Thermoregulation and the effect of body temperature on call temporal parameters in the cicada Diceroprocta olympusa (Homoptera: Cicadidae)

We investigated the thermoregulatory behavior, thermal responses (minimum flight, maximum voluntary tolerance and heat torpor temperatures) and the effect of body temperature (T(b)) on call parameters in the cicada Diceroprocta olympusa (Walker). Regression of T(b) as a function of ambient (T(a)) or perch temperatures (T(p)) suggests thermoregulation is occurring. Thermoregulation occurs through behavioral changes that alter the uptake of solar radiation. T(p) is a better predictor of T(b) than is T(a). Thermal responses (minimum flight temperature 20.4 degrees C, maximum voluntary tolerance temperature 37 degrees C, and heat torpor temperature 46.7 degrees C) may be related to the humid, grassland habitat of the species. In contrast to other acoustic insects, no significant relationship was found between the temporal parameters of the calling song and T(b) within the population of D. olympusa.     

Metabolism, body temperature and thermal conductance of fruit-doves (Aves: Columbidae, Treroninae)

Basal metabolic rate (MR), body temperature (T(b)) and wet thermal conductance (C(wet)) of three tropical species of fruit-doves were investigated at ambient temperatures (T(a)) of 11-33 degrees C in activity (alpha) and rest (rho) phases to investigate the possible effect of obligate frugivory on the physiology of columbids. The basal metabolic rates of Ptilinopus melanospila (black-naped fruit-dove, 94 g), Drepanoptila holosericea (cloven-feathered dove, 198 g) and Ducula pinon (Pinon's imperial pigeon, 748 g) are 20-38% lower than predicted for all birds, including granivorous columbid species from temperate and tropical regions. The MR was minimal at a T(a) value of approximately 30 degrees C (=lower critical temperature, T(lc)) for all three species, indicating that these rainforest birds are not able to withstand high ambient temperatures as well as arid-adapted members of the pigeon family. Minimal wet-thermal conductance was, on average, higher than expected, indicating poor insulation in these tropical birds. Body temperatures were as expected; however, below T(lc) the body temperatures decreased to levels of 35-36 degrees C (T(a)=12 degrees C).     

Hypothermia versus torpor in response to cold stress in the native Australian mouse Pseudomys hermannsburgensis and the introduced house mouse Mus musculus

This study compared torpor as a response to food deprivation and low ambient temperature for the introduced house mouse (Mus musculus) and the Australian endemic sandy inland mouse (Pseudomys hermannsburgensis). The house mouse (mass 13.0+/-0.48 g) had a normothermic body temperature of 34.0+/-0.20 degrees C at ambient temperatures from 5 degrees C to 30 degrees C and a basal metabolic rate at 30 degrees C of 2.29+/-0.07 mL O2 g(-1) h(-1). It used torpor with spontaneous arousal at low ambient temperatures; body temperature during torpor was 20.5+/-3.30 degrees C at 15 degrees C. The sandy inland mouse (mass 11.7+/-0.16 g) had a normothermic T(b) of 33.0+/-0.38 degrees C between T(a) of 5 degrees C to 30 degrees C, and a BMR of 1.45+/-0.26 mL O2 g(-1) h(-1) at 30 degrees C. They became hypothermic at low T(a) (T(b) about 17.3 degrees C at T(a)=15 degrees C), but did not spontaneously arouse. They did, however, survive and become normothermic if returned to room temperature (23 degrees C). We conclude that this is hypothermia, not torpor. Consequently, house mice (Subfamily Murinae) appear to use torpor as an energy conservation strategy whereas sandy inland mice (Subfamily Conilurinae) do not, but can survive hypothermia. This may reflect a general phylogenetic pattern of metabolic reduction in rodents. On the other hand, this may be related to differences in the social structure of house mice (solitary) and sandy inland mice (communal).     

Roles of endothermy in niche differentiation for ball-rolling dung beetles (Coleoptera: Scarabaeidae) along an altitudinal gradient

Abstract.  1. An analysis of whether niche differentiation in ball-rolling dung beetles can be explained by the way in which they regulate their body temperature was conducted.
2.  A priori assumptions were: (i) if thermoregulation affects niche partitioning, sympatric species must have different endothermic strategies that minimise encounters; or, alternatively (ii) if two co-occurring species show the same thermoregulation pattern and their flight periods overlap, they might be avoiding competition by exhibiting different resource preferences or different food relocation behaviour.
3. The ball-rolling dung beetles studied showed a hierarchical structure based on the species' endothermic capacity, measured as temperature excess [ T _ex= difference between body temperature ( T _b) and ambient temperature ( T _a)]. Those with a high T _ex (10–15 °C) were located exclusively at altitudes >1000 m a.s.l. On the coastal plains, species with a high T _ex were restricted to flying at night when the T _a was lower. Species with a lower T _ex (less than 10 °C higher than T _a) were found in the coastal plains zone.
4. Where there was sympatry with similar trophic habits, the species involved showed very different thermal niches, and where there was significant overlap of thermal niches between sympatric species, trophic habits of species were very different.
5. The results suggest that it is possible to use the concept of the thermal niche as a tool to explain interspecific interactions and the spatial distribution of species.     

Oxygen consumption and thermoregulatory responses in three species of South American marsupials

Oxygen consumption (VO(2)), body temperature (T(b)) and wet thermal conductance (C(wet)), under resting conditions, exposure to low ambient temperature (T(a)) and during sustained exercise (treadmill running) were measured in three phylogenetic related (same family; Didelphidae) South American marsupials possessing similar body masses: Caluromys philander (arboreal/fruit and insect eating), Philander opossum (terrestrial and arboreal/omnivore), and Metachirus nudicaudatus (terrestrial/omnivore). Our measurements of VO(2) and C(wet) under resting conditions agree with those previously reported for other marsupials. We expected that C. philander would show a lower maximal sustained VO(2), compared to the other two species, based on its more reduced skeletal muscle mass. However, the values obtained for C. philander were not statistically different (ANOVA) from those obtained for the other two species. When exposed to low ambient temperature (12 degrees C), differences among the three species were detected, i.e., M. nudicaudatus did not survive, while the other two species were able to reduce their T(b) under such conditions. C. philander gradually decreases its T(b) when cold exposed, and P. opossum shows a more pronounced T(b) drop only when exposure to low ambient temperatures occurs for a more prolonged period of time.     

Increased heat loss affects hibernation in golden-mantled ground squirrels

During hibernation at ambient temperatures (T(a)) above 0 degrees C, rodents typically maintain body temperature (T(b)) approximately 1 degrees C above T(a), reduce metabolic rate, and suspend or substantially reduce many physiological functions. We tested the extent to which the presence of an insulative pelage affects hibernation. T(b) was recorded telemetrically in golden-mantled ground squirrels (Spermophilus lateralis) housed at a T(a) of 5 degrees C; food intake and body mass were measured at regular intervals throughout the hibernation season and after the terminal arousal. Animals were subjected to complete removal of the dorsal fur or a control procedure after they had been in hibernation for 3-4 wk. Shaved squirrels continued to hibernate with little or no change in minimum T(b), bout duration, duration of periodic normothermic bouts, and food intake during normothermia. Rates of rewarming from torpor were, however, significantly slower in shaved squirrels, and rates of body mass loss were significantly higher, indicating increased depletion of white adipose energy stores. An insulative pelage evidently conserves energy over the course of the hibernation season by decreasing body heat loss and reducing energy expenditure during periodic arousals from torpor and subsequent intervals of normothermia. This prolongs the hibernation season by several weeks, thereby eliminating the debilitating consequences associated with premature emergence from hibernation.     

Influence of temperature on spring flight initiation for southwestern ponderosa pine bark beetles (Coleoptera: Curculionidae, Scolytinae)

Determination of temperature requirements for many economically important insects is a cornerstone of pest management. For bark beetles (Coleoptera: Curculionidae, Scolytinae), this information can facilitate timing of management strategies. Our goals were to determine temperature predictors for flight initiation of three species of Ips bark beetles, five species of Dendroctonus bark beetles, and two genera of bark beetle predators, Enoclerus spp. (Coleoptera: Cleridae) and Temnochila chlorodia (Mannerheim) (Coleoptera: Ostomidae), in ponderosa pine forests of northcentral Arizona. We quantified beetle flight activity using data loggers and pheromone-baited funnel traps at 18 sites over 4 yr. Ambient air temperature was monitored using temperature data loggers located in close proximity to funnel traps. We analyzed degree-day accumulation and differences between minimum, average, and maximum ambient temperature for the week before and week of first beetle capture to calculate flight temperature thresholds. Degree-day accumulation was not a good predictor for initiation of beetle flight. For all species analyzed other than D. adjunctus Blandford, beetles were captured in traps only when springtime temperatures exceeded 15.0 degrees C. D. adjunctus was collected when maximum temperatures reached only 14.5 degrees C. Once initial flights had begun, beetles were often captured when maximum ambient air temperatures were below initial threshold temperatures. Maximum and average air temperatures were a better predictor for beetle flight initiation than minimum temperature. We establish a temperature range for effective monitoring of bark beetles and their predators, and we discuss the implications of our results under climate change scenarios.     

Concurrent reductions in blood pressure and metabolic rate during fasting in the unrestrained SHR

Arctic ground squirrels (Spermophilus parryii) overwinter in hibernaculum conditions that are substantially below freezing. During torpor, captive arctic ground squirrels displayed ambient temperature (T(a))-dependent patterns of core body temperature (T(b)), metabolic rate (TMR), and metabolic fuel use, as determined by respiratory quotient (RQ). At T(a) 0 to -16 degrees C, T(b) remained relatively constant, and TMR rose proportionally with the expanding gradient between T(b) and T(a), increasing >15-fold from a minimum of 0.0115 +/- 0.0012 ml O(2). g(-1). h(-1). At T(a) 0-20 degrees C, T(b) increased with T(a); however, TMR did not change significantly from T(b) 0 to 12 degrees C, indicating temperature-independent inhibition of metabolic rate. The overall change in TMR from T(b) 4 to 20 degrees equates to a Q(10) of 2.4, but within this range of T(b), Q(10) changed from 1.0 to 14.1. During steady-state torpor at T(a) 4 and 8 degrees C, RQ averaged 0.70 +/- 0.013, indicating exclusive lipid catabolism. At T(a) -16 and 20 degrees C, RQ increased significantly to >0.85, consistent with recruitment of nonlipid fuels. RQ was negatively correlated with maximum torpor bout length. For T(a) values <0 degrees C, this relationship supports the hypothesis that availability of nonlipid metabolic fuels limits torpor duration in hibernating mammals; for T(a) values >0 degrees C, hypotheses linked to body temperature are supported. Because anterior body temperatures differ from core, overall, the duration torpor can be extended in hibernating mammals may be dependent on brain temperature.     

Water and heat balance during flight in the rose-colored starling (Sturnus roseus)

Water imbalance during flight is considered to be a potentially limiting factor for flight ranges in migrating birds, but empirical data are scarce. We studied flights under controlled ambient conditions with rose-colored starlings in a wind tunnel. In one experiment, we measured water fluxes with stable isotopes at a range of flight speeds (9-14 m s(-1)) at constant temperature (15 degrees C). In a second experiment, we measured evaporation rates at variable ambient temperatures (Ta = 5 deg -27 deg C) but constant speed (12 m s(-1)). During all flights, the birds experienced a net water loss. On average, water influx was 0.98 g h(-1) (SD = 0.16; n = 8), and water efflux was 1.29 g h(-1) (SD = 0.14; n = 8), irrespective of flight speed. Evaporation was related to temperature in a biphasic pattern. At temperatures below 18.2 degrees C, net evaporation was constant at 0.36 g h(-1) (SD = 0.18; n = 10), rising at higher temperatures with a slope of 0.11 per degree to about 1.5 g h(-1) at 27 degrees C. We calculated the relative proportion of dry and evaporative heat loss during flight. Evaporative heat loss at Ta < 18.2 deg C was 14% of total heat production during flight, and dry heat loss accounted for 84%. At higher temperatures, evaporative heat loss increased linearly with T(a) to about 25% at 27 degrees C. Our data suggest that for prolonged flights, rose-colored starlings should adopt behavioral water-saving strategies and that they cannot complete their annual migration without stopovers to replenish their water reserves.     

Patterns of daily flight activity in onitine dung beetles (Scarabaeinae: Onitini)

Different species of African dung beetles emerge from the soil at characteristic times of the day to fly and colonize the freshly-deposited dung of mammalian herbivores. Onitine dung beetles in their natural habitat displayed one of five distinctive daily flight behaviours: dusk crepuscular (Onitis alexis Klug, O. caffer Boheman, O. fulgidus Klug, O. tortuosus Houston, O. vanderkelleni Lansberge, O. westermanni Lansberge); dusk/dawn crepuscular (O. pecuarius Lansberge and O. viridulus Boheman); dusk/dawn crepuscular and nocturnal (O. aygulus (Fabricius), O. mendax Gillet, O. uncinatus Klug); late afternoon-dusk and dawn-early morning [Heteronitis castelnaui (Harold)]; or diurnal flight activity [O. belial (Fabricius), O. ion (Olivier)]. These diagnostic daily flight behaviours span a light intensity range of over 6 orders of magnitude and have been retained in selected species introduced into Australia. Ambient light intensity appears to be the primary determinant of the daily flight period in onitine dung beetles. Because the dung of mobile herbivores is rapidly exploited by onitine species for feeding and breeding purposes, different flight behaviours result in a spatial and temporal partitioning of species in the local dung beetle community. The timing of flight may contribute to, or lead to avoidance of, competition between species which may ultimately affect colonization success. Many onitines show a strong preference for dung of specific herbivores, which may further reduce interspecific competition. All crepuscular-nocturnal species examined raised their thoracic temperatures endothermically to between 35°C and 40°C before the onset of flight. In O. aygulus the thoracic temperature excess was as large as 19.3°C. The thermal threshold below which the frequency of flight onsets drops off rapidly is about 12°C for O. aygulus and 17°C for O. alexis and O. pecuarius. Radiant loss of body heat during cool nights and dawns may explain why smaller species (<0.4 g body weight), in particular, are adapted behaviourally so that they fly only during the day or early dusk.     

Respiratory water loss during rest and flight in European Starlings (Sturnus vulgaris)

Respiratory water loss in Starlings (Sturnus vulgaris) at rest and during flight at ambient temperatures (T(amb)) between 6 and 25 degrees C was calculated from respiratory airflow and exhaled air temperature. At rest, breathing frequency f (1.4+/-0.3 Hz) and tidal volume Vt (1.9+/-0.4 ml) were independent of T(amb), but negatively correlated with each other. Mean ventilation at rest was 156+/-28 ml min(-1) at all T(amb). Exhaled air temperature (T(exh)) at rest increased with T(amb) (T(exh) = 0.92.T(amb)+12.45). Respiratory water loss at rest averaged 0.18+/-0.09 ml h(-1) irrespective of T(amb). In flying Starlings f was 4.0+/-0.4 Hz and independent of T(amb). Vt during flight averaged 3.6+/-0.4 ml and increased with T(amb) (Vt = 0.06.T(amb)+2.83) as, correspondingly, did ventilation. T(exh) during flight increased with T(amb) (T(exh) = 0.85.T(amb)+17.29). Respiratory water loss during flight (average REWL(f) = 0.74+/-0.22 ml h(-1)) was significantly higher than at rest and increased with T(amb). Our measurements suggest that respiratory evaporation accounts for most water loss in flying Starlings and increases more than cutaneous evaporation with rising ambient temperature.     

Energy metabolism and evaporative water loss in the European free-tailed bat and Hemprich's long-eared bat (Microchiroptera): species sympatric in the Negev Desert

We compared the thermoregulatory abilities of two insectivorous bat species, Tadarida teniotis (mean body mass 32 g) and Otonycteris hemprichii (mean body mass 25 g), that are of different phylogenetic origins and zoogeographic distributions but are sympatric in the Negev Desert. At night, both were normothermic. By day, both were torpid when exposed to ambient temperatures (T(a)) below 25 degrees Celsius, with concomitant adjustments in metabolic rate (MR). Otonycteris hemprichii entered torpor at higher T(a) than T. teniotis, and, when torpid, their body temperatures (T(b)) were 1 degrees -2 degrees Celsius and 5 degrees -8 degrees Celsius above T(a), respectively; MR was correspondingly reduced. At night, the lower critical temperature of T. teniotis was 31.5 degrees Celsius, and that of O. hemprichii was 33 degrees Celsius. Mean nocturnal thermoneutral MR of T. teniotis was 37% greater than that of O. hemprichii. At high T(a), evaporative water loss (EWL) increased markedly in both species, but it was significantly higher in T. teniotis above 38 degrees Celsius. In both species, the dry heat transfer coefficient (thermal conductance) followed the expected pattern for small mammals, by day and by night. Total EWL was notably low in normothermic and torpid animals of both species, much lower than values reported for other bats, indicating efficient water conservation mechanisms in the study species. Comparing thermoregulatory abilities suggests that O. hemprichii is better adapted to hot, arid environments than T. teniotis, which may explain its wider desert distribution. By both standard and phylogenetically informed ANCOVA, we found no differences in basal metabolic rate (BMR) between desert and nondesert species of insectivorous bats, substantiating previous studies suggesting that low BMR is a characteristic common to insectivorous bats in general.     

The role of thermogenesis in the pollination biology of the Amazon waterlily Victoria amazonica

BACKGROUND AND AIMS: Several families of tropical plants have thermogenic flowers that show a 2-d protogynous sequence. Most are pollinated by large beetles that remain for the entire period in the flowers, where they compete for mates and feed. Active beetles require high body temperatures that they can achieve endogenously at great energy expense or attain passively and cheaply in a warm environment. Floral heating is therefore hypothesized to be a direct energy reward to endothermic beetles, in addition to its accepted role in enhancing scent production. METHODS: This study measures the pattern of floral heat production (as temperature in 20 flowers and respiration rates in five flowers) in Victoria amazonica at field sites in Guyana and correlates floral temperatures with body temperatures necessary for activity in visiting Cyclocephala hardyi beetles. KEY RESULTS: Thermogenesis occurred in a bimodal pattern, with peaks associated with the arrival and departure of beetles near sunset. Peak CO(2) production rates averaged 2.9 micromol s(-1), equivalent to a heat production of 1.4 W. Heat was generated mainly in the floral chamber on the first evening and by the stamen complex on the second. Mean chamber temperature remained between 29.3 and 34.7 degrees C during the first night, when ambient temperature was 23.5-25.2 degrees C. Beetles actively competed for mates and consumed stylar processes in the floral chamber, where their mean thoracic temperature was 33.2 degrees C. At the lower ambient temperatures outside of the flower, beetles capable of sustained flight had a similar mean temperature of 32.0 degrees C. CONCLUSIONS: Floral heating is not only associated with attraction, but continues throughout the night when beetles are active inside the flower and increases again when they leave. Floral chamber temperatures similar to activity temperatures of actively endothermic beetles imply that thermogenesis is an energy reward.     

Ambient temperature influences diet selection and physiology of an herbivorous mammal, Neotoma albigula

The whitethroat woodrat (Neotoma albigula) eats juniper (Juniperus monosperma), but the amount of juniper in its diet varies seasonally. We tested whether changes in juniper consumption are due to changes in ambient temperature and what the physiological consequences of consuming plant secondary compounds (PSCs) at different ambient temperatures might be. Woodrats were acclimated to either 20 degrees C or 28 degrees C. Later, they were given two diets to choose from (50% juniper and a nontoxic control) for 7 d. Food intake, resting metabolic rate (RMR), and body temperature (T(b)) were measured over the last 2 d. Woodrats at 28 degrees C ate significantly less juniper, both proportionally and absolutely, than woodrats at 20 degrees C. RMRs were higher for woodrats consuming juniper regardless of ambient temperature, and T(b) was higher for woodrats consuming juniper at 28 degrees C than for woodrats eating control diet at 28 degrees C. Thus, juniper consumption by N. albigula is influenced by ambient temperature. We conclude that juniper may influence thermoregulation in N. albigula in ways that are helpful at low temperatures but harmful at warmer temperatures in that juniper PSCs may be more toxic at warmer temperatures. The results suggest that increases in ambient temperature associated with climate change could significantly influence foraging behavior of mammalian herbivores.     

Re-evaluation of the allometry of wet thermal conductance for birds

Wet thermal conductance is an important thermoregulatory parameter for birds and mammals. It is generally calculated as C(wet) (ml O2 g(-1) h(-1) degrees C(-1)) = VO2/(T(b)-T(a)), where VO2 is metabolic rate measured in ml O2 g(-1) h(-1), T(b) is body and T(a) is ambient temperature measured in degrees C. Minimum C(wet) is measured at T(a) at or below the lower critical temperature (T(lc)) of the thermoneutral zone, and is strongly influenced by time of day (rest or activity phase) and body mass [J. Aschoff, Comp. Biochem. Physiol. 69A (1981) 611]. Allometric analyses indicate differences in C(wet) for passerine and non-passerine birds, in their rest and active phases (Aschoff, 1981). The allometric slope for non-passerine rest-phase (-0.583) is lower than that for non-passerine active-phase (-0.484), and passerine rest-phase (-0.461) and active-phase (-0.463), although none of these slopes are significantly different. This different-sloped relationship for non-passerine rest-phase C(wet) extrapolates to lower-than-expected values at high body mass, and so this allometric relationship may be inappropriate for predictive purposes. Consequently, we have reanalysed Aschoff's (1981) data, as well as more recent compilations, to determine a more useful allometric relationship for C(wet) of non-passerine rest-phase birds. Re-analyses of minimum thermal conductance data from Drent and Stonehouse [Comp. Biochem. Physiol. 40A (1971) 689], Aschoff (1981) and Gavrilov and Dolnik [Acta XVIII Congressus Internationalis Ornithologici Moscow (1982) 421] indicate that the most appropriate regressions for predicting C(wet) (ml O2 g(-1) h(-1) degrees C(-1)) of birds from body mass (M; g) are the pooled regressions for non-passerine and passerine birds, in the active (alpha) and resting (rho) phases, using data tabulated by Aschoff (1981): alpha, C(wet)=0.994M(-0.509); rho, C(wet)=0.702M(-0.519). C(wet) is approximately 40% higher in the active phase than the rest phase. Regressions of various data sets for C(wet) of birds and mammals indicate a similar slope of approximately -0.5 for the allometric relationship, but significantly higher elevations for mammals compared to birds. The approximately 50% higher C(wet) for mammals than birds indicates a better physical insulation for birds than mammals of the same body mass. The general scaling of C(wet) with M(-0.5) indicates that (T(b)-T(lc)) should scale with M(0.22), if mass-specific metabolic rate scales with M(-0.28) [Reynolds and Lee, Am. Nat. 147 (1996) 735]. The observed scaling for (T(b)-T(lc)) of M(0.183) (calculated from Gavrilov and Dolnik, 1985) is consistent with this expectation.     

Rewarming rates and thermogenesis in hibernating echidnas.

We measured body temperatures (T(b)) in 14 free-ranging echidnas (Tachyglossus aculeatus) using implanted data-loggers. An average of 1020+/-744 days of T(b) data was recorded from each animal. The average maximum T(b) was 35.3+/-0.7 degrees C (n=14), and the lowest T(b) was 4.7 degrees C. Detailed analysis of rewarming events from four echidnas showed rewarming time to be dependent on initial T(b) (rewarming time in hours=15.6-0.41T(initial), n=31) with an average rewarming rate of 1.9+/-0.4 degrees C h(-1). Based on an hourly sampling rate, the peak rewarming rate was found to be 7.2+/-0.8 degrees C h(-1) (n=12), which was measured at a mean T(b) of 26.2+/-2.4 degrees C. This rate of heating was calculated to be equivalent to a peak oxygen consumption rate of 1.4+/-0.2 ml O2 g h(-1), approximately 9 times the basal metabolic rate. We found that a plot of rate of change of T(b) against T(b) for the entire data set from an individual echidna provided a useful summary and analytical tool.     

Simple roost nests confer large energetic savings for sparrow-weavers

White-browed sparrow-weavers (Plocepasser mahali, body mass 40 g) are group-living passerines adapted to the semi-arid environment of north-eastern and south-western Africa. During winter, the nocturnal ambient temperature of these regions often falls below 0 degrees C. imposing conditions demanding an increase in thermoregulatory heat production. Individuals roost throughout the year in inverted U-shaped roost nests. We investigated the energetic advantages of roosting by measuring nest and ambient temperatures in the field, as well as the resting metabolic rate (RMR) of the birds. The sparrow-weavers exhibited a wide thermoneutral zone (13 degrees C - 32 degrees C). Although RMR at thermoneutrality (40.2 J g.h(-1)) conforms with those of other passerines. the value at 0 degrees C (74.8 J g.h(-1)) is significantly lower than expected. The slope of the line below the lower critical temperature is unexpectedly steep, however, and appears to cause the physiological requirement for nest roosting due to a high cost of thermoregulation at low temperatures, perhaps due to shivering or non-shivering thermogenesis. The nest temperature at 0 degrees C ambient is 5 degrees C. resulting in a saving of some 7% in the energy spent during winter nights when food resources are in short supply compared with the rest of the year.     

Rewarming rates and thermogenesis in hibernating echidnas

We measured body temperatures (T(b)) in 14 free-ranging echidnas (Tachyglossus aculeatus) using implanted data-loggers. An average of 1020+/-744 days of T(b) data was recorded from each animal. The average maximum T(b) was 35.3+/-0.7 degrees C (n=14), and the lowest T(b) was 4.7 degrees C. Detailed analysis of rewarming events from four echidnas showed rewarming time to be dependent on initial T(b) (rewarming time in hours=15.6-0.41T(initial), n=31) with an average rewarming rate of 1.9+/-0.4 degrees C h(-1). Based on an hourly sampling rate, the peak rewarming rate was found to be 7.2+/-0.8 degrees C h(-1) (n=12), which was measured at a mean T(b) of 26.2+/-2.4 degrees C. This rate of heating was calculated to be equivalent to a peak oxygen consumption rate of 1.4+/-0.2 ml O2 g h(-1), approximately 9 times the basal metabolic rate. We found that a plot of rate of change of T(b) against T(b) for the entire data set from an individual echidna provided a useful summary and analytical tool.     

Persistence of sleep-temperature coupling after suprachiasmatic nuclei lesions in rats

The suprachiasmatic nucleus (SCN) regulates the circadian rhythms of body temperature (T(b)) and vigilance states in mammals. We studied rats in which circadian rhythmicity was abolished after SCN lesions (SCNx rats) to investigate the association between the ultradian rhythms of sleep-wake states and brain temperature (T(br)), which are exposed after lesions. Ultradian rhythms of T(br) (mean period: 3.6 h) and sleep were closely associated in SCNx rats. Within each ultradian cycle, nonrapid eye movement (NREM) sleep was initiated 5 +/- 1 min after T(br) peaks, after which temperature continued a slow decline (0.02 +/- 0.006 degrees C/min) until it reached a minimum. Sleep and slow wave activity (SWA), an index of sleep intensity, were associated with declining temperature. Cross-correlation analysis revealed that the rhythm of T(br) preceded that of SWA by 2-10 min. We also investigated the thermoregulatory and sleep-wake responses of SCNx rats and controls to mild ambient cooling (18 degrees C) and warming (30 degrees C) over 24-h periods. SCNx rats and controls responded similarly to changes in ambient temperature. Cooling decreased REM sleep and increased wake. Warming increased T(br), blunted the amplitude of ultradian T(br) rhythms, and increased the number of transitions into NREM sleep. SCNx rats and controls had similar percentages of NREM sleep, REM sleep, and wake, as well as the same average T(b) within each 24-h period. Our results suggest that, in rats, the SCN modulates the timing but not the amount of sleep or the homeostatic control of sleep-wake states or T(b) during deviations in ambient temperature.     

Behavioral thermoregulation in obese and lean Zucker rats in a thermal gradient

Genetically obese Zucker (Z) rats have been reported to display a body core temperature (Tb) that is consistently below that of their lean littermates. We asked the question whether the lower Tb was a result of deficits in thermoregulation or a downward resetting of the set point for Tb. For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (T(a)) range of 15-35 degrees C, while subjected to a 12:12-h light-dark cycle. Tb was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (VO2) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred T(a)) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower Tb [37.1 +/- 0.1 degrees C (SD) vs. 37.3 +/- 0.1 degrees C, P < 0.001] and a lower VO2 (25.3 +/- 1.9 ml x kg(-1) x h(-1)) than lean controls [33.1 +/- 3.7 (A) and 33.9 +/- 3.9 (M) ml x kg(-1) x h(-1), P < 0.001]. Also, the obese Z rats consistently chose to occupy a cooler T(a) [20.9 +/- 0.6 degrees C vs. 22.7 +/- 0.6 degrees C (A) and 22.5 +/- 0.7 degrees C (M), P < 0.001] in the thermal gradient. This suggests a lower set point for Tb in the obese Z rat, as they refused the option to select a warmer T(a) that might allow them to counteract any thermoregulatory deficiency that could lead to a low Tb. Although all rats followed a definite circadian rhythm for both Tb and VO2, there was no discernible circadian pattern for preferred T(a) in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats.