Effects of repeated short-term cold exposures on cold induced thermogenesis of women

The effects of repeated exposures to resting cold air (10°C) on the shivering and thermogenic responses of women to standard cold stress were investigated. Ten women, aged 18 to 34 years, were divided into two groups of five women each. One group, the acclimated (A) was exposed ten times within 2 weeks, the first and the last exposures being the pre-and post-tests respectively. The second group, the control (C) was exposed twice within 18 days. Measurements of rectal and skin temperatures, oxygen uptake, time to onset of shivering (TOS), and perceived cold were performed during all exposures. Shivering responses were evaluated by electromyography and visually. A significant (P<0.05), increase was seen in TOS (from 26.2 min to 55.6 min), and a significant decrease was seen in thermoregulatory heat production (from 14.78 kcal/h to –2.64 kcal/h) in group A; these changes were evident after about five exposures. It is concluded that the women became cold acclimated as a result of the repeated short-term resting cold air exposures.Research supported by Capes/Brazil, and by the Universidade Federal de Minas Gerais/Brazil     

The physiology of hypothermia in the black-capped Chickadee,Parus atricapillus

Summary The shivering, body temperature, and metabolic response to stable and decreasing ambient temperature were measured in winter acclimatized Black-capped Chickadees,Parus atricapillus. Shivering activity, measured by duration and amplitude of bursts, increased curvilinearly from thermoneutral temperatures of 27°C down to 0°C. This parabolic shivering response may be a major component of the curvilinear response of metabolism to decreasing ambient temperature.Birds exposed to 0°C exhibited metabolism 32–45% lower than predicted for a 12-g homeotherm and body temperatures 10°C below the pre-experimental nocturnal body temperature. This hypothermia was not the result of a breakdown in thermoregulation, but was a controlled effort serving to reduce overnight energy expenditure. It is suggested that (1) hypothermia was achieved by decreased shivering by pectoral muscles during exposure to decreasing ambient temperatures, (2) the rate of body temperature decline was moderated by intermittent and reduced bursts during the cooling period, and (3) body temperature was maintained at a particular level during exposure to a stable low ambient temperature by intense bursts lasting one to three minutes.The physiology of hypothermia in chickadees is similar to torpor; however, chickadees did not arouse to a normal diurnal body temperature in the laboratory, and their hypothermia was not induced by inanition or prolonged exposure to cold, as reported for other species capable of torpor.     

A comparison of the responses of tropical and temperate flies (Diptera: Sarcophagidae) to cold and heat stress

Summary Two flesh fly species from the tropical lowlands (Peckia abnormis and Sarcodexia sternodontis) were more susceptible to both cold-shock and heatshock injury than temperate flies (Sarcophaga crassipalpis and S. bullata) and a fly from a tropical high altitude (Blaesoxipha plinthopyga). A brief (2-h) exposure to 0°C elicits a protective response against subsequent cold injury at–10°C in the temperate flies and in B. plinthopyga but no such response was found in the flies from the tropical lowlands. However, both tropical and temperate flies could be protected against heat injury (45°C) by first exposing them to a mild heat shock (2 h at 40°C). The supercooling point is not a good indicator of cold tolerance: supercooling points of pupae were similar in all species, ranging from–18.9 to–23.0°C, and no differences were found between the tropical and temperate species. Among the temperate species, glycerol, the major cryoprotectant, can be elevated by short-term exposure to 0°C, but glycerol could not be detected in the tropical flies. Low-temperature (0°C) exposure also increased hemolymph osmolality of the temperate species, but no such increase was observed in the tropical lowland species. Adaptations to temperature stress thus differ in tropical and temperate flesh flies: while flies from both geographic areas share a mechanism for rapidly increasing heat tolerance, only the temperate flies appear capable of responding rapidly to cold stress. The presence of a heat shock response in species that lack the ability to rapidly respond to cold stress indicates that the biochemical and physiological bases for these two responses are likely to differ.     

Heat and cold shock responses in different strains of Thiobacillus ferrooxidans

Different strains of Thiobacillus ferrooxidans were examined for their ability to produce a heat shock and a cold shock response. Strain A1, heat shocked from 20° to 35°C, acquired thermotolerance, as it showed a 1000-fold reduction in cell mortality when exposed to the supermaximum temperature of 42°C, as compared to a non-heat-shocked control. A heat shock from 25° to 35°C yielded similar results, although a higher degree of thermotolerance was achieved for the shorter exposure times. Cultures heat shocked for 5 h showed a five-log reduction in viable counts after 41 h at 42°C, whereas non-heat-shocked cultures showed a similar reduction in viability in 28 h. Conferred thermotolerance was immediate and sustained for the duration of the exposure to 42°C. Heat-shocked cultures were not significantly protected against loss of viability due to freezing (-15°C for 24 h). Strain S2, cold shocked from 25° to 10°C, and strain D6, cold shocked from 25° to 5°C, were not protected against freezing at-15°C. An analysis of proteins extracted from heat-shocked cells of strain A1 showed the presence of at least one newly induced protein and eight hyper-induced proteins. The molecular weights of the heat shock proteins were in the range of 15–80.3 kDa.     

Shivering modulation in humans: Effects of rapid changes in environmental temperature

During cold exposure, increase in heat production is produced via the activation of shivering thermogenesis and nonshivering thermogenesis, the former being the main contributor to compensatory heat production in non-acclimatized humans. In rats, it has been demonstrated that shivering thermogenesis is modulated solely by skin thermoreceptors but this modulation has yet to be investigated in humans. The aim of this study was to determine if cold-induced shivering in humans can be modulated by cutaneous thermoreceptors in conditions where increases in heat loss can be adequately compensated by increases in thermogenic rate. Using a liquid-conditioned suit, six non-acclimatized men were exposed to cold (6 °C) for four 30 min periods, each of them separated by 15 min of heat exposure (33 °C). Core temperature remained stable throughout exposures whereas skin temperatures significantly decreased by 12% in average during the sequential cold/heat exposures compared to baseline (p<0.0001). Shivering intensity and metabolic rate increased significantly during 6 °C exposures (3.3±0.7% MVC, 0.40±0.0 L O₂/min, respectively) and were significantly reduced during 33 °C exposure (0.5±0.1% MVC, 0.25±0.0 L O₂/min; p<0.005 for both). Most importantly, shivering could be quickly and strongly inhibited during 33 °C exposure although skin temperature often remained below baseline values. In conclusion, under compensatory conditions, cutaneous thermoreceptors appear to be a major modulator of the shivering response in humans and seem to react rapidly to changes in the microclimate right next to the skin and to skin temperature.     

Daily rhythm of oxygen consumption and thermoregulatory responses in some European winter- or summer-acclimatized finches at different ambient temperatures

The oxygen consumption of European finches, the siskin (Carduelis spinus), the brambling (Fringilla montifringilla), the bullfinch (Pyrhulla pyrhulla), the greenfinch (Carduelis chloris) and the hawfinch (Coccothraustes coccothraustes), was recorded continuously while ambient temperature was decreased stepwise from +30 down to-75°C. The oxygen consumption, body temperature (telemetrically), and shivering (integrated pectoral electromyography) of greenfinches were measured simultaneously at ambient temperatures between +30 and-75°C. Maximum heat production, cold limit, lower critical temperature, basal metabolic rate and thermal conductance (of the greenfinch) were determined. The diurnal variation of oxygen consumption of siskins and greenfinches was recorded at thermoneutrality and below the thermoneutral zone in winter- and summer-acclimatized birds. The diurnal variation of body temperature and thermal conductance of greenfinches were also determined. The diurnal variation of heat production was not seasonal or temperature dependent in the siskin and in the greenfinch. Nocturnal reduction of oxygen consumption saved 15–33% energy in the siskin and greenfinch. Body temperature of the greenfinch was lowered by 2.5–3.4°C. The nocturnal reduction of thermal conductance in the greenfinch was 39–48%. The basal metabolic rate was lowest in the largest bird (hawfinch) and highest in the smallest bird (siskin). The values were in the expected range. The heat production capacity of finches in winter was 4.7 times basal metabolic rate in the siskin, 4.2 times in the brambling, 3.5 times in the greenfinch and 2.9 times in the bullfinch and hawfinch. The heat production capacity of the siskin and greenfinch was not significantly lower in summer. The cold limit temperatures (°C) in winter were-61.2 in the siskin,-41.3 in the greenfinch,-37.0 in the bullfinch,-35.7 in the brambling and-28.9 in the hawfinch. The cold limit was 14.3°C higher in summer than in winter in the siskin and 8.7°C in the greenfinch. Thermal insulation of the greenfinch was significantly better in winter than in summer. The shivering of the greenfinch increased linearly when ambient temperature was decreased down to-40°C. Maintenance of shivering was coincident with season. In severe cold integrated pectoral electromyography did not correlate with oxygen consumption as expected. The possible existence of non-shivering thermogenesis in birds is discussed. It is concluded that the acclimatization of European finches is primarily metabolic and only secondly affected by insulation.Abbreviations AAT avian adipose tissue - bm body mass - BMR basal metabolic rate - C _t thermal conductance - EMG electromyogram - HP heat production - HP _max maximum heat production - MR metabolic rate - NST non-shivering thermogenesis - RMR resting metabolic rate - RQ respiratory quotient - T _a ambient temperature - T _b body temperature - T _c colonic temperature - T _1c lower critical temperature - TNZ thermoneutral zone - T _st shivering threshold temperature - V oxygen consumption     

Cold thermoregulatory changes induced by sleep deprivation in men

The aim of this study was to evaluate the thermoregulatory changes induced by 27-h of sleep deprivation (SD) in men at rest both in a comfortable ambient temperature and in cold air. A group of 12 male subjects were placed in a comfortable ambient temperature (dry bulb temperature,T _db = 25° C, relative humidity, rh = 40%–50% , clothing insulation = 1 clo) for 1 h and then they were submitted to a standard cold air test in a climatic chamber for 2h (T _db=1° C, rh = 40%–50%, wind speed = 0.8 m·s^–1, nude), before and after 27 h of sleep deprivation. Thermoregulatory changes (rectal temperature,T _re; mean skin temperature, _sk; metabolic heat production ) were monitored continuously. At comfortable ambient temperature, no significant change was observed after SD forT _re, _sk and . During the cold test,T _re did not change but _sk and were higher after SD (P<0.05). Increased (+ 6%,P < 0.05) was related to earlier and higher shivering, with a possible increase in the sensitivity of the thermoregulatory system as shown by the shorter time to onset of continous shivering (d): 8.66 (SEM 1.33) min versus 28.20 (SEM 1.33) min (P < 0.001) and by a higher _sk observed at d: 27.60 (SEM 1.40)° C versus 21.40 (SEM 0.60)° C (P < 0.001). These results were associated with higher cold sensations and shivering following SD. They also suggested that SD modified thermoregulatory responses at a central level especially in a cold environment.     

Rapid cold hardening in the olive fruit fly Bactrocera oleae under laboratory and field conditions

A rapid cold hardening response was studied in females and males of the olive fruit fly Bactrocera (Dacus) oleae. When laboratory-reared females and males were transferred and maintained from the rearing temperature of 24 °C for 2 h to –6.5 °C approximately 5% survived. However, conditioning of both females and males for 2 h at various temperatures from 0 to 10 °C before their exposure for 2 h to –6.5 °C increased survival to 80 to 92%. A similar rapid cold hardening response in both females and males was also induced through gradual cooling of the flies at a rate of approximately 0.4 °C per min. The rapid increase in cold tolerance after prior conditioning of the flies to low temperatures, was rapidly lost when they returned to a higher temperature of 24 °C. In the field, in late February and early March, females and males were capable of a rapid cold hardening response. After exposure to the critical temperature they suffered a high mortality when tested in the afternoon and low mortality early in the morning on consecutive days, probably because of differences in the prevailing field temperatures a few hours before testing. This plasticity of cold tolerance gained through rapid cold hardening may allow the flies to survive during periods of the year with great fluctuation in circadian temperatures.     

The effects of acclimation to alternating environmental temperature on metabolic and endocrine responses in guinea pigs,during acute heat and cold exposure

Male Guinea pigs (n=80) were divided into four groups and maintained in a climatic chamber for three weeks in one of the following environmental conditions: (1) T_a20°C and 55% RH; (2) T_a35°C and 30–35% RH from 08:00 to 20:00 h and 5°C; 60–65% RH, from 20:00 h to 08:00 h; (3) T_a5°C and 60–65% RH; (4) T_a35°C and 30–35% RH. At the end of this period the animals were exposed to either –5°C, 60–65% RH or 45°C, 30–35% RH, for a period of 20 min, following which T_re, plasma 11-OHCS, thyroxin, glucose, and FFA, and body and organ weights were determined. The cold-warm adapted animals seemed to develop a more efficient adaptability to acute heat and cold exposure. It is suggested that on acute exposure to severe environmental conditions the endocrine and the nervous system play a dominant role in maintaining optimal body temperature, while on chronic exposure the metabolic rate of the various organs becomes relatively more important.     

Temperature regulation in a burrow-nesting tropical seabird,the Wedge-tailed Shearwater (Puffinus pacificus)

Summary At low air temperatures (2.3–13.9°C), Wedge-tailed Shearwaters (Puffinus pacificus) shivered and their oxygen consumption increased to as much as 283% of the mean value (0.77 ml O₂/g·h) within the thermoneutral zone of air temperature (23–34°C). The minimal thermal conductance of the tissues and plumage was similar to the value predicted from the body mass (320.5 g). The oxygen consumption of the birds within their thermoneutral zone was lower than predictions based on body mass. At elevated air temperatures, the shearwaters panted at respiratory frequencies as high as 260 respirations/min; maximal respiratory frequencies were not invoked until the birds had become hyperthermic. During exposure to a hot environment, the oxygen consumption of the birds increased and in most instances the shearwaters were not able to lose heat equivalent to their concurrent metabolic heat production.Symbols and abbreviations BMR basal metabolic rate - C _total total thermal conductance - f respiratory frequency - TEWL total evaporative water loss - T _st stomach temperature - T _re rectal temperature     

Seasonal thermogenic capacity in a hibernator,Spermophilus richardsonii

Summary The maximum thermogenic capacity (HPmax) and the maximum capacity for non-shivering thermogenesis (NSTmax) were assessed in a hibernator, the Richardson's ground squirrel at different times of year. The HPmax was elicited by exposing animals to He–O₂ (21% oxygen, balance helium) at –10 to –25°C. The NSTmax was estimated by i.v. infusion of isoproterenol in anesthetized animals at thermoneutrality. Non-hibernating phase adults were collected and tested in April and June, and youngs in June and August for effects of seasonal acclimatization; animals were also tested after acclimation to cold (5°C) or warm (20°C). Hibernating phase animals were tested both shortly after the onset of hibernation season and after several months into the hibernation season. Although HPmax differed significantly between the lowest [101 cal (wt^0.73·h)^–1 in the June-Young group] and the highest [142 cal (wt^0.73·h)^–1 in the June-Adult group], it was not significantly different between other groups regardless of hibernation status or temperature acclimation (Fig. 4). The NSTmax, however, increased from 40–50 cal (wt^0.73·h)^–1 in the Warm-Acclimated, August-Young, June-Adult, and April-Adult to 66.5 and 79.2 cal (wt^0.73·h)^–1 in the two hibernating groups (Fig. 3). No significant difference in NSTmax was observed between Cold- and Warm-Acclimated groups. Since HPmax was maintained essentially constant at different times of year or after temperature acclimation, the increase of NSTmax during the hibernating phase can best be viewed as an adjustment for facilitation of periodic rewarmings from depressed body temperature during hibernation rather than to counter cold.Abbreviations HP heat production - HPmax maximum heat production - NST non-shivering thermogenesis - NSTmax maximum non-shivering thermogenesis - ST shivering thermogenesis - T _a ambient temperature - T _b body temperature     

Survival of the gemmules of Anheteromeyenia ryderi (Potts) following aerial exposure during winter in New England

Gemmules of Anheteromeyenia ryderi survived 24 h exposure to air temperatures as low as –20 °C under laboratory conditions. Drying the gemmules of A. ryderi at 5 °C under laboratory conditions resulted in a reduced viability and a slower germination rate following rehydration compared with undried control gemmules. Only 25% of the gemmules germinated after drying for one month. Up to 25% of the gemmules of A. ryderi that were tested survived aerial exposure from early November to early April when a pond in Connecticut inhabited by this sponge was drained. During this period air temperatures dropped to as low as –16 °C. Continued aerial exposure of the gemmules during the summer resulted in nearly complete gemmule mortality.     

Effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans

This study sought to investigate the effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans. The subjects were eight healthy males, from whom informed consent had been obtained. The experiments were carried out under three different sets of conditions: a control climate [air temperature (Ta)=26°C, relative humidity (RH)=50%] (C); a humid heat climate (Ta=32°C, RH=80%) (H); and a humid heat exposure in later sleep segments (C for the first 3 h 45 min, followed by a 30-min transition to H, which was then maintained for the last 3 h 45 min) (C–H). Electroencephalogram, EOG, and mental electromyogram, rectal temperature (Tre), and skin temperature (Tsk) were continuously measured. The total amount of wakefulness was significantly increased in H compared to C–H or C. Compared to C, wakefulness in C–H and H was significantly increased during later sleep segments. Tre and mean Tsk were significantly higher in H than in C–H or C. In C–H, Tsk and Tre increased to levels equal to those observed in H after Ta and RH increase. Whole body sweat loss was significantly lower in C–H and C than in H. These results suggest that humid heat exposure in the later sleep segment reduces thermal load as compared to full-night humid heat exposure. In daily life, the use of air conditioning in the initial sleep hours can protect sleep and thermoregulation.     

Effect of photoperiod and melatonin on cold resistance,thermoregulation and shivering/nonshivering thermogenesis in Japanese quail

Summary The effect of photoperiod and melatonin treatment on cold resistance and thermogenesis of quails was studied. The birds were acclimated for 8 weeks to short day (8L:16D) or long day (16L:8D) conditions, and 8 of 16 quails in each group were implanted with melatonin capsules. One group of quails was maintained outside in an aviary during winter. Oxygen consumption ( ) body temperature (T _b, recorded with temperature transmitters) and shivering (integrated pectoral EMG) were recorded continuously, and samples of heart rate and breathing rate were picked up when ambient temperature was decreased stepwise from 27 down to –75 °C. Heat production maximum (HP_max), cold limit, lower critical temperature, basal metabolic rate (BMR) and thermal conductance were determined.The results show that short day, cold and melatonin treatment improved cold resistance and thermal insulation of quils when compared with quails acclimated to long day conditions. An increase in HP_max was induced only by melatonin treatment. The results suggest that the acclimatization of quails is under control of the pineal gland.The linear increase of shivering intensity with at moderate cold load shows that shivering is the primary source for thermoregulatory heat production in the quail. AtT _a's below –40 °C shivering remained constant although , heart rate and breathing rate continued to increase with increasing cold load. This could indicate the existence of a nonshivering thermogenesis in birds. Unlike to mammals, this non-shivering thermogenesis in birds would serve as secondary source of heat supporting shivering thermogenesis in severe coldAbbreviations BMR basal metabolic rate - ECG electrocardiogram - EMG electromyogram - NST nonshivering thermogenesis - SMR standard metabolic rate     

Supramaximal heat production induced by aminophylline in temperature-acclimated rats

Previous studies have shown that aminophylline, a phosphodiesterase inhibitor (thereby increasing intracellular cyclic AMP concentration) elicits supramaximal heat production and improves cold tolerance in rats acclimated to 22°C. To test whether aminophylline-stimulated supramaximal thermogenesis is independent of both the thermogenic capacity (i.e. aerobic fitness) and the mode of thermogenesis (shivering vs. non-shivering), rats (adult male Sprague-Dawley, approximately 400 g) of two different ages (4–11 month and 9–17 month, n=12 for each) were acclimated to 5, 15, and 25°C in succession and their thermogenic responses to aminophylline subsequently assessed. Aminophylline elicited supramaximal thermogenesis and improved cold tolerance regardless of age or acclimating temperatures. Further, the absolute net increase in heat production stimulated by aminophylline was also similar for all acclimating temperatures. After acclimating to 15°C, a single injection of aminophylline in the older rats elicited thermogenesis greater than that of the controls acclimated to 5°C; in the younger rats, aminophylline duplicated 46% of the increase in thermogenesis observed after acclimating to 5°C. These results indicated that the aminophylline-stimulated extra heat production is independent of both the thermogenic capacity and the mode of thermogenesis. It is possible that an enhanced substrate mobilization consequent to increased intracellular cyclic AMP concentration by aminophylline underlies the common mechanism via which supramaximal thermogenesis is elicited in temperature-acclimated rats.     

Metabolic changes associated with sustained 48-Hr shivering thermogenesis in the newborn pig

Metabolic changes associated with sustained 48-hr shivering thermogenesis were studied in piglets maintained at 34 (thermoneutrality) or 25°C (cold) between 6 and 54 hr of life. Despite their high shivering activity and elevated heat production, cold-exposed piglets exhibited a slightly lower rectal temperature than thermoneutral animals (-1.1°C; P < 0.01) at the end of the treatment. The enhancement of heat production and shivering activity were associated with a decrease in muscle glycogen (− 47%; P < 0.05) and total lipid content (− 23%; P < 0.05), a reduction of blood lactate levels (P < 0.05) and an enhancement of muscle cytochrome oxidase activity (+20%;P < 0.05), which suggests that muscle oxidative potential was increased by cold exposure. Potential for capturing lipids (lipoprotein lipase activity) was also higher in the redrhomboideus muscle (+ 71%; P < 0.01) and lower in adipose tissue (−58%; P < 0.01) of the cold-exposed piglets. Measurements performed at the mitochondrial level show no changes inrhomboideus muscle, but respiratory capacities (state IV and FCCP-stimulated respiration) and intermyofibrillar mitochondria oxidative and phosphorylative (creatine kinase activity) capacities were enhanced inlongissimus dorsi muscle (P < 0.05). These changes may contribute to provide muscles with nonlimiting amount of readily oxidable substrates and ATP necessary for shivering thermogenesis. A rise in plasma norepinephrine levels was also observed during the second day of cold exposure (P < 0.05).     

Freeze or dehydrate: only two options for the survival of subzero temperatures in the arctic enchytraeid<Emphasis Type="Italic"> Fridericia ratzeli</Emphasis>

Hygrophilic soil animals, like enchytraeids, overwintering in frozen soil are unlikely to base their cold tolerance on supercooling of body fluids. It seems more likely that they will either freeze due to inoculative freezing, or dehydrate and adjust their body fluid melting point to ambient temperature as has been shown for earthworm cocoons and Collembola. In the present study we tested this hypothesis by exposing field-collected adult Fridericia ratzeli from Disko, West Greenland, to freezing temperatures under various moisture regimes. When cooled at –1 °C min^–1 under dry conditions F. ratzeli had a mean temperature of crystallisation (T_c) of –5.8 °C. However, when exposed to temperatures above standard T_c for 22 h, at –4 °C, most individuals (90%, n= 30) remained unfrozen. Slow cooling from –1 °C to –6 °C in vials where the air was in equilibrium with the vapour pressure of ice resulted in freezing in about 65% of the individuals. These individuals maintained a normal body water content of 2.7–3.0 mg mg^–1 dry weight and had body fluid melting points of about –0.5 °C with little or no change due to freezing. About 35% of the individuals dehydrated drastically to below 1.1 mg mg^–1 dry weight at –6 °C, and consequently had lowered their body fluid melting point to ca. –6 °C at this time. Survival was high in both frozen and dehydrated animals at –6 °C, about 60%. Approximately 25% of the animals (both frozen and dehydrated individuals) had elevated glucose concentrations, but the mean glucose concentration was not increased to any great extent in any group due to cold exposure. The desiccating potential of ice was simulated using aqueous NaCl solutions at 0 °C. Water loss and survival in this experiment were in good agreement with results from freezing experiments. The influence of soil moisture on survival and tendency to dehydrate was also evaluated. However, soil moisture ranging between 0.74 g g^–1 and 1.15 g g^–1 dry soil did not result in any significant differences in survival or frequency of dehydrated animals even though the apparent wetness and structure of the soil was clearly different in these moisture contents.Abbreviations DW dry weight - FW fresh weight - MP melting point - RH relative humidity - Tc crystallisation temperatures - WC water contentCommunicated by I.D. Hume     

Effects of chronic and intermittent cold exposure on metabolic capacity ofPeromyscus andMicrotus

The effect of 30 days of acclimation at 5°C and of a semiweekly series of short severe cold exposures (T_b 20–30°C) on metabolic capacity (M_max) was measured in Alaskan meadow voles(Microtus pennsylvanicus tananaensis) and Wisconsin deer mice(Peromyscus maniculatus bairdii). Meadow voles, with an M_max of 12–14 ml/(g.h) or 8–9 met (M_max/M_st), showed little response to either treatment. In deer mice, however, acclimation at 5°C increased M_max by about half (from 11.0 to 15.4 ml/(g.h) or from 6.0 to 9.1 met). In 25°C-acclimated deer mice 7 severe cold exposures produced a similar increase of which about half was seen with the first 2 exposures. In 5°C-acclimated deer mice, M_max averaged a 0.3 ml/(g.h) increase for each cold exposure to reach a level of 19 ml/(g.h) or 11 met after 6 weeks.     

Influence of acclimation temperature on the shivering behavior of the butterfly Danaus plexippus (L.)

Summary Most of the monarch butterflies kept at 4–5° C for a few days shivered when released at a test temperature of 15–16° C, whereas fewer of the butterflies kept at 23–24° C did so. Cold-acclimated butterflies shivered more readily, as indicated by the length of the interval between release at the test temperature and the onset of shivering, and they shivered for longer periods of time. The effects of cold acclimation were reversible, but in only 1 out of 3 replicates was the warm acclimation clearly reversed. Cool animals shivered at room temperature, indicating that body temperature and not ambient temperature is important in releasing the behavior. It is suggested that the acclimation involves alteration in the central neurons controlling the activity of muscles involved in shivering.I thank Miss Janice Ruppert and Mr. C. J. Doughty for their valuable technical assistance. The co-operation of the administrators of New Brighton Beach State Park in permitting me to collect in the park is appreciated.     

Metabolic and thermoregulatory responses to heat and cold in the Djungarian hamster,Phodopus sungorus

Summary Djungarian hamsters,Phodopus sungorus (31.1 g body weight) were exposed to ambient temperatures (T _a ) between –35°C and +34°C. They tolerated severe cold stress but were less able to withstand heat. At –35° CT _a , normal body temperature was maintained for several hours. Thereby maximum thermal insulation was calculated at 1.1 g·°C/mW, which is only slightly higher than expected from the hamsters body size. High levels of heat production (60 to 90 m W/g) were maintained for several hours, suggesting that well developed means of heat production are the main reason for cold tolerance of the Djungarian hamster.