Thermoregulation by Diabrotica undecimpunctata howardi and potential effects on overwintering biology

In North Carolina, southern corn rootworm (SCR), Diabrotica undecimpunctata howardi Barber, adults of both sexes (diapausing and nondiapausing) elevate body temperature (Tb) above ambient air temperature (Ta) by basking in direct sunlight on clear fall and winter days when Ta<13°C (Tb-Ta range: 0.8–13.3°C). On cloudy days, SCR adults did not exhibit basking behavior and Tb was more highly correlated with Ta, ground temperature (Tg), and substrate temperature (Ts) than on clear days. Ts was the best predictor of Tb regardless of Ta and the presence or absence of adult basking behavior (fall basking SCR, Ts vs. Tb: R²=0.94, Ta vs. Tb: R²=0.41; fall nonbasking SCR, Ts vs. Tb: R²=0.85, Ta vs. Tb: R²=0.55).These results suggest that SCR thermoregulation at low Ta is ectothermic regulation by microhabitat selection. Elevation of Tb by thermoregulation was often of sufficient magnitude to affect SCR behavior (flight, feeding, mating) and preovipositional development rate. This could explain why gravid SCR females have been found in North Carolina by 6 January when Ta's were too low for development. The relationship of Tb to Ta on cold sunny days would be important to include in any predictive day-degree model of SCR postdiapause development or SCR endoparasite development in the field.

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Résumé En Caroline du Nord, les adultes diapausants ou non des deux sexes de Diabrotica undecimpunctata howardi (SCR) élèvent la température de leurs corps (Tb) au-dessus de la température de l'air ambiant (Ta) en lézardant en plein soleil par jours clairs du printemps et de l'hiver, quand Ta<13°C (Tb-Ta de 0.8 à 13.3°C). Par temps nuageux, les adultes ne lézardent pas, et Tb dépend beaucoup plus de Ta, de la température du sol (Tg), et de la température du substrat (Ts), que par temps clair. Ts fournit la meilleure idée de Ta que les adultes lézardent ou non; quand SCR lézarde au printemps la corrélation de Ts avec Tb est R²=0.94, de Ta avec Tb:R²=0.41; au printemps, quand ils ne lézardent pas; Ts avec TbR²=0.85 et Ta avec Tb:R²=0.55. Ces résultats suggerent que la thermorégulation de SCR aux basses Ta est une régulation ectothermique par sélection du microhabitat. L'augmentation de Tb par thermorégulation est souvent suffisante pour modifier le comportement de SCR (vol, prise de nourriture, accouplement) et un développement préponte. Ceci pourrait expliquer pourquoi des femelles gravides ont été trouvées un 6 janvier en Caroline du Nord quand Ta était trop basse pour permettre le développement. II paraît important d'inclure la relation entre Tb et Ta par temps nuageux froid, dans tout modèle prédictif, basé sur la notion de degrés-jours, sur le développement de SCR après la diapause ou le dévelopment d'endoparasites de SCR dans la nature.

     

东亚飞蝗耐高温能力及其体温调节行为

为明确东亚飞蝗Locusta migratoria manilensis的耐高温能力和体温调节行为, 采用高温饲养、提供辐射热源和风力等方法, 对东亚飞蝗各发育期的高温耐受能力、体温（body temperature, Tb）与环境温度（air temperature, Ta）和辐射温度（radiant temperature, Tr）的关系、飞蝗对辐射热的选择行为、体温升高速率以及辐射角度和风力对体温的影响进行了研究。结果表明: 东亚飞蝗44℃下饲养, LT90 最长为326.4 h; 50℃下, LT90可达20.6 h。无辐射热条件下, Tb随Ta的上升而升高, 当Ta升至32℃, 蝗虫出现体温调节行为; Ta以0.5℃/min速率上升时, 出现体温波动的个体数占试虫总数的53.7%, Tb平均波动温差为1.15℃, 平均波动时间为5.2 min, Tb平均波动起始温度为47.2℃, 成虫致死时间略长于若虫。有辐射热条件下, 随笼顶辐射温度的逐渐升高, 飞蝗趋向选择温度相对较低的笼底, 试虫体温调节较无辐射热条件下强; 辐射角度和风力均对飞蝗的体温有显著影响。结果显示东亚飞蝗对高温的耐受能力较强, 并且具有明显的体温调节行为, 可调节体温达到最佳生理状态。     

Thermoregulation in the Angolan free-tailed bat Mops condylurus: A small mammal that uses hot roosts.

The Angolan free-tailed bat (Mops condylurus) uses roosts that often exceed 40 degrees C, an ambient temperature (Ta) that is lethal to many microchiropterans. We measured the physiological responses of this species at Ta's from 15 degrees to 45 degrees C. Torpor was commonly employed during the day at the lower Ta, but the bats generally remained euthermic at night, with a mean body temperature (Tb) of 35.2 degrees C. Metabolic rate reflected the pattern of Tb, increasing with falling Ta at night but decreasing during the day. Metabolic rate and evaporative losses were lower in torpid than in euthermic bats. Body temperature increased at each Ta >35 degrees C and was 43 degrees C at Ta of 45 degrees C. At Ta of 40 degrees C bats increased dry thermal conductance and evaporative heat loss compared to lower Ta. At 45 degrees C dry thermal conductance was lower than at 40 degrees C and evaporative heat loss was 132% of metabolic heat production. At high Ta there was only a slight increase in metabolic rate despite the employment of evaporative cooling mechanisms and an increase in Tb. Collectively our results suggest that M. condylurus is well suited to tolerate high Ta, and this may enable it to exploit thermally challenging roost sites and to colonise habitats and exploit food sources where less stressful roosts are limiting.     

Redefining physiological responses of moose (Alces alces) to warm environmental conditions

We tested the concept that moose (Alces alces) begin to show signs of thermal stress at ambient air temperatures as low as 14 °C. We determined the response of Alaskan female moose to environmental conditions from May through September by measuring core body temperature, heart rate, respiration rate, rate of heat loss from exhaled air, skin temperature, and fecal and salivary glucocorticoids. Seasonal and daily patterns in moose body temperature did not passively follow the same patterns as environmental variables. We used large changes in body temperature (≥1.25 °C in 24hr) to indicate days of physiological tolerance to thermal stressors. Thermal tolerance correlated with high ambient air temperatures from the prior day and with seasonal peaks in solar radiation (June), ambient air temperature and vapor pressure (July). At midday (12:00hr), moose exhibited daily minima of body temperature, heart rate and skin temperature (difference between the ear artery and pinna) that coincided with daily maxima in respiration rate and the rate of heat lost through respiration. Salivary cortisol measured in moose during the morning was positively related to the change in air temperature during the hour prior to sample collection, while fecal glucocorticoid levels increased with increasing solar radiation during the prior day. Our results suggest that free-ranging moose do not have a static threshold of ambient air temperature at which they become heat stressed during the warm season. In early summer, body temperature of moose is influenced by the interaction of ambient temperature during the prior day with the seasonal peak of solar radiation. In late summer, moose body temperature is influenced by the interaction between ambient temperature and vapor pressure. Thermal tolerance of moose depends on the intensity and duration of daily weather parameters and the ability of the animal to use physiological and behavioral responses to dissipate heat loads.     

Heat loss from the human head during exercise

Evaporative and convective heat loss from head skin and expired air were measured in four male subjects at rest and during incremental exercise at 5, 15, and 25 degrees C ambient temperature (Ta) to verify whether the head can function as a heat sink for selective brain cooling. The heat losses were measured with an open-circuit method. At rest the heat loss from head skin and expired air decreased with increasing Ta from 69 +/- 5 and 37 +/- 18 (SE) W (5 degrees C) to 44 +/- 25 and 26 +/- 7 W (25 degrees C). At a work load of 150 W the heat loss tended to increase with increasing Ta: 119 +/- 21 (head skin) and 82 +/- 5 W (respiratory tract) at 5 degrees C Ta to 132 +/- 27 and 103 +/- 12 W at 25 degrees C Ta. Heat loss was always higher from the head surface than from the respiratory tract. The heat losses, separately and together (total), were highly correlated to the increasing esophageal temperature at 15 and 25 degrees C Ta. At 5 degrees C Ta on correlation occurred. The results showed that the heat loss from the head was larger than the heat brought to the brain by the arterial blood during hyperthermia, estimated to be 45 W per 1 degree C increase above normal temperature, plus the heat produced by the brain, estimated to be up to 20 W. The total heat to be lost is therefore approximately 65 W during a mild hyperthermia (+1 degrees C) if brain temperature is to remain constant.(ABSTRACT TRUNCATED AT 250 WORDS)     

温州地区冬季北红尾鸲的代谢特征及体温调节

为探讨北方迁徙性鸟类北红尾鸲(Phoenicurus auroreus)在越冬环境的代谢特征及体温调节,本文采用开放式氧气分析仪和数字式温度测量仪测定了在环境温度(Ta) 5、10、15、20、25、27.5、30、32.5、35、37.5、40℃条件下的代谢率(MR)和体温(Tb),并计算不同温度的热传导(C)。结果表明:在Ta为5~35℃范围内,北红尾鸲的Tb基本维持恒定,平均为40.3±0.1℃;热中性区(TNZ)为25~35℃;基础代谢率(BMR)为50.25±1.35 mL O2·h^-1,是Londo1o等(2015)体重预期值的151%;在Ta为5~25℃范围内,MR随Ta的降低而显著增加,回归方程为:MR (m L O2·h^-1)=99.65-1.93Ta(r=-0.707,P<0.001);高于35℃时,MR随Ta升高增加;当Ta为40℃时,MR达到59.78±1.31 mL O2·h^-1;在Ta为5~25℃范围内,北红尾鸲的C最低且基本保持恒定,为0.17±0.01 mL O2·g^-1·h^-1·℃^-1,是Aschoff(1981)体重预测值的111%;北红尾鸲的基本热生物学特征为较高的Tb、BMR和C以及较宽的TNZ,具有北方地区小型鸟类的代谢特点。     

Thermal, metabolic, and cardiovascular responses to various degrees of cold stress.

The metabolic, thermal, and cardiovascular responses of two male Caucasians to 1 2 h exposure to ambient temperature ranging between 28 degrees C and 5 degrees C were studied and related to the respective ambient temperatures. The metabolic heat production increased linearly with decreasing ambient temperature, where heat production (kcal times m- minus 2 times h- minus 1) = minus 2.79 Ta degrees C + 103.4, r = -0.97, P smaller than 0.001. During all exposures below 28 degrees C, the rate of decrease in mean skin temperature (Tsk) was found to be an exponential function dependent upon the ambient temperature (Ta) and the time of exposure. Reestablishment of Tsk steady state occurred at 90-120 min of exposure, and the time needed to attain steady state was linearly related to decreasing Ta. The net result was that a constant ratio of 1.5 of the external thermal gradient to the internal thermal gradient was obtained, and at all experimental temperatures, the whole body heat transfer coefficient remained constant. Cardiac output was inversely related to decreasing Ta, where cardiac output (Q) = minus 0.25 Ta degrees C + 14.0, r = minus 0.92, P smaller than 0.01. However, the primary reason for the increased Q, the stroke output, was also described as a third-order polynomial, although the increasing stroke volume throughout the Ta range (28-5 degrees C) was linearly related to decreasing ambients. The non-linear response of this parameter which occurred at 20 degrees C larger than or equal to Ta larger than or equal to 10 degrees C suggested that the organism's cardiac output response was an integration of the depressed heart rate response and the increasing stroke output at these temperatures.     

Effects of passive heat adaptation and moderate sweatless conditioning on responses to cold and heat

Two series of experiments were performed in physically untrained subjects. In series A (heat adaptation, HA), seven male subjects were adapted to dry heat (five consecutive days at 55 degrees C ambient air temperature (Ta) for 1 h X day-1) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (Ta + 10 to 0 degrees C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120 b X min-1) under Ta conditions individually adjusted (Ta + 15 to +5 degrees C) to prevent both sweating and cold sensations. Before and after "sweatless training", the subjects were subjected to a combined cold and heat test. During HA the thresholds for shivering, cutaneous vasodilatation (thumb and forearm) and sweating were shifted significantly (p less than 0.05) towards lower mean body temperatures (Tb). The mean decrease in threshold Tb was 0.36 degrees C. "Sweatless training" resulted in a mean increase in work rate (at HR 120 b X min-1) and oxygen pulse of 13 and 8%, respectively. However, "sweatless training" did not change the threshold Tb for shivering or sweating. Neither HA nor "sweatless training" changed the slopes of the relationships of shivering and sweating to Tb. It is concluded that the previously reported lowering of shivering and sweating threshold Tb in long-distance runners is not due to an increased fitness level, but is essentially identical with HA. The decreased shivering threshold following HA is interpreted as "cross adaptation" produced by the stressors cold and heat.     

季节、环境温度与黄鼠冬眠的关系

观察了达乌尔黄鼠在实验室内冬眠的一般情况。常温黄鼠的体温有着规律性的年周期,与环境温度的年周期变动不完全呈依从关系。出眠初期(4月下旬),动物体温高而稳定。4月至6月常温黄鼠的平均体温(皮温)为35.6℃,波动菹围32—37.5℃。随着体重达到顶峰,体温逐渐降低。8月份部分黄鼠出现低于32℃的低常体温,表明部分黄鼠自8月盛夏开始冬眠。但就整个种群而言,北京地区实验室内黄鼠冬眠季自9月下半月开始。3月底止,共6.5个月。秋季室温下降,动物入眠趋势增长。浅低体温(31.9—15℃)的比数逐渐升高。9月至12月,低体温(低于31.9℃)的百分比从47％增至84.8％,反映了动物从浅冬眠向深冬眠过渡。1月至2月份,低体温占85％以上,深低体温(低于15℃)占绝对优势。标志着动物种群的深眠月份。秋季动物从常温期向冬眠期转化的界线是不清的,而春季从冬眠期向常温相转化的界限却比较明显。     

Role of brain nitric oxide in the thermoregulation of broiler chicks

Nitric oxide (NO) plays a key role in body temperature (Tb) regulation of mammals, acting on the brain to stimulate heat loss. Regarding birds, the putative participation of NO in the maintenance of Tb in thermoneutrality or during heat stress and the site of its action (periphery or brain) is unknown. Thus, we tested if NO participates in the maintenance of chicks' Tb in those conditions. We investigated the effect of intramuscular (im; 25, 50, 100 mg/kg) or intracerebroventricular (icv; 22.5, 45, 90, 180 µg/animal) injections of the non selective NO synthase inhibitor L-NAME on Tb of 5-day-old chicks at thermoneutral zone (TNZ; 31–32 °C) and under heat stress (37 °C for 5–6 h). We also verified plasma and diencephalic nitrite/nitrate levels in non-injected chicks under both conditions. At TNZ, 100 mg/kg (im) or 45, 90, 180 µg (icv) of L-NAME decreased Tb. A significant correlation between Tb and diencephalic, but not plasma, nitrite/nitrate levels was observed. Heat stress-induced hyperthermia was inhibited by all tested doses of L-NAME (im and icv). Tb was correlated neither with plasma nor with diencephalic nitrite/nitrate levels during heat stress. These results indicate the involvement of brain NO in the maintenance of Tb of chicks, an opposite action of that observed in mammals, and may modulate hyperthermia.     

Torpor and hibernation in a basal placental mammal, the Lesser Hedgehog Tenrec Echinops telfairi

The patterns of heterothermy were measured in Lesser Hedgehog Tenrecs, Echinops telfairi, under semi-natural conditions in an outdoor enclosure during the austral mid-winter in southwestern Madagascar. The animals were implanted with miniaturized body temperature (Tb) loggers (iButtons) that measured body temperature every 42 min for 2 months (May and June). The tenrecs entered daily torpor on all 60 consecutive days of measurement, that is, on 100% of animal days, with body temperature closely tracking ambient temperature (Ta) during the ambient heating phase. The mean minimum daily Tb of the tenrecs was 18.44 +/- 0.50 degrees C (n = 174, N = 3), and never exceeded 25 degrees C whereas, apart from a few hibernation bouts in one animal, the mean maximum daily Tb was 30.73 +/- 0.15 degrees C (n = 167, N = 3). Thus during winter, tenrecs display the lowest normothermic Tb of all placental mammals. E. telfairi showed afternoon and early evening arousals, but entered torpor before midnight and remained in torpor for 12-18 h each day. One animal hibernated on two occasions for periods of 2-4 days. We consider E. telfairi to be a protoendotherm, and discuss the relevance and potential of these data for testing models on the evolution of endothermy.     

Assessing heat tolerance through productive vs physiological indicators. Data from dairy sheep under on-farm conditions

The search for criteria that allow the quantification of the level of thermotolerance of an animal is a major challenge in animal production. Different criteria have been proposed to date, mainly the use of routine milk recording and weather information or the collection of physiological measures related with heat stress. This study aimed at quantifying the association between indicators of heat tolerance derived from productive and physiological traits. For this purpose, two physiological traits, rectal temperature (RT) and respiratory rate (RR), and nine productive traits (milk yield, fat, protein and lactose yields and contents, casein and urea contents) were measured from June to September of 2018 in three flocks of Manchega sheep. A total of 462 lactating ewes participated in the study. Air temperature (Ta), relative humidity (RH) and associated temperature and humidity index (THI) were recorded inside the barn and also obtained from the closest weather station from the national meteorological network, and used to produce several measurements of heat load on animals. Based on the results of fits for quadratic and cubic regressions on the alternative heat load measures, the cubic regression on Ta and THI obtained inside the barn at time of recording yielded the best fit for physiological and productive parameters. The use of weather information taken from the official weather station closest to the farm also produced similar estimates and could be considered as a good alternative when on-farm meteorological data are not available. Two-trait random regression models that involved individual intercept and slope of response to heat load were used to obtain correlations between basal levels and heat tolerance within and across traits. Estimated correlations showed that animals with smaller vs larger basal levels of RT and RR tend to be more vs less heat tolerant (correlations up to 0.46) and that slopes of increase for RR and RT under heat stress were highly correlated (0.82). Estimated correlations between tolerance criteria from production vs physiology were up to ?0.5 (between milk yield and RT), indicating that animals that show less increase in body temperature also tend to show a smaller decrease in production under heat stress. However, because of the non-unity correlation between the two types of indicators of heat tolerance, both sources of information, productive and physiological ought to be taken into account to ensure the long-term sustainability of selection programmes aiming at improving productive levels when heat stress is a concerning issue.     

The heat-acclimated pigeon: an ideal physiological model for a desert bird

Acclimation of rock pigeon (Columba livia) to high ambient temperature (Ta) 50 degrees C from the time of hatching resulted in a well-developed cutaneous evaporative cooling mechanism (CECM), which became the dominant mechanism for heat dissipation. After the age of 15 days and in adults, acclimated pigeons exposed to 48-60 degrees C Ta could regulate normal body temperature (Tb) without employing either panting or gular fluttering. Respiration rate varied between 36 +/- 12 (SD) and 35 +/- 14 breaths/min at moderate and at extreme high Ta's, respectively. During thermal stress (42, 45, and 47 degrees C) imposed in a metabolic chamber, nonpanting pigeons' heat balance was achieved by adjusting low-level heat production (46.2 +/- 6.8 W/m2) and by use of an efficient CECM that dissipated 145% of the metabolic heat. Tb was regulated between 40.7 +/- 0.5 and 41.8 +/- 0.4 degrees C over a wide range of Ta's (20-56 degrees C). The respiratory evaporative cooling mechanism (RECM) was effective since hatching. The CECM developed approximately 24 h later during the ontogeny of the altricial nestling pigeon. This trait, which exists in many bird species and may be a recent development, possibly evolved as an adaptation to hot environments. In the present study we have brought evidence for a multitrait physiological adaptation that takes preeminence in adjusting the processes involved in maintaining heat balance. This integrative complex creates a powerful, efficient tool for contending with the most extreme thermal conditions.     

Energetics and thermoregulation during digging in the rodent tuco-tuco (Ctenomys talarum)

For subterranean rodents, searching for food by extension of the tunnel system and maintenance of body temperature are two of the most important factors affecting their life underground. In this study we assess the effect of ambient temperature on energetics and thermoregulation during digging in Ctenomys talarum. We measured VO2 during digging and resting at ambient temperature (Ta) below, within, and above thermoneutrality. Digging metabolic rate was lowest at Ta within the thermoneutral zone and increased at both lower and higher temperatures, but body temperature (Tb) remained constant at all Tas. Below thermoneutrality, the cost of digging and thermoregulation are additive. Heat production for thermoregulation would be compensated by heat produced as a by-product of muscular activity during digging. Above thermoneutrality, conduction would be an important mechanism to maintain a constant Tb during digging.     

The dependence of the rate of heart contractions in the long-tailed Yakutsk suslik Citellus undulatus on environmental temperature]

Decrease of ambient temperature (Ta) leads to the increase of the heart rate (HR) in active ground squirrels C. undulatus by 5.3/min/1 degree C in summer and by 3.8/min/1 degree C in winter. In a hibernation state, the dependence of the HR on Ta was in a good agreement with equation HT = 2.53.exp.(0.1.Ta). On entering into hibernation and on arousal, the HR change outruns the corresponding body temperature (Tb) change by 1.5-2 hours. A maximum HR level (up to 400/min and more) was registered on arousal when Tb reached 17-20 degrees C. A minimal HR level (4-5/min) was observed during hibernation at Ta 2-5 degrees C. The maximum Ta, at witch C. undulatus was hibernating, reached 23-24 degrees C, the HR being 23-25/min.     

Field test of a paradigm: hysteresis of heart rate in thermoregulation by a free-ranging lizard (Pogona barbata).

The discovery that changes in heart rate and blood flow allow some reptiles to heat faster than they cool has become a central paradigm in our understanding of reptilian thermoregulation. However, this hysteresis in heart rate has been demonstrated only in simplistic laboratory heating and cooling trials, leaving its functional significance in free-ranging animals unproven. To test the validity of this paradigm, we measured heart rate and body temperature (Tb) in undisturbed, free-ranging bearded dragons (Pogona barbata), the species in which this phenomenon was first described. Our field data confirmed the paradigm and we found that heart rate during heating usually exceeded heart rate during cooling at any Tb. Importantly, however, we discovered that heart rate was proportionally faster in cool lizards whose Tb was still well below the 'preferred Tb range' compared to lizards whose Tb was already close to it. Similarly, heart rate during cooling was proportionally slower the warmer the lizard and the greater its cooling potential compared to lizards whose Tb was already near minimum operative temperature. Further, we predicted that, if heart rate hysteresis has functional significance, a 'reverse hysteresis' pattern should be observable when lizards risked overheating. This was indeed the case and, during heating on those occasions when Tb reached very high levels (> 40 degrees C), heart rate was significantly lower than heart rate during the immediately following cooling phase. These results demonstrate that physiological control of thermoregulation in reptiles is more complex than has been previously recognized.     

An infrared thermographic study of surface temperature in relation to external thermal stress in the Mongolian gerbil, Meriones unguiculatus

1. Temperatures of different body surface regions and deep body temperature (Tb) of unrestrained adult Mongolia gerbils exposed to ambient temperatures (Ta) of -10-35 degrees C were measured using infrared (i.r.) thermography and a thermocouple. 2. A strong positive linear relationship between the surface temperature and Ta was found. For Ta range -4-35 degrees C, the slope was lowest for the areas around the eyes and dorsal head, and steepest for the body extremities. At -10 degrees C, surface temperatures of the areas around the eyes and dorsal head were significantly lower then predicted. 3. Tb was lowest at Ta of 25 and 30 degrees C, increased at all temperatures above and up to Ta of -4 degrees C below this range, and began decline at -10 degrees C. 4. The thermoneutral zone (TNZ) is probably between 28 and 32 degrees C, and the absolute lower critical temperature (Tabsl) is probably -4 degrees C. 5. The Mongolian gerbil shows little control of surface temperature and in contrast to larger mammals it has not developed any special thermoregulatory surface areas to regulate heat exchange with its environment. At temperatures below -4 degrees C, this species is unable to maintain the surface temperature of body extremities above the freezing point. 6. It is suggested that the Mongolian gerbil uses mainly behavioral and ecological adaptive strategies to attenuate the stressful effects of its habitat.     

Thermoregulatory and metabolic changes during fever in young and old rats

We injected old and young rats with lipopolysaccharide (LPS; 50 microg/kg ip) at two ambient temperatures (Ta; 21 and 31 degrees C). Young rats mounted equivalent fevers at both Tas [peak body temperatures (Tb) of 38.3 and 38.7 degrees C, respectively]. The Tb of old rats was not different from baseline (37.3 degrees C) after LPS at Ta 21 degrees C, whereas, at 31 degrees C, their Tb rose to a mean peak of 38.4 degrees C. We also measured the associated thermoregulatory responses by use of calorimetry. At 21 degrees C, young rats developed a fever by increasing both O2 consumption and heat conservation. Old rats did not become febrile, and O2 consumption fell by 15%. Heat loss was the same in old and young rats. At 31 degrees C, young and old rats developed similar fevers with similar increases in heat production and conservation. Our results suggest that the lack of LPS fever in old rats at 21 degrees C is due mainly to the lowered metabolic rate.     

Thyrotropin-releasing hormone action in the preoptic/anterior hypothalamus decreases thermoregulatory set point in ground squirrels.

Earlier work has shown that thyrotropin releasing hormone (TRH) produces dose-dependent decreases in body temperature (Tb) and metabolic rate when microinjected into the dorsal hippocampus (HPC) or preoptic/anterior hypothalamus (PO/AH) of awake ground squirrels. This study employed a behavioral paradigm to investigate the possibility that TRH-induced hypothermia is associated with a decrease in thermoregulatory set point. Six animals were successfully trained to press a bar for radiant heat escape and cool air reinforcement in order to obtain a cooler ambient temperature (Ta). During experimental testing, the animals were microinjected remotely with TRH (10-1000 ng/microliters) or a control solution (sterile saline or TRH-OH) into the PO/AH. The micro-injections were delivered via bilateral injection cannulae inserted through chronic bilateral cannula guides that had been stereotaxically implanted under pentobarbital anesthesia. Cumulative and time-integrated bar presses were obtained on a computer generated display. Tb, measured in the brain via a bead-type thermistor, and chamber Ta were recorded continuously. Following TRH administration, a significant increase in mean bar-press rate was observed during the period in which Tb was falling, when compared to a comparable time period just prior to the microinjection. These findings complement results obtained from four animals that were trained to press a bar for heat reinforcement in a cold (- 10 degrees C) environment. In this alternative behavioral paradigm, microinjection of TRH into the PO/AH or HPC induced a decrease in mean bar-press rate as Tb was falling. The results support the hypothesis that TRH-induced hypothermia in golden-mantled ground squirrels is achieved by lowering thermoregulatory set point.     

Metabolic and hormonal acclimation to heat stress in domesticated ruminants

Environmentally induced periods of heat stress decrease productivity with devastating economic consequences to global animal agriculture. Heat stress can be defined as a physiological condition when the core body temperature of a given species exceeds its range specified for normal activity, which results from a total heat load (internal production and environment) exceeding the capacity for heat dissipation and this prompts physiological and behavioral responses to reduce the strain. The ability of ruminants to regulate body temperature is species- and breed-dependent. Dairy breeds are typically more sensitive to heat stress than meat breeds, and higher-producing animals are more susceptible to heat stress because they generate more metabolic heat. During heat stress, ruminants, like other homeothermic animals, increase avenues of heat loss and reduce heat production in an attempt to maintain euthermia. The immediate responses to heat load are increased respiration rates, decreased feed intake and increased water intake. Acclimatization is a process by which animals adapt to environmental conditions and engage behavioral, hormonal and metabolic changes that are characteristics of either acclimatory homeostasis or homeorhetic mechanisms used by the animals to survive in a new 'physiological state'. For example, alterations in the hormonal profile are mainly characterized by a decline and increase in anabolic and catabolic hormones, respectively. The response to heat load and the heat-induced change in homeorhetic modifiers alters post-absorptive energy, lipid and protein metabolism, impairs liver function, causes oxidative stress, jeopardizes the immune response and decreases reproductive performance. These physiological modifications alter nutrient partitioning and may prevent heat-stressed lactating cows from recruiting glucose-sparing mechanisms (despite the reduced nutrient intake). This might explain, in large part, why decreased feed intake only accounts for a minor portion of the reduced milk yield from environmentally induced hyperthermic cows. How these metabolic changes are initiated and regulated is not known. It also remains unclear how these changes differ between short-term v. long-term heat acclimation to impact animal productivity and well-being. A better understanding of the adaptations enlisted by ruminants during heat stress is necessary to enhance the likelihood of developing strategies to simultaneously improve heat tolerance and increase productivity.