Occurrence Of Torpor Research Articles

The effect of food timing on torpor propensity and characteristics in laboratory mice during a common food restriction paradigm

Background Many behavioural, pharmacological, and metabolic studies in mice require fasting, yet the possibility of fasting-induced torpor affecting the data is rarely considered. Torpor is a state characterised by depressed metabolism and profound alterations to physiology and behaviour. In this study we aimed to determine how the effects of torpor on experimental outcomes could be mitigated. Methods To this end, timing and characteristics of fasting-induced torpor in response to feeding in the morning versus feeding in the night were compared using non-invasive monitoring of peripheral body temperature. Results Night-fed mice entered significantly more torpor bouts per day compared to morning-fed mice (Morning: 2.79±0.197 (mean ± SEM); Night: 4.79±0.533 (mean ± SEM); p=0.0125), but these bouts were shorter on average by ~1.5h. Latency to the first torpor bout following feeding tended to be shorter during night feeding (Morning: 9.57±0.8h (mean ± SEM); Night: 6.66±1.2h (mean ± SEM); p=0.0928). Moreover, torpor bouts typically occurred during the dark phase in the morning-fed group, whilst night feeding resulted in a shift of torpor occurrence to earlier in the day (Morning: 14.2±0.4 ZT h (mean ± SEM); Night: 12.2±0.9 ZT h (mean ± SEM); p=0.0933). There was a high degree of variation in torpor occurrence within and between animals in each group. Conclusions We recommend that feeding time is kept consistent between days and the same across animals to minimise variation in torpor occurrence. Moreover, the timing of food provision may be optimised to allow measurements to be taken during euthermia, to mitigate the effects of torpor on the variables investigated. Finally, we recommend that body temperature is monitored non-invasively to determine when torpor is occurring, and that testing, or sample collection is conducted when the torpor history is comparable between animals.

Induction of torpor in response to a common chronic food restriction paradigm: implications for behavioural research using mice

Background Many behavioural, pharmacological, and metabolic studies in mice require fasting, yet the possibility of fasting-induced torpor affecting data is rarely considered. Torpor is a state characterised by depressed metabolism and profound alterations in physiology and behaviour. In this study, we aimed to determine whether a chronic food restriction paradigm, common in behavioural studies, was sufficient to induce torpor in mice. Methods Mice were food restricted to ~85-90% of their bodyweight, as is typically done, and monitored using continuous thermal imaging. Results We observed that body temperature significantly decreased over days of food restriction, and it was significantly related to the drop in bodyweight (r2=0.8989, p<0.0001). All mice reliably entered torpor daily from day 8 of food restriction which coincided with bodyweight stabilisation at ~85%. We found a strong positive relationship between the magnitude of the decrease of bodyweight and the proportion of mice entering torpor each day (r2=0.8715, p<0.0001). Conclusions Overall, we found that torpor is readily induced in response to food restriction. Considering that hunger is frequently used as a motivational drive in behavioural tasks, it is likely that torpor occurrence is common in such studies, while remaining undetected and unaccounted for. Due to the profound effect of torpor on physiology, it is possible that torpor induction may be confounding subsequent data and represents an important source of variation. We recommend that body temperature is always monitored noninvasively in studies where food restriction is employed, to determine when torpor is occurring, and that torpor history is appropriately controlled for within and across experimental groups.

Widespread Torpor Use in Hummingbirds from the Thermally Stable Lowland Tropics.

AbstractTorpor, the temporary reduction of metabolic rate and body temperature, is a common energy-saving strategy in endotherms. Because of their small body size and energetically demanding life histories, hummingbirds have proven useful for understanding when and why endotherms use torpor. Previous studies of torpor in hummingbirds have been largely limited to tropical montane species or long-distance migrants that regularly experience challenging thermal conditions. Comparatively little is known, however, about the use of torpor in hummingbirds of the lowland tropics, where relatively high and stable year-round temperatures may at least partially negate the need for torpor. To fill this knowledge gap, we tested for the occurrence of torpor in tropical lowland hummingbirds ( individuals of six species) from central Panama. In controlled experimental conditions simulating the local temperature regime, all six species used torpor to varying degrees and entered torpor at high ambient temperatures (i.e., ≥28°C), indicating that hummingbirds from the thermally stable lowland tropics regularly use torpor. Torpor reduced overnight mass loss, with individuals that spent more time in torpor losing less body mass during temperature experiments. Body mass was the best predictor of torpor depth and duration among and within species-smaller species and individuals tended to use torpor more frequently and enter deeper torpor. Average mass loss in our experiments (∼8%-10%) was greater than that reported in studies of hummingbirds from higher elevation sites (∼4%). We therefore posit that the energetic benefits accrued from torpor may be limited by relatively high nighttime temperatures in the lowland tropics, although further studies are needed to test this hypothesis.

Drivers of torpor during the active season of Hazel Dormouse

Torpor is a life history strategy within more than half of mammalian orders, to achieve the conservation of energy particularly when species are experiencing unfavourable conditions. Daily torpor is limited to a duration of less than 24 hours and occurs when an animal reduces its metabolic rate and lowers its body temperature between bouts of activity. Hazel Dormice utilise daily torpor as an energy saving strategy during their active period (April-October) and the prevalence of torpor varies from year to year. The intrinsic and extrinsic drivers of daily torpor in Hazel Dormice are not yet well understood. I have identified drivers of torpor among Hazel Dormice which include climatic variables and the life history of individual animals. Dormice were more likely to be found in torpor on colder and wetter days and during colder and wetter seasons. Weight was a strong predictor of torpor; lighter individuals were more likely to be torpid earlier in the year (April-August) and heavier individuals later in the year (September-October). Individuals found in breeding nest boxes rarely enter torpor and solitary animals are more likely to be torpid. Sex only affects the prevalence of torpor in two months, August and September; males were more likely to be torpid in August and females in September. Using the resulting model I can predict how daily torpor occurrence might change under a changing climate. This will have consequences for Hazel Dormice in the UK as it is likely to affect survival and breeding rates.

Heterothermy as the Norm, Homeothermy as the Exception: Variable Torpor Patterns in the South American Marsupial Monito del Monte (Dromiciops gliroides).

Hibernation (i.e., multiday torpor) is considered an adaptive strategy of mammals to face seasonal environmental challenges such as food, cold, and/or water shortage. It has been considered functionally different from daily torpor, a physiological strategy to cope with unpredictable environments. However, recent studies have shown large variability in patterns of hibernation and daily torpor (“heterothermic responses”), especially in species from tropical and subtropical regions. The arboreal marsupial “monito del monte” (Dromiciops gliroides) is the last living representative of the order Microbiotheria and is known to express both short torpor episodes and also multiday torpor depending on environmental conditions. However, only limited laboratory experiments have documented these patterns in D. gliroides. Here, we combined laboratory and field experiments to characterize the heterothermic responses in this marsupial at extreme temperatures. We used intraperitoneal data loggers and simultaneous measurement of ambient and body temperatures (TA and TB, respectively) for analyzing variations in the thermal differential, in active and torpid animals. We also explored how this differential was affected by environmental variables (TA, natural photoperiod changes, food availability, and body mass changes), using mixed-effects generalized linear models. Our results suggest that: (1) individuals express short bouts of torpor, independently of TA and even during the reproductive period; (2) seasonal torpor also occurs in D. gliroides, with a maximum bout duration of 5 days and a mean defended TB of 3.6 ± 0.9°C (one individual controlled TB at 0.09°C, at sub-freezing TA); (3) the best model explaining torpor occurrence (Akaike information criteria weight = 0.59) discarded all predictor variables except for photoperiod and a photoperiod by food interaction. Altogether, these results confirm that this marsupial expresses a dynamic form of torpor that progresses from short torpor to hibernation as daylength shortens. These data add to a growing body of evidence characterizing tropical and sub-tropical heterothermy as a form of opportunistic torpor, expressed as daily or seasonal torpor depending on environmental conditions.

Daily changes in food availability, but not long-term unpredictability, determine daily torpor-bout occurrences and frequency in stripe-faced dunnarts (Sminthopsis macroura)

Daily torpor, a short-term reduction in body temperature and metabolism, is an energy-saving strategy that has been interpreted as an adaptation to unpredictable resource availability. However, the effect of food-supply variability on torpor, separately from consistent food restriction, remains largely unexamined. In this study, we investigated the effect of unpredictable food availability on torpor in stripe-faced dunnarts (Sminthopsis macroura). After a control period of ad libitum feeding, dunnarts were offered 65% of their average daily ad libitum intake over 31 days, either as a constant restriction (i.e. as equal amount of food offered each day) or as an unpredictable schedule of feed offered, varied daily as 0%, 30%, 60%, 100% or 130% of ad libitum. Both feeding groups had increased torpor-bout occurrences (as a proportion of all dunnarts on a given day) and torpor-bout frequency (average number of bouts each day) when on a restricted diet compared with ad libitum feeding, but torpor frequency did not differ between the consistently restricted and unpredictably restricted groups. Most importantly, torpor occurrence and daily bout frequency by the unpredictably restricted group appeared to change in direct association with the amount of food offered on each day; torpor frequency was higher on days of low food availability. Our data do not support the interpretation that torpor is a response to unpredictable food availability per se, but rather that torpor allowed a rapid adjustment of energy expenditure to manage daily fluctuations in food availability.

Torpor in free-ranging antechinus: does it increase fitness?

Antechinus are small, insectivorous, heterothermic marsupial mammals that use torpor from late summer to early winter and reproduce once a year in late winter/early spring. Males die after mating, most females produce only a single litter, but some survive a second winter and produce another litter. As it is not known how these females manage to survive the second winter after the energetically demanding reproductive period and then reproduce a second time, we aimed to provide the first data on thermal biology of free-ranging antechinus by using temperature telemetry. Male Antechinus stuartii and Antechinus flavipes rarely entered torpor in autumn/early winter in the wild, expressing only shallow bouts of <2 h. Female A. stuartii used torpor extensively, employing bouts up to 16.7 h with body temperatures as low as 17.8 °C. Interestingly, although first and second year females used similar torpor patterns, torpor occurrence was almost twofold in second year (93 % of days) than first year females (49 %), and the proportion of the overall monitoring period animals spent torpid was 3.2-fold longer in the former with a corresponding shorter activity period. Our study suggests that intensive use of torpor is crucial for second year females for autumn and winter survival and production of a second litter. We provide the first evidence of an age-related pattern in daily torpor expression in free-ranging mammals and show that torpor use is a complex process that is affected not only by the current energy availability and thermal conditions but also by the reproductive history and age of individuals.

Interspecific Competition and Torpor in Golden Spiny Mice: Two Sides of the Energy-Acquisition Coin

We studied the occurrence of torpor in golden spiny mice in a hot rocky desert near the Dead Sea. In this rodent assemblage, a congener, the nocturnal common spiny mouse, competitively excluded the golden spiny mouse from the nocturnal part of the diel cycle and forced it into diurnal activity; this temporal partitioning allows the two species to partition their prey populations, particularly in summer when the diet of the two species is comprised mainly of arthropods, and largely overlap. We studied the effect of the presence of the common spiny mice at two resource levels (natural food availability and food added ad libitum) on populations of golden spiny mice in four large outdoor enclosures: two with common spiny mice removed and two enclosures with populations of both species. We hypothesized that with interspecific competition and/or reduced resources, golden spiny mice will increase their use of torpor. As we expected, supplemented food reduced the total time spent torpid. In summer, when the different activity periods of the two species results in prey species partitioning, removal of the congener did not affect torpor in the golden spiny mouse. However, in winter, when insect populations are low and the two species of mice overlap in a largely vegetarian diet, removal of the common spiny mouse reduced torpor in golden spiny mice, whether food was supplemented or not. This result suggests that torpor, a mechanism that allows small mammals to sustain periods of low availability of resources or high energetic requirements, may also help them to tolerate periods of enhanced interspecific competition. This may be a significant short-term mechanism that reduces competition and hence increases fitness, in particular of individuals of the subordinate species whose accessibility to resources may be limited.

Does torpor of elephant shrews differ from that of other heterothermic mammals?

Torpor bouts of elephant shrews are intermediate in duration to those of daily heterotherms and hibernating mammals, but their body temperatures (Tbs) and metabolic rates are very low and similar to those of hibernating mammals. We quantified the thermal physiology of the Cape rock elephant shrew (Elephantulus edwardii), a species endemic to high-altitude regions of South Africa, where winters are cold and wet, and tested whether it displays multiday torpor characteristic of hibernators at low ambient temperature (Ta). E. edwardii regularly displayed torpor over a wide range of Tas. Occurrence of torpor and duration of torpor bouts increased with decreasing Ta. Whereas normothermic Tb was stable, Tb in torpid individuals fell with Ta. The mean Tb 2 Ta differential at the minimum Tb was 0.7uC, and the mean minimum Tb at Ta 8.9uC was 9.3uC. Duration of torpor bouts ranged from 6.5 to 44 h and was correlated negatively with Ta and Tb during torpor. Time required for the reduction of Tb to a Tb 2 Ta differential of ,2.0uC was faster for .1-day torpor bouts than those lasting 1 day, suggesting that the duration of a bout might be determined at the beginning, not during, a bout. The nature of heterothermy in E. edwardii seems qualitatively similar to that of other elephant shrews, although torpor is somewhat deeper and longer in this species. Temporal patterns of torpor in E. edwardii differ from those of most cold-climate hibernators, likely for ecological rather than physiological reasons.

Torpor and energetic consequences in free-ranging grey mouse lemurs (Microcebus murinus): a comparison of dry and wet forests

Many endotherms save energy during food and water shortage or unpredictable environment using controlled reductions in body temperature and metabolism called torpor. In this study, we measured energy metabolism and water turnover in free-ranging grey mouse lemurs Microcebus murinus (approximately 60 g) using doubly labelled water during the austral winter in the rain forest of southeastern Madagascar. We then compared patterns of thermal biology between grey mouse lemurs from the rain forest and a population from the dry forest. M. murinus from the rain forest, without a distinct dry season, entered daily torpor independent of ambient temperature (T (a)). There were no differences in torpor occurrence, duration and depth between M. murinus from the rain and dry forest. Mouse lemurs using daily torpor reduced their energy expenditure by 11% in the rain forest and by 10.5% in the dry forest, respectively. There was no significant difference in the mean water flux rates of mouse lemurs remaining normothermic between populations of both sites. In contrast, mean water flux rate of individuals from the dry forest that used torpor was significantly lower than those from the rain forest. This study represents the first account of energy expenditure, water flux and skin temperature (T (sk)) in free-ranging M. murinus from the rain forest. Our comparative findings suggest that water turnover and therefore water requirement during the austral winter months plays a more restricting role on grey mouse lemurs from the dry forest than on those from the rain forest.

Basking and diurnal foraging in the dasyurid marsupial Pseudantechinus macdonnellensis

Several mammal species bask to passively rewarm during arousal from torpor, a strategy that can decrease energetic costs. Nothing is known about basking behaviour in these species or the trade-offs between energetic benefits of basking and potential costs associated with changes in activity patterns and increased predation risk. We assessed basking during winter in Pseudantechinus macdonnellensis, an Australian arid-zone marsupial that belongs to a family (Dasyuridae) that is typically nocturnal. Animals were implanted with temperature-sensitive transmitters to assess body temperatures and to assist in visually locating animals active during the day. Tagged animals regularly exhibited diurnal foraging. Foraging bouts occurred throughout the day; however, most bouts were observed within 3 h of sunset. By comparison, basking occurred much more frequently in the morning. Basking and a shift towards diurnal foraging in winter is associated with a decrease in richness and abundance of predators. P. macdonnellensis appears to compensate for the occurrence of torpor during the active phase (i.e. night) in winter by changing activity patterns such that foraging commences during what is usually the rest phase. These activity patterns are not expected to occur during the remainder of the year.

Torpor in an African caprimulgid, the freckled nightjar Caprimulgus tristigma

Recent data suggest that facultative hypothermic responses such as torpor are more important in the energy balance of birds from tropical and sub‐tropical regions than previously thought. We used telemetric measurements of skin temperature (Tskin) for five individuals on 151 bird‐nights to investigate the occurrence of torpor during winter in an 81 g African caprimulgid, the freckled nightjar Caprimulgus tristigma. We found that freckled nightjars have the capacity to enter torpor, with a minimum observed Tskin of 12.8°C. During the torpor bouts we observed, complete rewarming typically occurred after sunrise, and coincided with the availability of solar radiation. There was considerable inter‐individual variability in the frequency and depth of torpor bouts, with one female nightjar exhibiting particularly frequent and deep torpor. Our results confirm the ability to use torpor by a nocturnal aerial insectivore from the Afrotropics, and reiterate the variability in patterns of torpor that can exist within a population.

Thermal biology, torpor and behaviour in sugar gliders: a laboratory-field comparison

Most studies on animal physiology and behaviour are conducted in captivity without verification that data are representative of free-ranging animals. We provide the first quantitative comparison of daily torpor, thermal biology and activity patterns, conducted on two groups of sugar gliders (Petaurus breviceps, Marsupialia) exposed to similar thermal conditions, one in captivity and the other in the field. Our study shows that activity in captive gliders in an outdoor aviary is restricted to the night and largely unaffected by weather, whereas free-ranging gliders omit foraging on cold/wet nights and may also forage in the afternoon. Torpor occurrence in gliders was significantly lower in captivity (8.4% after food deprivation; 1.1% for all observations) than in the field (25.9%), mean torpor bout duration was shorter in captivity (6.9 h) than in the field (13.1 h), and mean body temperatures during torpor were higher in captivity (25.3 degrees C) than in the field (19.6 degrees C). Moreover, normothermic body temperature as a function of air temperature differed between captive and free-ranging gliders, with a >3 degrees C difference at low air temperatures. Our comparison shows that activity patterns, thermal physiology, use of torpor and patterns of torpor may differ substantially between the laboratory and field, and provides further evidence that functional and behavioural data on captive individuals may not necessarily be representative of those living in the wild.

To use or not to use torpor? Activity and body temperature as predictors

When food is limited and/or environmental conditions are unfavourable, many mammals reduce activity and use torpor to save energy. Nevertheless, reliable predictors for torpor occurrence, especially in the wild, are currently not available. Interrelations between torpor use and other energy conserving strategies are also poorly understood. We tested the hypothesis that reductions in normothermic body temperature (T(b)) and the period of activity before torpor events could be used as predictors for torpor occurrence in sugar gliders, Petaurus breviceps (body mass, approximately 125 g), known to display daily torpor in the wild. Occurrence of torpor was preceded by significant (approximately 10-25%) reductions of the duration of the activity phase. Moreover, the normothermic resting T(b) fell by an average of 1.2 degrees C over 3 days before a torpor event, relative to individuals that did not display torpor. Our new findings suggest that before entering torpor, sugar gliders, which appear to use torpor as an emergency measure rather than a routine energy saving strategy, systematically reduce activity times and normothermic resting T(b)s to lower energy expenditure and perhaps to avoid employing torpor. Thus, reduced activity and normothermic T(b) may provide a predictive tool for the occurrence of daily torpor in the wild.

Photoperiod affects daily torpor and tissue fatty acid composition in deer mice

Photoperiod and dietary lipids both influence thermal physiology and the pattern of torpor of heterothermic mammals. The aim of the present study was to test the hypothesis that photoperiod-induced physiological changes are linked to differences in tissue fatty acid composition of deer mice, Peromyscus maniculatus ( approximately 18-g body mass). Deer mice were acclimated for >8 weeks to one of three photoperiods (LD, light/dark): LD 8:16 (short photoperiod), LD 12:12 (equinox photoperiod), and LD 16:8 (long photoperiod). Deer mice under short and equinox photoperiods showed a greater occurrence of torpor than those under long photoperiods (71, 70, and 14%, respectively). The duration of torpor bouts was longest in deer mice under short photoperiod (9.3 +/- 2.6 h), intermediate under equinox photoperiod (5.1 +/- 0.3 h), and shortest under long photoperiod (3.7 +/- 0.6 h). Physiological differences in torpor use were associated with significant alterations of fatty acid composition in approximately 50% of the major fatty acids from leg muscle total lipids, whereas white adipose tissue fatty acid composition showed fewer changes. Our results provide the first evidence that physiological changes due to photoperiod exposure do result in changes in lipid composition in the muscle tissue of deer mice and suggest that these may play a role in survival of low body temperature and metabolic rate during torpor, thus, enhancing favourable energy balance over the course of the winter.

Daily torpor in relation to photoperiod in a subtropical blossom-bat, Syconycteris australis (Megachiroptera)

Daily torpor in many temperate-zone mammals is affected by photoperiod. As little is known about the effects of photoperiod on torpor in subtropical species, we investigated whether, and if so how, torpor use, duration, and depth are affected by acclimation to three photoperiods (short, intermediate, long) in the blossom-bat Syconycteris australis. In contrast to many other studies, torpor occurrence, duration, and depth did not significantly respond to photoperiod acclimation in S. australis. Interestingly, the trend of a decline in torpor use under long photoperiod was the opposite of that observed previously in S. australis, which had been captured from the wild in summer and winter. Our study suggests that some species living in low latitude areas with unpredictable weather like S. australis may not use photoperiod for seasonal adjustments in physiology because it is not a reliable cue for food availability.

Torpor in Birds: Taxonomy, Energetics, and Ecology

Recent reports on patterns and occurrence of torpor and other natural hypothermic states in birds have prompted a revision of many longstanding opinions. For instance, a larger assortment of birds covering a wider range of body mass than previously recognized uses energy-conserving mechanisms in the face of abundant food supplies. Thus, although acute energetic stress triggers the occurrence of hypometabolic states in many birds, energy deficits can no longer be regarded as the sole stimulus for avian torpor. Additionally, the phenology of this phenomenon (phases, duration, depth) shows more interspecific variation than previously appreciated, and traditional concepts of the phases of torpor are not always adequate. Hence, inclusive definitions of torpor based on physiological and/or behavioral criteria have become more difficult to formulate. However, average nighttime body temperature, which is highly consistent throughout the class Aves (38.54 degrees +/- 0.96 degrees C, n = 202), provides a convenient reference for detecting natural hypothermic states. Of the putative ecological factors associated with avian torpor, food specialization seems a prime candidate as an ultimate factor in the occurrence of this state in birds. With few exceptions, all of these animals capable of torpor are either frugivorous, nectarivorous, or insectivorous, suggesting a correlation between thermoregulatory pattern and predictability of food supply. To date, no clear answer exists as to whether the variety of thermoregulatory patterns evident in birds involves discrete mechanisms or merely steps in a physiological continuum. However, I suggest that the occurrence of differences in torpor patterns among closely related species (e.g., within families) favors the latter interpretation.

Correlation between torpor frequency and capacity for non-shivering thermogenesis in the Siberian hamster ( Phodopus sungorus)

We investigated the correlation between torpor frequency and capacity for non-shivering thermogenesis (NST) in Siberian hamsters ( Phodopus sungorus) during 25 weeks of acclimation to cold and short days. We hypothesized that torpor use is conditioned on the development of brown adipose tissue (BAT) capacity for NST. We found that (1) the degree of noradrenaline (NA)-induced hyperthermia was positively correlated with torpor frequency and its length and depth, and (2) the maximum response to NA occurred at the time of day when hamsters naturally arouse from torpor. The present study quantifies the correlation between torpor frequency and NST capacity and we suggest that a well-developed NST capacity is a prerequisite for the occurrence of torpor.

Patterns of activity and inactivity in echidnas (Tachyglossus aculeatus) free-ranging in a hot dry climate: correlates with ambient temperature, time of day and season

Echidnas occur throughout Australia. They exhibit daily fluctuations in body temperature (Tb) and use torpor to various degrees throughout much of their range. Echidnas elsewhere are commonly diurnal except during hot weather. This study used temperature-sensitive radio-transmitters to investigate the activity patterns and temperature relations of echidnas in the relatively hot, dry climate of south-west Queensland with respect to temperature and photoperiod. During activity, echidnas were characterised by rising, but not necessarily high, Tbs. Activity was seen only within an ambient temperature range (as measured in a nearby Stevenson Screen: Tss) of 9–33�C so that activity was seen during the day and at night during the cool weather but only at night in summer. Echidnas used caves, burrows and logs when inactive. Tbs of inactive echidnas declined except when affected by rising ambient temperatures, as determined within these shelters (Ta). In summer, Tbs of echidnas in these shelters changed little or rose with increasing Ta to levels even higher than in active echidnas.Torpor was used by echidnas for periods up to nine days during winter and occasionally for up to one day during summer. Due to the difficulty of identifying the occurrence of torpor from Tb alone in warm conditions, the possibility that echidnas utilise torpor for less than one day remains inconclusive. Nevertheless, at least five bouts of torpor were identified in four (of eight) echidnas during winter/spring and two bouts of torpor from two echidnas in summer.

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 (Tsk) 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 (Ta) 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 Ta 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.