Hibernating patterns of free-ranging Columbian ground squirrels

Summary The patterns of torpor and euthermy during hibernation was documented for 28 free-ranging Columbian ground squirrels (Spermophilus columbianus) fitted with temperature-sensitive radio transmitter collars. Adult males began hibernation earlier, were euthermic for a greater proportion of the hibernating season and emerged earlier than other age and sex classes. The patterns of hibernation of adult females did not differ significantly from those of juveniles. Emergence from the hibernaculum was preceded by a long (3–12 d) euthermic interval in adult males but not in adult females or juveniles. Changes in soil temperature did not appear to initiate emergence. The greater time spent euthermic by adult males is interpreted as a significantly greater energy cost of hibernation for adult males than for other age and sex classes. The benefits offsetting these costs may be increased reproductive potential in spring and avoidance of predation in late summer.     

Maintenance of biological rhythms during hibernation in Eastern woodchucks (<Emphasis Type="Italic">Marmota monax</Emphasis>)

We undertook a study to determine presence of circadian rhythms during woodchuck hibernation using continuously monitored body temperatures. Males had shorter torpor and longer euthermic periods than females. Circular statistics revealed a significant mean vector for males entering into torpor (10:21 h), but not for females. No significant mean vector was found for male or female arousal from torpor. A contingency test was applied to the torpor bout durations. All 7 males tested had significant τ’s between 24 and 26 h, while 6 of the 13 females tested had significant τ’s with a range of 22–27 h. These results implicate a free-running circadian clock during torpor bouts. Overall, the data support the existence of biological rhythms during hibernation in woodchucks, especially for males during arousals. Since entries into torpor appear to be synchronized for males, arousal periods may be used to resynchronize their circadian system. The persistence of biological rhythms during hibernation may help to insure successful mating in the spring after emergence.     

Intraspecific differences in the pattern of hibernation in the ground squirrelSpermophilus beldingi

Summary Intraspecific differences in the patterns of heterothermy were found in captive Belding's ground squirrels that hibernated undisturbed at ambient temperatures of 5°C, 10°C, and 15°C. The timing of all entrances into and arousals from hibernation was determined from records of copper-constantan thermocouples that were mounted on the floor of each animal's nest box and connected to continuously recording potentiometers. In the absence of food, large adult males terminated hibernation spontaneously in the spring. In contrast, females and small non-breeding males (yearlings) did not stop hibernating but instead they shortened their bouts of torpor in the spring so that they aroused every three or four days. This interval of frequent arousals, termed the emergence period, lastel until the squirrels became emaciated, and it was only in the 2 or 3 weeks preceeding death (starvation period) that arousal frequency once again decreased towards midwinter values (Fig. 3). These animals terminated hibernation when fed during the emergence or starvation periods, but they were able to resume torpor if that feeding lasted less than a week.Termination of hibernation and reproductive development in males were related to the size of the animals, not their age. Males two years and older which did not deposit normal quantities of fat were like yearlings in that they had open-ended hibernation seasons and showed little testicular enlargement. Likewise, juvenile males that grew to near adult size in their first summer spontaneously terminated hibernation and had well developed testes like most older individuals. In addition, both the frequency and duration of arousals in the emergence period were related, in part, to the extent of the animals' fat reserves, such that large squirrels spent more time at high body temperatures than small individuals (Figs. 5, 6). This trend was most pronounced at high ambient temperatures.These intraspecific differences in hibernation physiology are consistent with the sex and agerelated differences in the timing of emergence above ground in nature. The increase in the time spent euthermic in the spring isinterpreted as an adaptation for increasing the opportunities for environmental assessment. The intraspecific differences in the extent of this euthermia appear to be associated with differences in the balance between the energy supplies available to an animal and its need to accelerate the use of that energy (i.e., arouse) in order to achieve an early and accurately-time emergence from the hibernaculum.     

Testis development and testosterone secretion in captive European ground squirrels before, during, and after hibernation

In some hibernating species, an extended euthermic period before spring emergence has been reported during which testicular maturation occurred. In this study, we investigated whether male European ground squirrels Spermophilus citellus (Linnaeus, 1766) develop scrotal testes before or after the termination of hibernation. The course of testis development and testosterone concentrations were examined in young-of-year male ground squirrels (n = 4) before, during, and after their first hibernation. In the environmental chamber, all animals showed regular hibernation patterns with decreasing torpor bout lengths prior to the end of hibernation. Four weeks before hibernation ended, testosterone levels were elevated during spontaneous arousals. Testosterone concentrations peaked during the first 2 months post heterothermy and decreased thereafter. In 2 experimental males, testes descended from the inguinal region into the scrotum shortly before the end of heterothermy. The state of testis development and testosterone secretion corresponded to that found in semi-free-living males at spring emergence. Testis size increased in active animals to maximum widths during weeks 3 and 4 post heterothermy. Scrotal skin pigmentation was absent until heterothermy ended and developed during the first month thereafter. The data suggest that male S. citellus do not need a pre-emergence euthermic period for reproductive development.     

Changing seasonality and phenological responses of free-living male arctic ground squirrels: the importance of sex

Many studies have addressed the effects of climate change on species as a whole; however, few have examined the possibility of sex-specific differences. To understand better the impact that changing patterns of snow-cover have on an important resident Arctic mammal, we investigated the long-term (13 years) phenology of hibernating male arctic ground squirrels living at two nearby sites in northern Alaska that experience significantly different snow-cover regimes. Previously, we demonstrated that snow-cover influences the timing of phenological events in females. Our results here suggest that the end of heterothermy in males is influenced by soil temperature and an endogenous circannual clock, but timing of male emergence from hibernation is influenced by the timing of female emergence. Males at both sites, Atigun and Toolik, end heterothermy on the same date in spring, but remain in their burrows while undergoing reproductive maturation. However, at Atigun, where snowmelt and female emergence occur relatively early, males emerge 8 days earlier than those at Toolik, maintaining a 12-day period between male and female emergence found at each site, but reducing the pre-emergence euthermic period that is critical for reproductive maturation. This sensitivity in timing of male emergence to female emergence will need to be matched by phase shifts in the circannual clock and responsiveness to environmental factors that time the end of heterothermy, if synchrony in reproductive readiness between the sexes is to be preserved in a rapidly changing climate.     

Plasma androgen and gonadotropin levels during hibernation and testicular maturation in golden-mantled ground squirrels

The 4-5-mo hibernation season of golden-mantled ground squirrels consists of extended torpor bouts interspersed with brief, periodic intervals of normothermic arousal. Plasma levels of testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) and degree of scrotal pigmentation were measured in torpid and aroused male ground squirrels throughout a season of hibernation and in active animals after the termination of torpor. T was basal in torpid animals; beginning 3 weeks before torpor ended, T was elevated in normothermic males during the first half of periodic arousals but returned to basal levels before animals reentered torpor. After the last (terminal) arousal from torpor, T levels were moderately elevated for 4 wk and maximal for the next 6 wk before they returned to basal values. LH patterns were similar to those of T; however, levels of T and LH were positively correlated only in aroused or posthibernation males. FSH levels remained constant and low during most of the heterothermic season but increased in several torpid males within 3 days of terminal arousal. FSH levels peaked 2 wk after the end of heterothermy. Scrotal pigmentation developed over the first 4 wk after terminal arousal. Maturation of reproductive function occurs during the 4 wk after termination of heterothermy, but elevated levels of T during arousals and variable levels of FSH in the last days of torpor suggest that activation or increased sensitivity of the hypothalamic-pituitary-gonadal axis is important in the termination of heterothermy in ground squirrels.     

Ecology of natural hibernation in the marsupial mountain pygmy-possum (Burramys parvus)

The hibernating marsupial mountain pygmy-possum (Burramys parvus, 40 g) has to raise its slow-growing offspring during a short alpine summer. Only females provide parental care, while after mating males emigrate to marginal habitats often at lower altitudes which can sustain only low possum densities. We predicted that the hibernation strategies in mountain pygmy-possums are distinct from those of similar-sized placental hibernators, because of the developmental constraints in marsupials and because hibernation differs between the sexes. Using temperature-sensitive radio transmitters, we studied the hibernation patterns of free-living male and female mountain pygmy-possums living in a north- and a south-facing boulder field (Kosciusko National Park) for two consecutive winters. Individual possums commenced hibernation several months before the snow season. As in other hibernators, torpor in the mountain pygmy-possum was interrupted by periodic arousals which occurred most often during the late afternoon. Torpor bouts initially lasted a few days when the hibernacula temperature (T _hib) ranged from 4 to 7°C. As the hibernation season progressed, torpor bouts became longer and possum body temperatures (T _b) approached 2°C. The T _bs of females were significantly lower and torpor bouts were longer in the second half of the hibernation season than in males. Between torpor bouts, both sexes were often active and left hibernacula for periods of up to 5 days. Especially during the first months of the hibernation season, possums also frequently changed hibernacula sites probably in an attempt to select a site with a more suitable microclimate. Emergence from hibernation was closely coupled with the disappearance of snow from the possum habitat (September 1995, October 1996) and the limited fat stores probably dictate an opportunistic spring emergence. However, in 1995, spring was early and males emerged significantly earlier than females. In 1996, when snow melt was delayed, this difference vanished. Testes are regressed in males during hibernation and the time needed for testes growth and spermatogenesis favours an earlier emergence for males which was probably achieved by their preference for the more sun exposed north-facing boulder field. A sexual dimorphism in hibernation strategies and spring emergence therefore enables mountain pygmy-possums to cope with their harsh alpine environment. Received: 22 May 1997 / Accepted: 21 August 1997     

Energy expenditure increases during the active season in the small,free-living hibernator Muscardinus avellanarius

Little is known about strategies employed by small mammals to reduce energy expenditure during the summer. To understand whether ambient conditions impact euthermic energy demands in a small free-living hibernator, we measured metabolic rate of hazel dormice (Muscardinus avellanarius) in the field. Furthermore, we aimed to reveal which variables influence torpor use. Our results show that hazel dormice altered euthermic energy expenditure during summer but not as expected as a response to environmental conditions. Euthermic resting metabolic rate was lowest directly after emergence from hibernation and increased by about 95% until the end of August. A considerable part of this increase was presumably caused by the changing influence of gender and rain on energy demands during different months, variation in food quality and quantity, and reversible size changes of organs that had been atrophied during hibernation. Torpor use in hazel dormice occurred more frequently when it was colder, earlier during the day, and in lighter individuals. Torpor was used routinely in males and non-reproductive females. We show that torpor is used more frequently than previously suggested by studies that only used visual proof of torpor use by surveying nest boxes.     

Hibernation and circadian rhythms of body temperature in free-living Arctic ground squirrels

In mammals, the circadian master clock generates daily rhythms of body temperature (T(b)) that act to entrain rhythms in peripheral circadian oscillators. The persistence and function of circadian rhythms during mammalian hibernation is contentious, and the factors that contribute to the reestablishment of rhythms after hibernation are unclear. We collected regular measures of core T(b) (every 34 min) and ambient light conditions (every 30 s) before, during, and following hibernation in free-living male arctic ground squirrels. Free-running circadian T(b) rhythms at euthermic levels of T(b) persisted for up to 10 d in constant darkness after animals became sequestered in their hibernacula in fall. During steady state torpor, T(b) was constant and arrhythmic for up to 13 d (within the 0.19°C resolution of loggers). In spring, males ended heterothermy but remained in their burrows at euthermic levels of T(b) for 22-26 d; patterns of T(b) were arrhythmic for the first 10 d of euthermia. One of four squirrels exhibited a significant free-running T(b) rhythm (τ = 22.1 h) before emergence; this squirrel had been briefly exposed to low-amplitude light before emergence. In all animals, diurnal T(b) rhythms were immediately reestablished coincident with emergence to the surface and the resumption of surface activity. Our results support the hypothesis that clock function is inhibited during hibernation and reactivated by exposure to light, although resumption of extended surface activity does not appear to be necessary to reinitiate T(b) cycles.     

Late-born intermittently fasted juvenile garden dormice use torpor to grow and fatten prior to hibernation: consequences for ageing processes

Torpor is thought to slow age-related processes and to sustain growth and fattening of young individuals. Energy allocation into these processes represents a challenge for juveniles, especially for those born late in the season. We tested the hypothesis that late-born juvenile garden dormice (Eliomys quercinus) fed ad libitum (‘AL’, n = 9) or intermittently fasted (‘IF’, n = 9) use short torpor bouts to enhance growth and fat accumulation to survive winter. IF juveniles displayed more frequent and longer torpor bouts, compared with AL individuals before hibernation. Torpor frequency correlated negatively with energy expenditure and water turnover. Hence, IF juveniles gained mass at the same rate, reached similar pre-hibernation fattening and displayed identical hibernating patterns and mass losses as AL animals. We found no group differences in relative telomere length (RTL), an indicator of ageing, during the period of highest summer mass gain, despite greater torpor use by IF juveniles. Percentage change in RTL was negatively associated with mean and total euthermic durations among all individuals during hibernation. We conclude that torpor use promotes fattening in late-born juvenile dormice prior to hibernation. Furthermore, we provided the first evidence for a functional link between time spent in euthermy and ageing processes over winter.     

The role of energy availability in Mammalian hibernation: an experimental test in free-ranging eastern chipmunks

Reduced torpor expression by hibernating mammals is often attributed to physiological constraints that limit their hibernation ability but may instead reflect adaptive, plastic responses to surplus energy availability. We evaluated this hypothesis by supplementing the food hoards of free-ranging eastern chipmunks (Tamias striatus) before hibernation and then documenting their use of torpor during the subsequent winter. In both years of study, chipmunks that received additional food were euthermic more than twice as frequently as nonsupplemented individuals. Furthermore, when food-supplemented individuals did express torpor, their minimum collar temperature was 5 degrees -10 degrees C warmer than nonsupplemented animals. These results indicate that reduced torpor expression by hibernators can result from an absence of energetic necessity rather than a lack of physiological capability and suggest that even endotherms sequestered in a hibernaculum may benefit from maintaining an elevated body temperature whenever possible.     

Sex differences in the use of daily torpor and foraging time by big brown bats (Eptesicus fuscus) during the reproductive season

Temperate zone bats can use daily torpor as a means of saving energy. Some argue, however, that torpor is costly for both males and females and that individuals should only use it during times of poor foraging conditions. Others hypothesize that the costs are greater for females and that males should enter torpor more regularly. We tested these alternative hypotheses by using temperature-sensitive radiotransmitters to monitor use of torpor and foraging by free-ranging big brown bats ( Eptesicus fuscus ). During the pregnancy period, males used torpor at night more and foraged less often than did females. Males also went into deep torpor more often and remained in torpor longer than did females. When they foraged, males and females were away from the roosts for equal periods of time. During the lactation period, males and females rarely failed to forage and foraging times were again no different between the sexes, although males may roost at night away from the maternity colonies. Males again used torpor and deep torpor more often and for longer than females did. These results support the hypothesis that the fitness costs of using torpor are lower for males than for reproductive females and that males regularly use torpor as an energy-saving mechanism. Females enter torpor only when foraging conditions are poor, presumably because torpor prolongs gestation and slows neonatal growth thereby leaving less time for females and their young to prepare for hibernation.     

Group hibernation does not reduce energetic costs of young yellow-bellied marmots

We investigated mechanisms of energy conservation during hibernation. The amount of time torpid was significantly less for groups of three young marmots than for marmots hibernating singly. Mean daily mass loss (DML; as mg d(-1) g(-1) immergence mass) averaged 1.33 for single marmots and 1.46 for grouped young. Animals were active 17.3% of the time, which used 82.4% of the energy, and were torpid 82.7% of the time, which used 17.6% of the energy expenditure. During longer torpor bouts, more time was spent in deep torpor, which decreased the hourly cost of a complete bout. Bout oxygen consumption V dot o2, percent time in deep torpor, and body temperature (T(B)) during deep torpor changed seasonally and were curvilinearly related to when in the hibernation period the measurements were made and probably represent a stage in the circannual metabolic cycle. The decrease of environmental temperature (T(E)) to 2 degrees C significantly increased metabolism. Potential costs of low T(E) were reduced by allowing T(B) to decrease, thereby reducing the T(B) to T(E) gradient. Average monthly metabolic rate was high early and late in the hibernation period when time spent euthermic was greater and when VO2 was higher. Over the hibernation period, energy saved averaged 77.1% and 88.0% of the costs for winter and summer euthermic metabolism, respectively. Hibernation costs were reduced by the seasonal changes, the high percentage of time in torpor, the rapid decline in V dot o2 following arousal, and allowing T(B) to decline at lower T(E). Asynchrony in the torpor cycles increased energy expenditures in group hibernators, which negated possible beneficial effects of group hibernation.     

Daily torpor and hibernation in birds and mammals

Many birds and mammals drastically reduce their energy expenditure during times of cold exposure, food shortage, or drought, by temporarily abandoning euthermia, i.e. the maintenance of high body temperatures. Traditionally, two different types of heterothermy, i.e. hypometabolic states associated with low body temperature (torpor), have been distinguished: daily torpor, which lasts less than 24 h and is accompanied by continued foraging, versus hibernation, with torpor bouts lasting consecutive days to several weeks in animals that usually do not forage but rely on energy stores, either food caches or body energy reserves. This classification of torpor types has been challenged, suggesting that these phenotypes may merely represent extremes in a continuum of traits. Here, we investigate whether variables of torpor in 214 species (43 birds and 171 mammals) form a continuum or a bimodal distribution. We use Gaussian‐mixture cluster analysis as well as phylogenetically informed regressions to quantitatively assess the distinction between hibernation and daily torpor and to evaluate the impact of body mass and geographical distribution of species on torpor traits. Cluster analysis clearly confirmed the classical distinction between daily torpor and hibernation. Overall, heterothermic endotherms tend to be small; hibernators are significantly heavier than daily heterotherms and also are distributed at higher average latitudes (～35°) than daily heterotherms (～25°). Variables of torpor for an average 30 g heterotherm differed significantly between daily heterotherms and hibernators. Average maximum torpor bout duration was >30‐fold longer, and mean torpor bout duration >25‐fold longer in hibernators. Mean minimum body temperature differed by ～13°C, and the mean minimum torpor metabolic rate was ～35% of the basal metabolic rate (BMR) in daily heterotherms but only 6% of BMR in hibernators. Consequently, our analysis strongly supports the view that hibernators and daily heterotherms are functionally distinct groups that probably have been subject to disruptive selection. Arguably, the primary physiological difference between daily torpor and hibernation, which leads to a variety of derived further distinct characteristics, is the temporal control of entry into and arousal from torpor, which is governed by the circadian clock in daily heterotherms, but apparently not in hibernators.     

Cytoskeletal Regulation Dominates Temperature-Sensitive Proteomic Changes of Hibernation in Forebrain of 13-Lined Ground Squirrels

13-lined ground squirrels, Ictidomys tridecemlineatus, are obligate hibernators that transition annually between summer homeothermy and winter heterothermy – wherein they exploit episodic torpor bouts. Despite cerebral ischemia during torpor and rapid reperfusion during arousal, hibernator brains resist damage and the animals emerge neurologically intact each spring. We hypothesized that protein changes in the brain underlie winter neuroprotection. To identify candidate proteins, we applied a sensitive 2D gel electrophoresis method to quantify protein differences among forebrain extracts prepared from ground squirrels in two summer, four winter and fall transition states. Proteins that differed among groups were identified using LC-MS/MS. Only 84 protein spots varied significantly among the defined states of hibernation. Protein changes in the forebrain proteome fell largely into two reciprocal patterns with a strong body temperature dependence. The importance of body temperature was tested in animals from the fall; these fall animals use torpor sporadically with body temperatures mirroring ambient temperatures between 4 and 21°C as they navigate the transition between summer homeothermy and winter heterothermy. Unlike cold-torpid fall ground squirrels, warm-torpid individuals strongly resembled the homeotherms, indicating that the changes observed in torpid hibernators are defined by body temperature, not torpor per se. Metabolic enzymes were largely unchanged despite varied metabolic activity across annual and torpor-arousal cycles. Instead, the majority of the observed changes were cytoskeletal proteins and their regulators. While cytoskeletal structural proteins tended to differ seasonally, i.e., between summer homeothermy and winter heterothermy, their regulatory proteins were more strongly affected by body temperature. Changes in the abundance of various isoforms of the microtubule assembly and disassembly regulatory proteins dihydropyrimidinase-related protein and stathmin suggested mechanisms for rapid cytoskeletal reorganization on return to euthermy during torpor-arousal cycles.     

Hibernation in the mountain pygmy possum Burramys parvus (Marsupialia)

Mountain pygmy possums Burramys parvus (40 g) disappear from their Mt. Kosciusko boulder fields from May to October/November and it is assumed that they hibernate during this time. However, laboratory studies did not observe the characteristic hibernation pattern of placentals, which, throughout the hibernation season, show long bouts of torpor (several days to weeks) that are interrupted by short (< 1 day) normothermic periods. We investigated the pattern of hibernation in juvenile (N = 8) and adult (N = 8) male and female B. parvus in the laboratory at an air temperature that was similar to that in the field during winter. Adults commenced hibernation earlier and hibernated longer (about seven months) than juveniles (about five months). All adult individuals hibernated whereas only six of the eight juveniles did so. Hibernating animals showed distinct seasonal changes in the duration of torpor bouts. Torpor bouts were short (about five days) at the beginning, long (12–20 days) during the middle, and short again at the end of the hibernation season. Normothermic periods were usually shorter than one day. The pattern of the seasonal change of torpor bout duration differed between juveniles and adults and between sexes. Body temperature during mid-hibernation was regulated at about 2 ° c in females and 3 ° C in males and the metabolic rate was similar to that of hibernating placentals.     

Heterothermy in Afrotropical mammals and birds: a review

Recent years have seen a rapid increase in the number of Afrotropical endotherms known to avoid mismatches between energy supply and demand by using daily torpor and/or hibernation. Among mammals, heterothermy has been reported in 40 species in six orders, namely Macroscelidea, Afrosoricida, Rodentia, Eulipotyphla, Primates and Chiroptera. These species span a range in body mass of 7-770?g, with minimum heterothermic body temperatures ranging from 1-27°C and bout length varying from 1?h to 70 days. Daily torpor is the most common form of heterothermy, with true hibernation being observed in only seven species, Graphiurus murinus, Graphiurus ocularis, Atelerix frontalis, Cheirogaleus medius, Cheirogaleus major, Microcebus murinus and Microcebus griseorufus. The traditional distinction between daily torpor and hibernation is blurred in some species, with free-ranging individuals exhibiting bouts of > 24?h and body temperatures < 16 °C, but none of the classical behaviours associated with hibernation. Several species bask in the sun during rewarming. Among birds, heterothermy has been reported in 16 species in seven orders, and is more pronounced in phylogenetically older taxa. Both in mammals and birds, patterns of heterothermy can vary dramatically among species occurring at a particular site, and even among individuals of a single species. For instance, patterns of heterothermy among cheirogalid primates in western Madagascar vary from daily torpor to uninterrupted hibernation for up to seven months. Other examples of variation among closely-related species involve small owls, elephant shrews and vespertilionid bats. There may also be variation in terms of the ecological correlates of torpor within a species, as is the case in the Freckled Nightjar Caprimulgus tristigma.     

Extreme plasticity in thermoregulatory behaviors of free-ranging black-tailed prairie dogs

In the natural environment, hibernating sciurids generally remain dormant during winter and enter numerous deep torpor bouts from the time of first immergence in fall until emergence in spring. In contrast, black-tailed prairie dogs (Cynomys ludovicianus) remain active throughout winter but periodically enter short and shallow bouts of torpor. While investigating body temperature (T(b)) patterns of black-tailed prairie dogs from six separate colonies in northern Colorado, we observed one population that displayed torpor patterns resembling those commonly seen in hibernators. Five individuals in this population experienced multiple torpor bouts in immediate succession that increased in length and depth as winter progressed, whereas 16 prairie dogs in five neighboring colonies remained euthermic for the majority of winter and entered shallow bouts of torpor infrequently. Our results suggest that these differences in torpor patterns did not result from differences in the physiological indicators that we measured because the prairie dogs monitored had similar body masses and concentrations of stored lipids across seasons. Likewise, our results did not support the idea that differences in overwinter T(b) patterns between prairie dogs in colonies with differing torpor patterns resulted from genetic differences between populations; genetic analyses of prairie dog colonies revealed high genetic similarity between the populations and implied that individuals regularly disperse between colonies. Local environmental conditions probably played a role in the unusual T(b) patterns experienced by prairie dogs in the colony where hibernation-like patterns were observed; this population received significantly less rainfall than neighboring colonies during the summer growing seasons before, during, and after the year of the winter in which they hibernated. Our study provides a rare example of extreme plasticity in thermoregulatory behaviors of free-ranging prairie dogs and provides evidence contrary to models that propose a clear delineation between homeothermy, facultative torpor, and hibernation.     

The timing of hibernation in Tasmanian echidnas: why do they do it when they do?

We investigated the patterns of hibernation and arousals in seven free-ranging echidnas Tachyglossus aculeatus setosus (two male, five female) in Tasmania using implanted temperature data loggers. All echidnas showed a ‘classical’ pattern of mammalian hibernation, with bouts of deep torpor interrupted by periodic arousals to euthermia (mean duration 1.04±0.05 (n=146). Torpor bout length increased as body temperature fell during the hibernation season, and became more variable as temperature rose again. Hibernation started in late summer (February 28±5 days, n=6) and males aroused just before the winter solstice (June 15±3 days, n=3), females that subsequently produced young aroused 40 days later (July 25±3, n=4) while females that did not produce young hibernated for a further two months (arousal Sept 27±5, n=7). We suggest that hibernation in Tasmanian echidnas can be divided into two phases, the first phase, marked by declining minimum body temperatures as ambient temperature falls, appears to be obligatory for all animals, while the second phase is ‘optional’ and is utilised to varying amounts by females. We suggest that early arousal and breeding is the favoured option for females in good condition, and that the ability to completely omit breeding in some years, and hibernate through to spring is an adaptation to an uncertain climate.