Comparison of hibernation, estivation and daily torpor in the edible dormouse, Glis glis

Three major forms of dormancy in mammals have been classified: hibernation in endotherms is characterised by reduced metabolic rate (MR) and body temperature (T _b) near ambient temperature (T _a) over prolonged times in the winter. Estivation is a similar form of dormancy in a dry and hot environment during summertime. Daily torpor is defined as reduced MR and T _b lower than 32 °C, limited to a duration of less than 24 h. The edible dormouse (Glis glis) is capable for all three distinct forms of dormancy. During periods of food restriction and/or low T _a, daily torpor is displayed throughout the year, alternating with hibernation and estivation in winter and summer respectively. We recorded T _b, O₂-consumption and CO₂-production in unrestrained dormice at different T _a's for periods of up to several months. Cooling rate and rate of metabolic depression during entrance into the torpid state was identical in all three forms of dormancy. The same was true for thermal conductance, maximum heat production, duration of arousal and cost of an arousal. The only difference between hibernation and daily torpor was found in the bout duration. A daily torpor bout lasted 3–21 h, a hibernation bout 39–768 h. As a consequence of prolonged duration, MR, T _b and also the T _b − T _a gradient decreased to lower values during hibernation bouts when compared to daily torpor bouts. Our findings suggest that all three forms of dormancy are based on the same physiological mechanism of thermal and metabolic regulation. Accepted: 27 June 2000     

Daily torpor and energetics in a tropical mammal, the northern blossom-bat Macroglossus minimus (Megachiroptera)

Little is known about torpor in the tropics or torpor in megachiropteran species. We investigated thermoregulation, energetics and patterns of torpor in the northern blossom-bat Macroglossus minimus (16 g) to test whether physiological variables may explain why its range is limited to tropical regions. Normothermic bats showed a large variation in body temperature (T _b) (33 to 37 °C) over a wide range of ambient temperatures (T _as) and a relatively low basal metabolic rate (1.29 ml O₂ g⁻¹h⁻¹). Bats entered torpor frequently in the laboratory at T _as between 14 and 25 °C. Entry into torpor always occurred when lights were switched on in the morning, independent of T _a. MRs during torpor were reduced to about 20–40% of normothermic bats and T _bs were regulated at a minimum of 23.1 ± 1.4 °C. The duration of torpor bouts increased with decreasing T _a in non-thermoregulating bats, but generally terminated after 8 h in thermoregulating torpid bats. Both the mean minimum T _b and MR of torpid M. minimus were higher than that predicted for a 16-g daily heterotherm and the T _b was also about 5 °C higher than that of the common blossom-bat Syconycteris australis, which has a more subtropical distribution. These observations suggest that variables associated with torpor are affected by T _a and that the restriction to tropical areas in M. minimus to some extent may be due to their ability to enter only very shallow daily torpor. Accepted: 22 September 1997     

Evidence of facultative daytime hypothermia in a small passerine wintering at northern latitudes

The use of hypothermia as a means to save energy is well documented in birds. This energy‐saving strategy is widely considered to occur exclusively at night in diurnally active species. However, recent studies suggest that facultative hypothermia may also occur during the day. Here, we document the use of daytime hypothermia in foraging Black‐capped Chickadees Poecile atricapillus wintering in eastern Canada. We measured the body temperature (T_b) of 126 individuals (plus 48 repeated measures) during a single winter and related values to ambient temperature (T_a) at the time of capture. We also tested whether daytime hypothermia was correlated with the size of body reserves (residuals of mass on structural size and fat score) and levels of metabolic performance (basal metabolic rate and maximum thermogenic capacity). We found that T_b of individual birds was lower when captured at low T_a, reaching values as low as 35.5 °C in actively foraging individuals. T_b was unrelated to metabolic performance or measures of body reserves. Therefore, daytime hypothermia does not result from individuals being unable to maintain T_b during cold spells or to a lack of body reserves. Our data also demonstrated a high level of individual variation in the depth of hypothermia, the causes of which remain to be explored.     

THE TEMPORAL ORGANIZATION OF DAILY TORPOR AND HIBERNATION: CIRCADIAN AND CIRCANNUAL RHYTHMS

Mammals and birds have evolved the ability to maintain a high and constant body temperature T_b over a wide range of ambient temperatures T_a using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of T_b and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T_b is reduced substantially and may even fall below 0°C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and T_a. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low T_b. (Chronobiology International, 17(2), 103–128, 2000)     

Hibernation patterns of Turkish hamsters: influence of sex and ambient temperature

Turkish hamsters (Mesocricetus brandti) are a model organism for studies of hibernation, yet a detailed account of their torpor characteristics has not been undertaken. This study employed continuous telemetric monitoring of body temperature (T _b) in hibernating male and female Turkish hamsters at ambient temperatures (T _as) of 5 and 13 °C to precisely characterize torpor bout depth, duration, and frequency, as well as rates of entry into and arousal from torpor. Hamsters generated brief intervals of short (<12 h), shallow test bouts (T _b > 20 °C), followed by deep torpor bouts lasting 4–6 days at T _a = 5 °C and 2–3 days at T _a = 13 °C. Females at T _a = 5 °C had longer bouts than males, but maintained higher torpor T _b; there were no sex differences at T _a = 13 °C. Neither body mass loss nor food intake differed between the two T _as. Hamsters entered torpor primarily during the scotophase (subjective night), but timing of arousals was highly variable. Hamsters at both T _as generated short, shallow torpor bouts between deep bouts, suggesting that this species may be capable of both hibernation and daily torpor.     

Seasonal energetics and torpor use in North American flying squirrels

Seasonal cold temperatures require mammals to use morphological, behavioural, or physiological traits to survive periods of extreme cold and food shortage. Torpor is a physiological state that minimizes energy requirements by decreasing resting metabolic rate (MR) and body temperature (T_b). Many rodent species are capable of torpor, however, evidence in northern and southern flying squirrels (Glaucomys sabrinus and Glaucomys volans, respectively) has remained anecdotal. We experimentally attempted to induce torpor in wild-caught flying squirrels by lowering ambient temperature (T_a) and measuring MR using open-flow respirometry. We also studied seasonal differences in MR and T_b at various T_a. Both MR and T_b provided evidence for torpor in flying squirrels, but only infrequent, shallow torpor. MR decreased infrequently and any decreases were rarely sustained for longer than one hour. We found a significant positive relationship between T_a and T_b only in G. volans, which suggests that G. volans is more susceptible to low T_a compared with G. sabrinus, possibly due to their small body size. We observed no substantive seasonal or interspecific differences in the relation between MR and T_a, with the exception that northern flying squirrels expended more energy at cold T_a during warm season trials than other species-season combinations. The infrequency of torpor use in our experiments suggests that other energy-saving strategies, such as social thermoregulation, may limit the reliance on torpor in this lineage.     

Adaptive mechanisms during food restriction in <Emphasis Type="Italic">Acomys russatus</Emphasis>: the use of torpor for desert survival

The golden spiny mouse (Acomys russatus) is an omnivorous desert rodent that does not store food, but can store large amounts of body fat. Thus, it provides a good animal model to study physiological and behavioural adaptations to changes in food availability. The aim of this study was to investigate the time course of metabolic and behavioural responses to prolonged food restriction. Spiny mice were kept at an ambient temperature of 27°C and for 3 weeks their food was reduced individually to 30% of their previous ad libitum food intake. When fed ad libitum, their average metabolic rate was 82.77±3.72 ml O₂ h^–1 during the photophase and 111.19±4.30 ml O₂ h^–1 during the scotophase. During food restriction they displayed episodes of daily torpor when the minimal metabolic rate gradually decreased to 16.07±1.07 ml O₂ h^–1, i.e. a metabolic rate depression of approximately 83%. During the hypometabolic bouts the minimum average body temperature T_b, decreased gradually from 32.6±0.1°C to 29.0±0.4°C, with increasing duration of consecutive bouts. In parallel, the animals increased their activity during the remaining daytime. Torpor as well as hyperactivity was suppressed immediately by refeeding. Thus golden spiny mice used two simultaneous strategies to adapt to shortened food supply, namely energysaving torpor during their resting period and an increase in locomotor activity pattern during their activity period.     

Daily torpor in the gray mouse lemur (Microcebus murinus) in Madagascar: energetic consequences and biological significance

Patterns and energetic consequences of spontaneous daily torpor were measured in the gray mouse lemur (Microcebus murinus) under natural conditions of ambient temperature and photoperiod in a dry deciduous forest in western Madagascar. Over a period of two consecutive dry seasons, oxygen consumption (VO₂) and body temperature (T _b) were measured on ten individuals kept in outdoor enclosures. In all animals, spontaneous daily torpor occurred on a daily basis with torpor bouts lasting from 3.6 to 17.6 h, with a mean torpor bout duration of 9.3 h. On average, body temperatures in torpor were 17.3±4.9°C with a recorded minimum value of 7.8°C. Torpor was not restricted to the mouse lemurs’ diurnal resting phase: entries occurred throughout the night and arousals mainly around midday, coinciding with the daily ambient temperature maximum. Arousal from torpor was a two-phase process with a first passive, exogenous heating where the T _b of animals increased from the torpor T _b minimum to a mean value of 27.1°C before the second, endogenous heat production commenced to further raise T _b to normothermic values. Metabolic rate during torpor (28.6±13.2 ml O₂ h^–1) was significantly reduced by about 76% compared to resting metabolic rate (132.6±50.5 ml O₂ h^–1). On average, for all M. murinus individuals measured, hypometabolism during daily torpor reduced daily energy expenditure by about 38%. In conclusion, all these energy-conserving mechanisms of the nocturnal mouse lemurs, with passive exogenous heating during arousal from torpor, low minimum torpor T _bs, and extended torpor bouts into the activity phase, comprise an important and highly adapted mechanism to minimize energetic costs in response to unfavorable environmental conditions and may play a crucial role for individual fitness. Received: 8 July 1999 / Accepted: 3 December 1999     

Timing of torpor bouts during hibernation in European hamsters (Cricetus cricetus L.)

Body temperature (T _b) of seven European hamsters maintained at constant ambient temperature (T _a = 8 °C) and constant photoperiod (LD 8:16) was recorded throughout the hibernating season using intraperitoneal temperature-sensitive HF transmitters. The animals spent about 30% of the hibernation season in hypothermia and 70% in inter-bout normothermy. Three types of hypothermia, namely deep hibernation bouts (DHBs), short hibernation bouts (SHBs), and short and shallow hibernation bouts (SSHBs), were distinguished by differences in bout duration and minimal body temperature (T _m). A gradual development of SSHBs from the diel minimum of T _b during normothermy could be seen in individual hamsters, suggesting a stepwise decrease of the homeostatic setpoint of T _b regulation during the early hibernation season. Entry into hibernation followed a 24-h rhythm occurring at preferred times of the day in all three types of hypothermia. DHBs and SHBs were initiated approximately 4 h before SSHBs, indicating a general difference in the physiological initiation of SSHBs on the one hand and DHBs and SHBs on the other. Arousals from SHBs and SSHBs also followed a 24-h rhythm, whereas spontaneous arousals from DHBs were widely scattered across day and night. Statistical analyses of bout length and the interval between arousals revealed evidence for a free-running circadian rhythm underlying the timing of arousals. The results clearly demonstrate that entries into hypothermia are linked to the light/dark-cycle. However, the role of the circadian system in the timing of arousals from DHBs remains unclear. Accepted: 11 December 1996     

How to budget metabolic energy: torpor in a small Neotropical mammal

Neotropical nectar-feeding bats (Glossophaginae) are highly specialized in the exploitation of floral nectar and have one of the highest mass-specific metabolic rates among mammals. Nevertheless, they are distributed throughout the tropics and subtropics over a wide elevational range, and thus encounter many extreme and energetically challenging environmental conditions. Depressing their otherwise high metabolic rate, e.g., in situations of food restriction, might be an important adaptive physiological strategy in these dietary specialists. We investigated the thermoregulatory behavior of captive 10-g nectar feeding bats (Glossophaga soricina; Chiroptera, Phyllostomidae) under variable ambient temperatures (T _a) and feeding regimes and predicted that bats would use torpor as an energy-conserving behavior under energetic constraints. All tested animals entered torpor in response to energetic restrictions and the depth of torpor was dependent on the body condition of the animals and hence on their degree of physiological constraints. Periods of torpor with body temperatures (T _b) below 34°C were precisely adjusted to the photoperiod. The median length of diurnal torpor was 11.43 h. The lowest T _b measured was 21°C at a T _a of 19°C. Estimated energy savings due to torpor were considerable, with reductions in metabolic rate to as low as 5% of the metabolic rate of normothermic bats at the same T _a. However, contrary to temperate zone bats that also employ diurnal torpor, G. soricina regulated their T _b to the highest possible levels given the present energetic supplies. To summarize, G. soricina is a precise thermoregulator, which strategically employs thermoregulatory behavior in order to decrease its energy expenditure when under energetic restrictions. This adaptation may play a crucial role in the distribution and the assembly of communities of nectar-feeding bats and may point to a general capacity for torpor in tropical bats.     

Body temperature and metabolic rate during natural hypothermia in endotherms

During daily torpor and hibernation metabolic rate is reduced to a fraction of the euthermic metabolic rate. This reduction is commonly explained by temperature effects on biochemical reactions, as described by Q ₁₀ effects or Arrhenius plots. This study shows that the degree of metabolic suppression during hypothermia can alternatively be explained by active downregulation of metabolic rate and thermoregulatory control of heat production. Heat regulation is fully adequate to predict changes in metabolic rate, and Q ₁₀ effects are not required to explain the reduction of energy requirements during hibernation and torpor.Abbreviations BMR basal metabolic rate - BW body weight - C thermal conductance - C_HL thermal conductance as derived from HL - C_HP thermal conductance as derived from HP - HL heat loss - HP heat production - MR metabolic rate - RQ respiratory quotient - T_a ambient temperature - T_b body temperature     

Thermoregulation in free‐ranging ground woodpeckers Geocolaptes olivaceus: no evidence of torpor

Heterothermic responses characterised by pronounced hypometabolism and reductions in body temperature (T_b) are one of the most effective ways in which small endotherms can offset the energetic cost of endothermic homeothermy. It remains unclear, therefore, why daily torpor and hibernation are restricted to only a subset of avian lineages. To further our understanding of the phylogenetic distribution of avian torpor, we investigated winter thermoregulation in the southern African ground woodpecker Geocolaptes olivaceus. We considered this species a good candidate for heterothermy, because it is resident year‐round in mountainous regions with cold winters and reliant on small ectothermic prey. We recorded T_b patterns in free‐ranging individuals and measured T_b and metabolic rates in captive individuals. Neither free‐ranging nor captive woodpeckers showed any indication of daily torpor or even shallow rest‐phase hypothermia. All birds maintained bimodally distributed T_b characteristic of classic endothemic homeothermy, with a mean rest‐phase T_b of 37.9 ± 0.2°C and no data below 37.0°C. The mean circadian amplitude of T_b was 4.2°C, equivalent to approximately twice the expected value. There was some evidence of seasonal acclimatisation in T_b, with a small decrease in rest‐phase T_b with the onset of the austral winter. Captive birds showed patterns of resting metabolic rate and T_b consistent with the classic model of endothermic homeothermy. The apparent absence of torpor in G. olivaceus supports the notion that, unlike the case in mammals, many avian taxa that may a priori be expected to benefit from deep heterothermy do not use it.     

Cleavage site of a major yolk protein (MYP) determined by cDNA isolation and amino acid sequencing in sea urchin,Hemicentrotus pulcherrimus

The grey mouse lemur (Microcebus murinus) is a small nocturnal primate exhibiting daily torpor. In constant ambient temperature (22-24 degrees C), body temperature (Tb) and locomotor activity were monitored by telemetry in animals exposed to short (SP: 10 h light/day) or long (LP: 14 light/day) photoperiods. They were first fed ad libitum for 8 days and then subjected to 80% restricted feeding for 8 more days. During ad libitum feeding, locomotor activity was significantly lower in SP-exposed animals than in LP-exposed animals. Whatever the photoperiod, animals entered daily hypothermia within the first hours following the light onset. Depth of daily hypothermia increased irregularly under SP exposure, whereas minimal daily Tb was constantly above 35 degrees C under LP exposure. After the transfer from long photoperiod to short photoperiod corresponding to the induction of seasonal fattening, locomotor activity and depth of controlled daily hypothermia did not change significantly. In contrast, food restriction led to a significant increase in locomotor activity and in frequency of daily torpor (Tb<33 degrees C) and body temperature reached minimum values averaging 25 degrees C. However, SP-exposed animals exhibited lower minimal daily Tb and higher torpor duration than LP exposed animals. Therefore, daily torpor appears as a rapid response to food restriction occurring whatever the photoperiod, although enhanced by short photoperiod.     

Exogenous passive heating during torpor arousal in free-ranging rock elephant shrews, <Emphasis Type="Italic">Elephantulus myurus</Emphasis>

In the laboratory rock elephant shrews (Elephantulus myurus; mean body mass 56.6 g) displayed the lowest torpor T_{b min} yet recorded (ca. 5°C) in a placental daily heterotherm. It was unknown whether these low T_bs were characteristic of daily heterothermy in free-ranging animals. It was also unclear how cost effective these low T_bs were since considerable energy is required to arouse from low T_bs on a daily basis. We continuously measured body temperature once every hour for 85 days in 13 free-ranging E. myurus from May to August 2001 (winter) in Weenen Game Reserve, KwaZulu-Natal, South Africa. We recorded a total of 412 torpor bouts. Free-ranging E. myurus had a high propensity for torpor with females displaying higher torpor frequency than males. The lowest T_b recorded was 7.5°C at T_a=2.7°C and the minimum torpor T_b was strongly correlated with ambient temperature. Torpor arousal was tightly coupled with ambient temperature cycles. Low torpor T_{b min} at low T_as was therefore cost-effective because the animals offset the high cost of arousal through exogenous passive heating. Laboratory studies under constant ambient temperatures may therefore underestimate the energetic benefits of torpor in free-ranging small mammals that inhabit regions where seasonality is moderate.     

Overwinter body temperature patterns in captive jerboas (<Emphasis Type="Italic">Jaculus orientali</Emphasis>s): influence of sex and group

The jerboa (Jaculus orientalis) has been described in the past as a hibernator, but no reliable data exist on the daily and seasonal rhythmicity of body temperature (T _b). In this study, T _b patterns were determined in different groups of jerboas (isolated males and females, castrated males and grouped animals) maintained in captivity during autumn and winter, and submitted to natural variations of light and ambient temperature (T _a). T _b and T _a variations were recorded with surgically implanted iButton temperature loggers at 30-min intervals for two consecutive years. About half (6/13) of isolated female jerboas hibernated with a T _b < 33°C, with hibernation bouts interspersed with short periods of normothermy from November to February. Hibernation bout durations were longer (4–5 days) than those of normothermia phases (1–4 days). During hibernation, the minimum T _b was low (T _bmin ~10.7°C). In contrast, one of the 12 isolated males showed short hibernation bouts of ca. 2 days late in the hibernation season, February–March. The males had T _bmin values of 15.1°C. In contrast to predictions, no castrated males hibernated. When jerboas were grouped, females and males exhibited concomitant torpor bouts. In males, the longest bouts were observed during the late hibernation season. These data suggest complex regulation of hibernation in jerboas.     

Body temperature variation in free-living and food-deprived yellow-necked mice sustains an adaptive framework for endothermic thermoregulation

In many mammalian species, variation in body temperature (T_b) exceeds the values suitable for defining homeothermy, making it justifiable and even necessary to resort to the term “heterothermic”. However, T_b data are only available for ca. 1% of extant mammalian species. We investigated variations in T_b in wild free-living and experimentally food-deprived yellow-necked mice Apodemus flavicollis, during the temperate-zone autumn-winter period. In line with the adaptive framework for endothermic thermoregulation, we hypothesised that T_b in the mice should be adjustable with the energetic cost-benefit trade-off associated with maintaining homeothermy. In laboratory conditions, mice clearly entered a state of daily torpor when food-deprived. Our study thus makes it clear that A. flavicollis is a heterothermic species in which food deprivation results in switching between endothermic and poikilothermic thermoregulation. We also assumed that, in free-living mice, heterothermy increases with elevated environmental challenges, e.g. when the ambient temperature (T_a) decreases. Consistent with this was the inverse correlation noted between variation in T_b in free-living mice and T_a, with most individuals clearly becoming torpid when T_a decreases below 0 °C. It is the increased cost of food hoarding under cold conditions that most likely triggers a state of torpor as a last result. Overall, our study indicates that yellow-necked mice can provide a further example of species sustaining an adaptive framework for endothermic thermoregulation.     

The energetic cost of arousal from torpor in the marsupial Sminthopsis macroura : benefits of summer ambient temperature cycles

The costs of arousal from induced torpor were measured in the striped-faced dunnart (Sminthopsis macroura; ca. 25 g) under two experimental ambient temperature cycles. The sinusoidal-type temperature cycles were designed to evaluate the effects of passive, ambient temperature heating during arousal from torpor in these insectivorous marsupials. It was hypothesised that diel ambient temperature cycles may offer significant energy savings during arousal in animals that employ daily torpor in summer as a response to unpredictable food availability. The cost of arousal in animals in which passive, exogenous heating occurred was significantly lower than that in animals not exposed to an ambient temperature cycle. The total cost of all three phases of torpor (entry, maintenance and arousal) was almost halved when animals were exposed to an ambient heating cycle from 15 °C to 25 °C over a 24-h period. In all animals, irrespective of the experimental ambient temperature cycle employed, the minimum torpor body temperature was 17–18 °C. The body temperature (T_b) of animals exposed to exogenous heating increased from the torpor T_b minimum to a mean value of 22.59 °C before endogenous heat production commenced. This relatively small increase in T_b of ca. 5 °C through `free' passive heating was sufficient to account for the significant ca. three-fold decrease in the cost of arousal and may represent an important energetic aid to free-ranging animals. Accepted: 4 October 1998     

Interactive effects of temperature and photoperiod on the daily activity and energy metabolism of pouched mice (Saccostomus campestris: Cricetidae) from southern Africa

The daily activity and energy metabolism of pouched mice (Saccostomus campestris) from two localities in southern Africa was examined following warm (25 °C) and cold (10 °C) acclimation under long (LD 14:10) and short (LD 10:14) photoperiol. There was no differential effect of photoperiod on the daily activity or metabolism of pouched mice from the two localities examined, which suggests that reported differences in photoresponsivity between these two populations were not the result of differences in daily organisation. Neverthe-less, there was a significant increase in metabolism at 10 °C, irrespective of photoperiod, even though seven cold-acclimated animals displayed bouts of spontaneous torpor and saved 16.4–36.2% of their daily energy expenditure. All but one of these bouts occurred under short photoperiod, which suggests that short photoperiod facilitated the expression of torpor and influenced the daily energy metabolism of these individuals. As expected for a noctureal species, the amount of time spent active increased following acclimation to short photoperiod at 25 °C. However, there was a reduction in mean activity levels under short photoperiod at 10 °C, possibly because the stimulation of activity by short photoperiod was masked by a reduction in activity during bouts of spontaneous torpor. Cold temperature clearly had an overriding effect on the daily activity and metabolism of this species by necessitating an increase in metabolic heat production and eliciting spontaneous torpor which overrode the effect of short photoperiod on activity at an ambient temperature of 10 °C.Abbreviations 3-ANOVA three-way analysis of variance - %ACT percentage of time spent active - ADMR average daily metabolic rate - M _b body mass - MR metabolic rate - MR_dark metabolic rate recorded during the dark phase - MR_light metabolic rate recorded during the light phase - NST non-shivering thermogenesis - RQ respiratory quotient - STPD standard temperature and pressure, dry - T _a ambient temperature - T _b body temperature - VO₂ oxygen consumption     

Daily Torpor in Free-Ranging Gray Mouse Lemurs (Microcebus murinus) in Madagascar

I aimed to determine when and under which seasonal environmental conditions gray mouse lemurs (Microcebus murinus), a small nocturnal primate species endemic to Madagascar, utilize daily torpor. Using temperature-sensitive radio collars, I measured skin temperature (T _sk ) of free-ranging mouse lemurs under natural conditions. My results showed that male and female mouse lemurs in the wild enter torpor spontaneously over a wide range of ambient temperatures (T _a ) during the dry season, but not during the rainy season. Mouse lemurs that remained normothermic had significantly lower body masses (mean: 59.7 g) than individuals that used torpor (mean: 80.2 g). Skin temperatures dropped to 20.9°C and the mean torpor bout duration is 10.3 h. The use of torpor on a given night varied among individuals, whereas the propensity for torpor did not differ significantly between males and females. I found no evidence that T _a can be used to predict whether mouse lemurs will remain normothermic or enter torpor. It appears that the most reliable indicator for the occurrence of torpor in free-ranging Microcebus murinus is time of the year, i.e., photoperiod.