation are repeatable among individuals of Phodopus sungorus.

Energy conservation is a clear function of torpor. Although many studies imply that torpor is also a water-saving strategy, the experimental evidence linking water availability with torpor is inconclusive. We tested the relative roles of water and energy shortages in driving torpor, using the Siberian hamster Phodopus sungorus as a model species. To account for the seasonal development of spontaneous heterothermy, we used male hamsters acclimated to short (8L:16D, SP; n = 40) and long (16L:8D, LP; n = 36) photoperiods. We continuously measured body temperature (Tb) during consecutive 32 h of complete removal of water, food, or both, separated by 7.5 d recovery periods. We predicted that all deprivation types would increase the frequency of spontaneous torpor in SP, and induce torpor in LP-acclimated hamsters. Individuals underwent each deprivation type twice in random orders. Food and water deprivation did not induce torpor in LP-acclimated P. sungorus. Patterns of torpor expression varied among deprivation types in SP individuals. Torpor frequency was significantly lower, but bouts were ∼2 h longer and 2.5 °C deeper, during water deprivation compared to food and food-and-water deprivation. Heterothermic responses to all deprivation types were repeatable among individuals. Different torpor patterns during water and food deprivation suggest that water and energy shortages are distinct physiological challenges. Deeper and longer bouts during water deprivation likely led to higher energy and water savings, while shorter and shallower bouts during fasting may reflect a trade-off between energy conservation and food-seeking activity. The lack of a difference between food- and food-and-water-deprived hamsters suggests a higher sensitivity to food than water shortage. This supports the traditional view that energy conservation is the major function of torpor, but suggests that water shortages may also modulate torpor use. The high repeatability of thermoregulatory responses to resource deprivation suggests that these may be heritable traits subject to natural selection.

Polymorphism of winter phenotype in Siberian hamster: consecutive litters do not differ in photoresponsiveness but prolonged acclimation to long photoperiod inhibits winter molt.

BACKGROUND: The theory of delayed life history effects assumes that phenotype of adult individual results from environmental conditions experienced at birth and as juvenile. In seasonal environments, being born late in the reproductive season affects timing of puberty, body condition, longevity, and fitness. We hypothesized that late-born individuals are more prone to respond to short photoperiod (SP) than early born ones. We used Siberian hamsters Phodopus sungorus, a model species characterized by high polymorphism of winter phenotype. We experimentally distinguished the effect of litter order (first or third) from the effect of exposure to long photoperiod (LP) before winter (3 months or 5 months) by manipulating the duration of LP acclimation in both litters. We predicted that, irrespective of the litter order, individuals exposed to long photoperiod for a short time have less time to gather energy resources and consequently are more prone to developing energy-conserving phenotypes. To assess effect of litter order, duration of acclimation to long days, and phenotype on basal cost of living we measured basal metabolic rate (BMR) of hamsters. RESULTS: Individuals born in third litters had faster growth rates and were bigger than individuals from first litters, but these differences vanished before transfer to SP. Litter order or duration of LP acclimation had no effects on torpor use or seasonal body mass changes, but prolonged acclimation to LP inhibited winter molting both in first and third litters. Moreover, individuals that did not molt had significantly higher BMR in SP than those which molted to white fur. Although one phenotype usually predominated within a litter, littermates were often heterogeneous. We also found that over 10% of individuals presented late response to short photoperiod. CONCLUSIONS: Our data indicate that duration of postnatal exposure to LP may define propensity to photoresponsiveness, regardless of the litter in which animal was born. Existence of littermates presenting different phenotypes suggests a prudent reproductive strategy of investing into offspring of varied phenotypes, that might be favored depending on environmental conditions. This strategy could have evolved in response to living in stochastic environment.

Photoresponsiveness affects life history traits but not oxidative status in a seasonal rodent.

BACKGROUND: Shortening photoperiod triggers seasonal adjustments like cessation of reproduction, molting and heterothermy. However there is a considerable among-individual variation in photoresponsiveness within one population. Although seasonal adjustments are considered beneficial to winter survival, and natural selection should favor the individuals responding to changes in photoperiod (responders), the phenotype non-responding to changes in day length is maintained in population. Assuming the same resource availability for both phenotypes which differ in strategy of winter survival, we hypothesized that they should differ in life history traits. To test this we compared reproductive traits of two extreme phenotypes of Siberian hamster Phodopus sungorus - responding and non-responding to seasonal changes in photoperiod. We bred individuals of the same phenotype and measured time to first parturition, time interval between litters, offspring body mass 3, 10 and 18 days after birth and their growth rate. We also analyzed nest-building behavior. Additionally, we estimated the correlation between reproduction, and basal metabolic rate (BMR) and oxidative status in both phenotypes to infer about the effect of reproductive output on future investments in somatic maintenance. RESULTS: Prior to reproduction responding individuals were smaller than non-responding ones, but this difference disappeared after reproduction. Responding pairs commenced breeding later than non-responding ones but there was no difference in time interval between consecutive litters. Responders delivered smaller offspring than non-responders and more out of responding individuals built the nest during winter than non-responding ones. Reproduction did not affect future investments in somatic maintenance. Phenotypes did not differ in BMR and oxidative status after reproduction. However, concentration of reactive oxygen metabolites (ROM) was highest in responding males, and biological antioxidant potential (BAP) was higher in males of both phenotypes than in females. CONCLUSIONS: Delayed breeding in responding Siberian hamsters and high ROM concentration in male responders support our hypothesis that differences in adjustment to winter result in different life history characteristics which may explain coexistence of both phenotypes in a population. We propose that polymorphism in photoresponsiveness may be beneficial in stochastic environment, where environmental conditions differ between winters. We suggest that non-responding phenotype may be particularly beneficial during mild winter, whereas responders would be favored under harsh conditions. Therefore, none of the phenotypes is impaired when compared to the other.

Specialist-generalist model of body temperature regulation can be applied at the intraspecific level.

According to theoretical predictions, endothermic homeotherms can be classified as either thermal specialists or thermal generalists. In high cost environments, thermal specialists are supposed to be more prone to using facultative heterothermy than generalists. We tested this hypothesis at the intraspecific level using male laboratory mice (C57BL/cmdb) fasted under different thermal conditions (20 and 10°C) and for different time periods (12-48 h). We predicted that variability of body temperature (Tb) and time spent with Tb below normothermy would increase with the increase of environmental demands (duration of fasting and cold). To verify the above prediction, we measured Tb and energy expenditure of fasted mice. We did not record torpor bouts but we found that variations in Tb and time spent in hypothermia increased with environmental demands. In response to fasting, mice also decreased their energy expenditure. Moreover, animals that showed more precise thermoregulation when fed had more variable Tb when fasted. We postulate that the prediction of the thermoregulatory generalist-specialist trade-off can be applied at the intraspecific level, offering a valid tool for identifying mechanistic explanations of the differences in animal responses to variations in energy supply.

Melatonin attenuates phenotypic flexibility of energy metabolism in a photoresponsive mammal, the Siberian hamster.

The duration of melatonin (MEL) secretion conveys information about day length and initiates a cascade of seasonal phenotypic adjustments in photoresponsive mammals. With shortening days, animals cease reproduction, minimize energy expenditure, enhance thermoregulatory capacity and adjust functioning of the hypothalamic-pituitary-adrenal (HPA) axis to match the winter increase in energy demands. Within each season, stress plays an important role in the flexible adjustments of a phenotype to environmental perturbations. Recent studies have shown that thermal reaction norms of energy metabolism were narrower in winter-acclimated Siberian hamsters, Phodopus sungorus We tested the hypothesis that physiological changes occurring in response to prolonged MEL signals, including changes in the secretion of stress hormones, are responsible for the seasonal decrease in phenotypic flexibility of energy metabolism in photoresponsive mammals. To quantify reaction norms for basal metabolic rate (BMR) and cortisol (CORT) secretion, male Siberian hamsters maintained at a long (16 h:8 h light:dark) photoperiod were acclimated repeatedly for 12 days to 10 and 28°C. As predicted, the phenotypic flexibility of BMR decreased when animals were supplemented with MEL. However, at the same time, mean CORT concentration and the reaction norm for its secretion in response to changes in acclimation temperature increased. These results suggest that decreased sensitivity of HPA axis to CORT signal, rather than changes in CORT level itself, is responsible for the decreased phenotypic flexibility in photoresponsive species. Our results suggest that decreased phenotypic flexibility in winter, together with increased stress hormone secretion, make photosensitive species more vulnerable to climate change.

Selection for high activity-related aerobic metabolism does not alter the capacity of non-shivering thermogenesis in bank voles.

An intriguing question is how the capacity of non-shivering thermogenesis (NST)-a special mechanism supporting endothermic thermoregulation in mammals-is affected by selection for high exercise metabolism. It has been proposed that high NST could be a mechanism to compensate for a low basal production of heat. On the other hand, high basal or activity metabolism is associated with physiological characteristics such as high performance of the circulatory system, which are also required for achieving a high NST. Here we tested whether selection for high aerobic exercise performance, which correlates with an increased basal metabolic rate, led to a correlated evolution of maximum and facultative NST. Therefore, we measured the NST of bank voles, Myodes (= Clethrionomys) glareolus, from lines selected for 13-14 generations (n=46) for high aerobic metabolism achieved during swimming and from unselected, control lines (n=46). Open-flow respirometry was used to measure the rate of oxygen consumption (V(·)O2) in anesthetized bank voles injected with noradrenaline (NA). After adjusting for body mass, maximum NST (maximum V(·)O2 recorded after injection of NA) did not differ between the selected (2.38±0.08 mLO2min(-1)) and control lines (2.36±0.08 mLO2min(-1); P=0.891). Facultative NST (= maximum NST minus resting metabolic rate of anesthetized animals) did not differ between the selected (1.49±0.07 mLO2min(-1)) and control lines (1.50±0.07 mLO2min(-1); P=0.985), either. Therefore, our results suggest that NST capacity is not strongly linked to maximum activity-related aerobic metabolic rate.

Huddling reduces evaporative water loss in torpid Natterer's bats, Myotis nattereri.

Periodic arousals during hibernation consume most of the winter energy budget for hibernating mammals. Evaporative water loss (EWL) is thought to affect the frequency of arousals and thus energy balance, and might have dramatic implications for over-winter survival and fitness. We hypothesized that huddling affects EWL and energy expenditure in torpid mammals. We tested this hypothesis using bats as a model and predicted that, during torpor, EWL and energy expenditure of huddling individuals would be lower than in individuals that are not in a huddle. We measured EWL and metabolic rate of torpid Myotis nattereri (Kuhl, 1817) huddling in groups or roosting individually. Evaporative water loss in huddling individual bats was almost 30% lower than in solitary animals (P=0.03), even after correcting for the effects of metabolic rate. Our results suggest that conservation of water is a substantial benefit underlying huddling by bats during hibernation. Ultimately, huddling could reduce the total cost of hibernation by reducing the number of expensive periodic arousals from torpor caused by the need to supplement water.

Housing conditions modify seasonal changes in basal metabolism and body mass of the Siberian hamster, Phodopus sungorus.

Proper housing conditions are important aspects of animal welfare. Animals housed in enriched environments show less stereotypic behaviours than animals kept in barren cages. However, different types of cage enrichment may affect the results of experimental studies and hinder comparative analyses of animal physiology and behaviour. We investigated whether access to a running wheel, availability of nesting material, and pair housing affect basal metabolic rate (BMR) of Siberian hamsters (Phodopus sungorus) under various acclimation conditions. We used 70 adult hamsters (35 males and 35 females) divided into five groups housed under different cage conditions. All individuals experienced the same acclimation procedure: first a winter (L8:D16) then a summer (L16:D8) photoperiod, at air temperatures of first 20 °C then 7 °C under both photoperiods. We found that nesting material and pair housing did not affect hamster BMR, while access to a running wheel increased BMR and body mass regardless of photoperiod and ambient temperature. Thus, we suggest that cage enrichment should be applied with caution, especially in studies on energetics or thermoregulation, particularly in seasonal animals.

Social thermoregulation and torpor in the Siberian hamster.

Social thermoregulation and huddling bring about energy benefits to animals sharing a nest because of the smaller surface-to-volume ratio of a huddle and the higher local temperature in the nest. We tested whether living in groups and huddling affect daily torpor, metabolic rate and seasonal changes in the body mass of a small heterothermic rodent, the Siberian hamster (Phodopus sungorus), housed under semi-natural conditions both singly and in groups of four litter-mates. We predicted that in hamsters housed in groups: (1) synchronized torpor bouts would be longer and deeper than non-synchronized ones but shallower than in solitary hamsters, (2) seasonal variations in metabolic rate would be lower than in solitary hamsters, and (3) the winter decrease in body mass would be smaller in grouped than in singly housed hamsters. We found that group housing led to a smaller decrease in body mass in winter, and affected the length and depth of daily torpor. In group-living hamsters more than 50% of all torpor episodes were synchronized and torpid animals were often found in huddles formed of all cage-mates. The longest and deepest torpor bouts in groups were recorded when all animals in a group entered torpor simultaneously. Although the minimum body temperature during torpor was higher, torpor duration was slightly longer than in solitary hamsters. We did not record significant differences in the body mass-adjusted rate of oxygen consumption between solitary and grouped animals, either in the cold or at the lower critical temperature. We conclude that social thermoregulation enables maintenance of a larger body mass, and thus a larger body fat content, which can ensure better body condition at the beginning of the reproductive season.

Diet affects resting, but not basal metabolic rate of normothermic Siberian hamsters acclimated to winter.

We examined the effect of different dietary supplements on seasonal changes in body mass (m(b)), metabolic rate (MR) and nonshivering thermogenesis (NST) capacity in normothermic Siberian hamsters housed under semi-natural conditions. Once a week standard hamster food was supplemented with either sunflower and flax seeds, rich in polyunsaturated fatty acids (FA), or mealworms, rich in saturated and monounsaturated FA. We found that neither of these dietary supplements affected the hamsters' normal winter decrease in m(b) and fat content nor their basal MR or NST capacity. NST capacity of summer-acclimated hamsters was lower than that of winter-acclimated ones. The composition of total body fat reflected the fat composition of the dietary supplements. Resting MR below the lower critical temperature of the hamsters, and their total serum cholesterol concentration were lower in hamsters fed a diet supplemented with mealworms than in hamsters fed a diet supplemented with seeds. These results indicate that in mealworm-fed hamsters energy expenditure in the cold is lower than in animals eating a seed-supplemented diet, and that the degree of FA unsaturation of diet affects energetics of heterotherms, not only during torpor, but also during normothermy.

Phenotypic flexibility in heat production and heat loss in response to thermal and hydric acclimation in the zebra finch, a small arid-zone passerine.

To maintain constant body temperature (Tb) over a wide range of ambient temperatures (Ta) endothermic animals require large amounts of energy and water. In hot environments, the main threat to endothermic homeotherms is insufficient water to supply that necessary for thermoregulation. We investigated flexible adjustment of traits related to thermoregulation and water conservation during acclimation to hot conditions or restricted water availability, or both, in the zebra finch, Taeniopygia guttata a small arid-zone passerine. Using indirect calorimetry, we measured changes in whole animal metabolic rate (MR), evaporative heat loss (EHL) and Tb before and after acclimation to 23 or 40 °C, with different availability of water. Additionally, we quantified changes in partitioning of EHL into respiratory and cutaneous avenues in birds exposed to 25 and 40 °C. In response to heat and water restriction zebra finches decreased MR, which together with unchanged EHL resulted in increased efficiency of evaporative heat loss. This facilitated more precise Tb regulation in heat-acclimated birds. Acclimation temperature and water availability had no effect on the partitioning of EHL into cutaneous or respiratory avenues. At 25 °C, cutaneous EHL accounted for ~ 60% of total EHL, while at 40 °C, its contribution decreased to ~ 20%. Consistent among-individual differences in MR and EHL suggest that these traits, provided that they are heritable, may be a subject to natural selection. We conclude that phenotypic flexibility in metabolic heat production associated with acclimation to hot, water-scarce conditions is crucial in response to changing environmental conditions, especially in the face of current and predicted climate change.

Influence of Algae Supplementation on the Concentration of Glutathione and the Activity of Glutathione Enzymes in the Mice Liver and Kidney.

Algae are potential and natural source of long-chain polyunsaturated fatty acids like eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The diatom Pinnularia borealis accumulates high levels of EPA and may be considered as a source for commercial production of dietary supplements. In this study we asked the question whether diet supplementation with P. borealis may augment antioxidant defense and ameliorate risk factors for cardiovascular diseases. We fed mice (Mus musculus) with lyophilized diatom solutions of different concentrations (1%, 3%, and 5%) for 7 days. Then we measured glutathione content and the activity of glutathione redox system enzymes, total cholesterol and triacylglycerol concentrations, and malondialdehyde concentration in the liver and kidney. We found that cholesterol and triacylglycerol concentrations in the liver and kidneys were the lowest in mice who were fed with the highest concentration of Pinnularia borealis, suggesting protective properties of algae. Additionally, the lowest concentration of Pinnularia borealis was sufficient to improve antioxidant capacity. Our results suggest that P. borealis may be used as a source for dietary supplements rich in EPA, but the amount supplied to the organism should be limited.

Predictive and reactive changes in antioxidant defence system in a heterothermic rodent.

Living in a seasonal environment requires periodic changes in animal physiology, morphology and behaviour. Winter phenotype of small mammals living in Temperate and Boreal Zones may differ considerably from summer one in multiple traits that enhance energy conservation or diminish energy loss. However, there is a considerable variation in the development of winter phenotype among individuals in a population and some, representing the non-responding phenotype (non-responders), are insensitive to shortening days and maintain summer phenotype throughout a year. Differences in energy management associated with the development of different winter phenotypes should be accompanied by changes in antioxidant defence capacity, leading to effective protection against oxidative stress resulting from increased heat production in winter. To test it, we analysed correlation of winter phenotypes of Siberian hamsters (Phodopus sungorus) with facultative non-shivering thermogenesis capacity (NST) and oxidative status. We found that in both phenotypes acclimation to winter-like conditions increased NST capacity and improved antioxidant defence resulting in lower oxidative stress (OS) than in summer, and females had always lower OS than males. Although NST capacity did not correlate with the intensity of OS, shortly after NST induction responders had lower OS than non-responders suggesting more effective mechanisms protecting from detrimental effects of reactive oxygen metabolites generated during rewarming from torpor. We suggest that seasonal increase in antioxidant defence is programmed endogenously to predictively prevent oxidative stress in winter. At the same time reactive upregulation of antioxidant defence protects against reactive oxygen species generated during NST itself. It suggests that evolution of winter phenotype with potentially harmful characteristics was counterbalanced by the development of protective mechanisms allowing for the maintenance of phenotypic adjustments to seasonally changing environment.

Individual Differences in the Phenotypic Flexibility of Basal Metabolic Rate in Siberian Hamsters Are Consistent on Short- and Long-Term Timescales.

Basal metabolic rate (BMR) correlates with the cost of life in endothermic animals. It usually differs consistently among individuals in a population, but it may be adjusted in response to predictable or unpredictable changes in the environment. The phenotypic flexibility of BMR is considered an adaptation to living in a stochastic environment; however, whether it is also repeatable it is still unexplored. Assuming that variations in phenotypic flexibility are evolutionarily important, we hypothesized that they are consistently different among individuals. We predicted that not only BMR but also its flexibility in response to changes in ambient temperature (Ta) are repeatable on short- and long-term timescales. To examine this, we acclimated Siberian hamsters (Phodopus sungorus) for 100 d to winterlike and then to summerlike conditions, and after each acclimation we exposed them interchangeably to 10° and 28°C for 14 d. The difference in BMR measured after each exposure defined an individual's phenotypic flexibility (ΔBMR). BMR was repeatable within and among seasons. It was also flexible in both seasons, but in winter this flexibility was lower in individuals responding to seasonal changes than in nonresponding ones. When we accounted for individual responsiveness, the repeatability of ΔBMR was significant in winter (τ = 0.48, P = 0.01) and in summer (τ = 0.55, P = 0.005). Finally, the flexibility of BMR in response to changes in Ta was also repeatable on a long-term timescale, that is, among seasons (τ = 0.31, P = 0.008). Our results indicate the evolutionary importance of the phenotypic flexibility of energy metabolism and suggest that it may be subject to selection.

Phenotypic flexibility of energetics in acclimated Siberian hamsters has a narrower scope in winter than in summer.

As photoperiod shortens with the approach of winter, small mammals should reduce their energy expenditure to survive periods of food limitation. However, within seasons, animals should balance their energy budgets as abiotic conditions change, sometimes unpredictably; cold spells should increase heat production, while warm spells should do the opposite. Therefore, we addressed specific questions about the possible interactions between seasonal acclimatization and the intra-seasonal phenotypic flexibility of metabolic rate. We hypothesized that phenotypic flexibility in small mammals differs seasonally and is greater in summer than in winter, and predicted that seasonal adjustments in energetics, which are driven by photoperiod, overwhelm the influence of variations in the thermal environment. We measured body mass, basal metabolic rate (BMR), facultative non-shivering thermogenesis (fNST), body temperature, and calculated minimum thermal conductance in Siberian hamsters Phodopus sungorus. Animals were acclimated to winter-like, and then to summer-like conditions and, within each season, were exposed twice, for 3 weeks to 10, 20 or 28 °C. We used differences between values measured after these short acclimation periods as a measure of the scope of phenotypic flexibility. After winter acclimation, hamsters were lighter, had lower whole animal BMR, higher fNST than in summer, and developed heterothermy. After these short acclimations to the above-mentioned temperatures, hamsters showed reversible changes in BMR and fNST; however, these traits were less flexible in winter than in summer. We conclude that seasonal acclimation affects hamster responses to intra-seasonal variations in the thermal environment. We argue that understanding seasonal changes in phenotypic flexibility is crucial for predicting the biological consequences of global climate changes.

Effect of dietary fatty acids on metabolic rate and nonshivering thermogenesis in golden hamsters.

Hibernating rodents prior to winter tend to select food rich in polyunsaturated fatty acids (PUFA). Several studies found that such diet may positively affect their winter energy budget by enhancing torpor episodes. However, the effect of composition of dietary fatty acids (FA) on metabolism of normothermic heterotherms is poorly understood. Thus we tested whether diets different in FA composition affect metabolic rate (MR) and the capacity for nonshivering thermogenesis (NST) in normothermic golden hamsters (Mesocricetus auratus). Animals were housed in outdoor enclosures from May 2010 to April 2011 and fed a diet enriched with PUFA (i.e., standard food supplemented weekly with sunflower and flax seeds) or with saturated and monounsaturated fatty acids (SFA/MUFA, standard food supplemented with mealworms). Since diet rich in PUFA results in lower MR in hibernating animals, we predicted that PUFA-rich diet would have similar effect on MR of normothermic hamsters, that is, normothermic hamsters on the PUFA diet would have lower metabolic rate in cold and higher NST capacity than hamsters supplemented with SFA/MUFA. Indeed, in winter resting metabolic rate (RMR) below the lower critical temperature was higher and NST capacity was lower in SFA/MUFA-supplemented animals than in PUFA-supplemented ones. These results suggest that the increased capacity for NST in PUFA-supplemented hamsters enables them lower RMR below the lower critical temperature of the thermoneural zone.