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Abstract

The species of interest in this chapter, the Djungarian dwarf hamster (Phodopus sungorus), is sometimes also called the hairy-footed hamster. This name, which refers to fur covering even the soles of the feet in this species, indicates an adaptation to extremely harsh climatic conditions in its natural habitat, the subarid steppes of continental Asia. Among these steppes, the Djungarian (or
... However, later studies on long-term energy budgets revealed that torpor bouts can have a much higher energy saving potential. Ruf and Heldmaier (2000) calculated that a torpor bout during the day also saves a considerable amount of energy during the night because of a reduced need to forage for food ). ...
... Although exposure to SP is sufficient to induce acclimation processes including SDT expression in Djungarian hamsters, it has been shown that energetically challenging ambient conditions can additionally facilitate SDT expression. Both a natural and artificial lowering of T a led to an advanced onset of the torpor season, a higher proportion of hamsters showing torpor, a higher individual torpor frequency as well as to deeper and longer torpor bouts compared to control hamsters held at thermoneutrality Ruf and Heldmaier 2000, 1991. Despite the strong impact of T a on torpor expression, it is not possible to induce torpor under long photoperiod (LP) solely by lowering T a (Ruf 1991), demonstrating that this ambient variable merely acts as modulating factor, rather than as factor of ultimate or proximate torpor induction. ...
... In contrast, smaller endotherms such as the Djungarian hamster are not able to accumulate a sufficient amount of internal energy stores to survive the winter in deep hibernation (Heldmaier 1989). Consequently, they need to forage for food during their activity phase and use spontaneous daily torpor (SDT) as a shorter and shallower form of reduced metabolism and body temperature to save energy during their daily resting phase (Ruf and Heldmaier 2000). Although SDT occurs under thermoneutral conditions and with abundant food (Heldmaier and Steinlechner 1981), energetic challenges like low ambient temperatures or reduced food availability have been shown to facilitate torpor expression ). ...
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Mammals that inhabit temperate and arctic latitudes are confronted with considerable seasonal fluctuations in ambient temperature and food availability. During winter, low ambient temperatures facilitate heat loss via the body surface of the animals and cause an increased energy demand. This energetic challenge has led to the development of a large variety of seasonal acclimatizations that predominantly reduce energy expenditure. One important acclimatization trait is a controlled temporal reduction of metabolic rate and body temperature, which occurs in two basic forms: daily torpor and hibernation. Daily torpor represents a rather moderate reduction in metabolic rate and body temperature, which lasts for several hours and is usually restricted to the animals’ resting phase. The Djungarian hamster (Phodopus sungorus) has long been a model organism for investigating seasonal changes and daily torpor. Interestingly, this species exhibits seasonal spontaneous daily torpor (SDT) induced by exposure to winter-like short photoperiod, but also fasting-induced torpor (FIT) in response to prolonged food restriction. While FIT has to be considered as an acute response to an energetic challenge, SDT has been shown to occur despite abundance of food. In the present study, we used predominantly whole-animal-physiology to evaluate SDT as energy saving mechanism a) in comparison to FIT and b) in response to environmental factors that modulate the hamsters’ energy balance. The results of our comparative study in combination with a comprehensive literature review show that SDT expressing hamsters are in energetic balance, even under environmental energetic challenges. In contrast, FIT occurs independently of any energy-saving seasonal acclimatization, but appears to serve as last resort when body fat is severely depleted. However, we could also observe a certain degree of acclimation in FIT expressing hamsters, as they showed an increased mucosal glucose absorption capacity, which tended to be negatively correlated with FIT frequency. Interestingly, food restriction during SDT expression forced some hamsters to show characteristics of FIT expression, which again allows for the differentiation between SDT as part of a seasonal long-term “energy budget” and FIT as acute “emergency shut down”. As not only food quantity, but also food quality influences SDT expression, another experiment investigated the effect of unsaturated fatty acids on SDT expression. Unsaturated fatty acids have been shown to increase energy saving efficiency during hibernation, which was mainly attributed to their positive effect on membrane and tissue functionality at low Tb. In contrast to these studies, neither a high ratio of poly- to monounsaturated fatty acids, nor a high ratio of n-6 to n-3 polyunsaturated fatty acids had a considerable facilitating effect on SDT expression and thus SDT energy saving efficiency. In contrast to earlier experiments under constantly lowered ambient temperature, a semi-natural daily temperature cycle with cold nights and warmer days did not facilitate SDT expression in adult hamsters and even decreased SDT expression in juvenile hamsters. As the juveniles showed a high proportion of unexpectedly short SDT bouts, we assume that the daily increase in ambient temperature interrupted SDT expression. In contrast, adult hamsters appeared to be more resistant to the disturbing external stimuli. In all experiments, we observed the already described high individual variability of SDT expression within the different treatment groups. Although energetic challenges facilitate SDT expression in general, the individual relative contribution of SDT to the overall energy saving potential of seasonal acclimatization differs dramatically. All results underline the theory of a predetermined endogenous SDT proneness. Although the reasons for the high variability in SDT expression in Djungarian hamsters remain unknown, we can at least say that this enigma can only be solved when regarding SDT as just one, albeit important facet of seasonal acclimatization.
... The 127 torpor bouts analyzed had their onset at ZT1.6 ± 1.4 h. It is important to notice that the reduction of the metabolic rate below the resting metabolic rate during torpor entrance precedes the reduction of body temperature below 32 • C by almost an hour (Ruf and Heldmaier, 2000;Heldmaier et al., 2004). Consequently, all torpor onsets determined by body temperature measurements before ZT01 already occurred before the end of the scotophase and thus without light as a proximate induction stimulus. ...
... Djungarian hamsters should be more active in LP than in SP since they expect mating and the intense care for their litters. The resulting high energy demand requires a high foraging activity, which can be reduced during the early stages of SP adaptation as a function of decreasing food intake, body mass, and reproductive activity (Ruf and Heldmaier, 2000). Indeed, a high activity and body temperature in LP and lower values when SP-adapted have been reported (Hamann, 1987;Prendergast et al., 2013) and were confirmed in this study. ...
Article
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To survive the Siberian winter, Djungarian hamsters (Phodopus sungorus) adjust their behavior, morphology, and physiology to maintain energy balance. The reduction of body mass and the improvement of fur insulation are followed by the expression of spontaneous daily torpor, a state of reduced metabolism during the resting phase to save additional energy. Since these complex changes require time, the upcoming winter is anticipated via decreasing photoperiod. Yet, the extent of adaptation and torpor use is highly individual. In this study, adaptation was triggered by an artificially changed light regime under laboratory conditions with 20°C ambient temperature and food and water ad libitum. Two approaches analyzed data on weekly measured body mass and fur index as well as continuously recorded core body temperature and activity during: (1) the torpor period of 60 hamsters and (2) the entire adaptation period of 11 hamsters, aiming to identify parameters allowing (1) a better prediction of torpor expression in individuals during the torpor period as well as (2) an early estimation of the adaptation extent and torpor proneness. In approach 1, 46 torpor-expressing hamsters had a median torpor incidence of 0.3, covering the spectrum from no torpor to torpor every day within one representative week. Torpor use reduced the body temperature during both photo- and scotophase. Torpor was never expressed by 14 hamsters. They could be identified by a high, constant body temperature during the torpor period and a low body mass loss during adaptation to a short photoperiod. Already in the first week of short photoperiod, approach 2 revealed that the hamsters extended their activity over the prolonged scotophase, yet with reduced scotophase activity and body temperature. Over the entire adaptation period, scotophase activity and body temperature of the scoto- and photophases were further reduced, later accompanied by a body mass decline and winter fur development. Torpor was expressed by those hamsters with the most pronounced adaptations. These results provide insights into the preconditions and proximate stimuli of torpor expression. This knowledge will improve experimental planning and sampling for neuroendocrine and molecular research on torpor regulation and has the potential to facilitate acute torpor forecasting to eventually unravel torpor regulation processes.
... It may seem that this association merely reflects the fact that dormice may use torpor to compensate for low foraging duration and hence reduced food intake, induced by low T a (Bright et al., 1996). However, daily heterotherms may have low rates of activity during periods of high torpor use even when food is abundant (Ruf et al., 1991;Ruf and Heldmaier, 2000). Arguably, this is because frequent torpor lowers total daily energy requirements, which in turn lowers the need for foraging. ...
... Arguably, this is because frequent torpor lowers total daily energy requirements, which in turn lowers the need for foraging. It is this complementary reinforcement which explains that energy savings via torpor and the associated reduction in activity are much higher than to be expected from hypometabolism in torpor alone (Ruf et al., 1991;Ruf and Heldmaier, 2000). ...
Article
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The edible dormouse (Glis glis, formerly Myoxus glis) is a small arboreal mammal inhabiting deciduous forests in Europe. This rodent shows behavioral and physiological adaptations to three types of environmental fluctuations: (i) predictable seasonal variation in climate and food resources (ii) unpredictable year-to-year fluctuation in seed-production by trees and (iii) day-to-day variation in ambient temperature and precipitation. They cope with seasonally fluctuating conditions by seasonal fattening and hibernation. Dormice have adjusted to tree-mast fluctuations, i.e., pulsed resources, by sensing future seed availability in spring, and restricting reproduction to years with at least some seed production by beech and oak trees, which are a crucial food-resource for fast-growing juveniles in fall. Finally, dormice respond to short-term drops in ambient temperature by increased use of daily torpor as well as by huddling in groups of up to 24 conspecifics. These responses to environmental fluctuations strongly interact with each other: Dormice are much more prone to using daily torpor and huddling in non-reproductive years, because active gonads can counteract torpor and energy requirements for reproduction may prevent the sharing of food resources associated with huddling. Accordingly, foraging activity in fall is much more intense in reproductive mast years. Also, depending on their energy reserves, dormice may retreat to underground burrows in the summers of non-reproductive years, causing an extension of the hibernation season to up to 11.4 months. In addition to these interactions, responses to environmental fluctuations are modulated by the progression of life-history stages. With increasing age and diminishing chances of future reproduction, females reproduce with increasing frequency even under suboptimal environmental conditions. Simultaneously, older dormice shorten the hibernation season and phase-advance the emergence from hibernation in spring, apparently to occupy good breeding territories early, despite increased predation risk above ground. All of the above adaptions, i.e., huddling, torpor, hibernation, and reproduction skipping do not merely optimize energy-budgets but also help to balance individual predation risk against reproductive success, which adds another layer of complexity to the ability to make flexible adjustments in this species.
... Possibly, torpor is avoided during this time because high rates of digestion of food and fat deposition require high body temperatures. It has been suggested that the use of torpor generally has a negative impact on activity because the reduction of energy expenditure during torpor allows animals to minimise foraging (Ruf and Heldmaier 2000;Turbill and Stojanovski 2018). When reproductive activity prevents torpor, but also in non-reproductive years, dormice use an alternative avenue of energy savings: they utilise huddling and communal nesting of up to 16 animals at a time (Ruf and Bieber 2020b). ...
Article
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We address the question of ultimate selective advantages of hibernation. Biologists generally seem to accept the notion that multiday torpor is primarily a response to adverse environmental conditions, namely cold climate and low food abundance. We closely examine hibernation, and its summer equivalent estivation, in the edible dormouse, Glis glis. We conclude that in this species, hibernation is not primarily driven by poor conditions. Dormice enter torpor with fat reserves in years that are unfavourable for reproduction but provide ample food supply for animals to sustain themselves and even gain body energy reserves. While staying in hibernacula below ground, hibernators have much higher chances of survival than during the active season. We think that dormice enter prolonged torpor predominantly to avoid predation, mainly nocturnal owls. Because estivation in summer is immediately followed by hibernation, this strategy requires a good body condition in terms of fat reserves. As dormice age, they encounter fewer occasions to reproduce when calorie-rich seeds are available late in the year, and phase advance the hibernation season. By early emergence from hibernation, the best territories can be occupied and the number of mates maximised. However, this advantage comes at the cost of increased predation pressure that is maximal in spring. We argue the predator avoidance is generally one of the primary reasons for hibernation, as increased perceived predation pressure leads to an enhanced torpor use. The edible dormouse may be just an example where this behaviour becomes most obvious, on the population level and across large areas.
... The hamsters reduce their body size when exposed to a winter-like short photoperiod to decrease their overall energy demand (Ruf and Heldmaier, 1992). However, they need to forage for food during their activity phase and use spontaneous daily torpor to save energy during their daily resting phase (Ruf and Heldmaier, 2000). Although low ambient temperatures or reduced food availability have been shown to facilitate torpor expression (Ruf et al., 1993), under short photoperiod, spontaneous daily torpor also occurs without acute energetic challenges (Heldmaier and Steinlechner, 1981). ...
Article
Small mammals exhibit seasonal changes in intestinal morphology and function via increased intestine size and resorptive surface and/or nutrient transport capacity to increase energy yield from food during winter. This study investigated whether seasonal or acute acclimation to anticipated or actual energetic challenges in Djungarian hamsters also resulted in higher nutrient resorption capacities due to changes in small intestine histology and physiology. The hamsters show numerous seasonal energy saving adjustments in response to short photoperiod. As spontaneous daily torpor represents one of these adjustments related to food quality and quantity, it was hypothesized that the hamsters’ variable torpor expression patterns are influenced by their individual nutrient uptake capacity. Hamsters under short photoperiod showed longer small intestines and higher mucosal electrogenic transport capacities for glucose relative to body mass. Similar observations were made in hamsters under long photoperiod and food restriction. However, this acute energetic challenge caused a stronger increase of glucose transport capacity. Apart from that, neither fasting-induced torpor in food-restricted hamsters nor spontaneous daily torpor in short photoperiod-exposed hamsters clearly correlated with mucosal glucose transport capacity. Both seasonally anticipated and acute energetic challenges caused adjustments in the hamsters’ small intestine. Short photoperiod appeared to induce an integration of these and other acclimation processes in relation to body mass to achieve a long-term adjustment of energy balance. Food restriction seemed to result in a more flexible, short-term strategy of maximizing energy uptake possibly via mucosal glucose transport and reducing energy consumption via torpor expression as emergency response.
... Furthermore, by reducing the proportion of the daily energy budget allocated to maintenance, torpor might also be used to reduce the indirect negative effects of predation risk on rates of growth and reproduction [4]. Torpor use has been suggested to permit an increased allocation of the energy budget to costs of foraging [29,30], fattening prior to migration [31], growth [32] and reproductive output [33,34]. In the current study, we aimed to test whether torpor permits mice to exhibit stronger behavioural responses to perceived predation risk, and hence, more generally, how the metabolic physiology of small endotherms integrates with their behavioural ecology. ...
Article
Foraging activity is needed for energy intake but increases the risk of predation, and antipredator behavioural responses, such as reduced activity, generally reduce energy intake. Hence, the mortality and indirect effects of predation risk are dependent on the energy requirements of prey. Torpor, a controlled reduction in resting metabolism and body temperature, is a common energy-saving mechanism of small mammals that enhances their resistance to starvation. Here we test the hypothesis that torpor could also reduce predation risk by compensating for the energetic cost of antipredator behaviours. We measured the foraging behaviour and body temperature of house mice in response to manipulation of perceived predation risk by adjusting levels of ground cover and starvation risk by 24 h food withdrawal every third day. We found that a voluntary reduction in daily food intake in response to lower cover (high predation risk) was matched by the extent of a daily reduction in body temperature. Our study provides the first experimental evidence of a close link between energy-saving torpor responses to starvation risk and behavioural responses to perceived predation risk. By reducing the risk of starvation, torpor can facilitate stronger antipredator behaviours. These results highlight the interplay between the capacity for reducing metabolic energy expenditure, optimal decisions about foraging behaviour and the life-history ecology of prey. © 2018 The Author(s) Published by the Royal Society. All rights reserved.
... However, increasing activity may lead to resource exhaustion (Russell et al., 1987), and it increases the risk of predation (Lima and Dill, 1990); nevertheless, it brings about an opportunity to find food (Overton and Williams, 2004;Sakurada et al., 2000). Torpor, in turn, is a state of regulated decrease of T b and metabolic rate (MR) (Heldmaier and Ruf, 1992;Ruf and Geiser, 2015;Snyder and Nestler, 1990), which brings about benefits when food is unavailable or when costs of foraging are too high (Hudson and Scott, 1979;Ruf and Heldmaier, 2000;Schubert et al., 2010; but see Humphries et al., 2003 andWojciechowski et al., 2011 for a discussion of increased predation risk associated with torpor). In recent decades, several studies have focused on torpor use as a response to energy deficit (Bae et al., 2003;Gutman et al., 2006;Nespolo et al., 2010;Schubert et al., 2010Schubert et al., , 2008. ...
Article
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According to theoretical predictions endothermic homeotherms can be classified as either thermal specialists and thermal generalists. In high cost environments thermal specialists are supposed to be more prone to use facultative heterothermy than generalists. We tested this hypothesis on the intraspecific level using laboratory male mice (C57BL/cmdb) fasted under different thermal conditions (20 and 10°C), and for different time periods (12-48 h). We predicted that variability of body temperature (Tb) and time spent with Tb below normothermy increase with the increase of environmental demands (duration of fasting and cold). To verify the above prediction, we measured Tb and energy expenditure of fasted mice. We did not record torpor bouts but we found that variations in Tb and time spent in hypothermia increased with environmental demands. In response to fasting, mice also decreased their energy expenditure. Moreover, we found that animals that showed more precise thermoregulation when fed, had more variable Tb when fasted. We postulate that the prediction of the thermoregulatory generalist-specialist trade-off can be applied on the intraspecific level, offering a valid tool to seek for mechanistic explanations of the differences in animal responses to variations in energy supply.
... Whereas they are active during the day in semi-natural field enclosures, all individuals are nocturnal under laboratory conditions, with or without access to runningwheels [8][9][10] suggesting that a fundamental feature of their natural environment is not reproduced in the laboratory. Both ecological and physiological studies indicate the critical role of daily energy balance in constraining the timing of activity, which is primarily determined by the circadian clock [7,[11][12][13][14][15][16][17]. Thus, differences in energy demand between field and laboratory conditions could be the fundamental feature ultimately leading to inversion in the timing of daily activity [7,11,17]. ...
Article
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Several rodent species that are diurnal in the field become nocturnal in the lab. It has been suggested that the use of running-wheels in the lab might contribute to this timing switch. This proposition is based on studies that indicate feed-back of vigorous wheel-running on the period and phase of circadian clocks that time daily activity rhythms. Tuco-tucos (Ctenomys aff. knighti) are subterranean rodents that are diurnal in the field but are robustly nocturnal in laboratory, with or without access to running wheels. We assessed their energy metabolism by continuously and simultaneously monitoring rates of oxygen consumption, body temperature, general motor and wheel running activity for several days in the presence and absence of wheels. Surprisingly, some individuals spontaneously suppressed running-wheel activity and switched to diurnality in the respirometry chamber, whereas the remaining animals continued to be nocturnal even after wheel removal. This is the first report of timing switches that occur with spontaneous wheel-running suppression and which are not replicated by removal of the wheel
... We studied Siberian hamsters because they inhabit environments that undergo extreme seasonal changes during which they are routinely exposed to thermal stress and dramatic shifts in food resources. During winter, invertebrate prey items are not available and Siberian hamsters must subsist on the seeds of shrubs and grass (Ruf and Heldmaier, 2000) that can be low in protein and/or lipid content. Although Siberian hamsters do not hibernate, they use several potentially adaptive strategies to survive winter. ...
Article
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During acclimatization to winter, changes in morphology and physiology combined with changes in diet may affect how animals use the nutrients they ingest. To study (a) how thermal acclimation and (b) nutritional history affect the rates at which Siberian hamsters (Phodopus sungorus) oxidize different classes of dietary nutrients, we conducted two trials in which we fed hamsters one of three 13C-labeled compounds, that is, glucose, leucine, or palmitic acid. We predicted that under acute cold stress (3 hr at 2°C) hamsters previously acclimated to cold temperatures (10°C) for 3 weeks would have higher resting metabolic rate (RMR) and would oxidize a greater proportion of dietary fatty acids than animals acclimated to 21°C. We also investigated how chronic nutritional stress affects how hamsters use dietary nutrients. To examine this, hamsters were fed four different diets (control, low protein, low lipid, and low-glycemic index) for 2 weeks. During cold challenges, hamsters previously acclimated to cold exhibited higher thermal conductance and RMR, and also oxidized more exogenous palmitic acid during the postprandial phase than animals acclimated to 21°C. In the nutritional stress trial, hamsters fed the low protein diet oxidized more exogenous glucose, but not more exogenous palmitic acid than the control group. The use of 13C-labeled metabolic tracers combined with breath testing demonstrated that both thermal and nutritional history results in significant changes in the extent to which animals oxidize dietary nutrients during the postprandial period. J. Exp. Zool. 9999A: XX–XX, 2014. © 2014 Wiley Periodicals, Inc.
Article
In addition to morphological and physiological traits of short-day acclimatization, Djungarian hamsters (Phodopus sungorus) from Central Asia exhibit spontaneous daily torpor to decrease energy demands during winter. Environmental factors such as food scarcity and low temperatures have been shown to facilitate the use of this temporal reduction in metabolism and body temperature. We investigated the effect of a daily cycle in ambient temperature on short-day acclimation and torpor expression in juvenile and adult Djungarian hamsters. The animals were exposed to a cold dark phase (6 °C) and a warmer light phase (18 °C) and were compared with control hamsters kept at a constant ambient temperature of 18 °C. Under constant conditions, torpor expression did not differ between adult and juvenile hamsters. Although the daily temperature cycle evoked an increased metabolic rate in adult and juvenile hamsters during the dark phase and strengthened the synchronization between torpor entrance and the beginning of the light phase, it did not induce the expected torpor facilitation. In adult hamsters, torpor expression profiles did not differ from those under constant conditions at all. In contrast, juvenile hamsters showed a delayed onset of torpor season, a decreased torpor frequency, depth and duration, as well as an increased number of early torpor terminations coinciding with the rise in ambient temperature after the beginning of the light phase. While the temperature challenge appeared to be of minor importance for energy balance and torpor expression in adult hamsters, it profoundly influenced the overall energy saving strategy of juvenile hamsters, promoting torpor-alleviating active foragers over torpor-prone energy-savers. In addition, our data suggest a more efficient acclimation in juvenile hamsters under additional energy challenges, which reduces the need for torpor expression.
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Continuous records of body temperature ( $T_{b}$ ) and oxygen consumption were used to investigate the influence of daily torpor on energy requirements in Djungarian hamsters (Phodopus sungorus) exposed to ambient temperatures ( $T_{a}'s$ ) between O° C and 20° C During torpor $T_{b}$ was lowered to values between 14.0 °C and 31.3°C for variable periods (0.3-9.2 h; mean, 5.4 ± 0.23 h). Average daily metabolic rates (ADMRs) were inversely related to $T_{a}$ in both hamsters exhibiting torpor and animals staying normothermic and can be predicted from their linear relation to mean daily ( $T_{a}-T_{b}$ )-gradients. The torpor-induced reduction of ADMRs below normothermic values was directly related to torpor depth and duration, withprolonged torpor episodes (>4 h) causing a 20.2% ± 0.9% saving in energy. Thus, daily torpor reduces metabolic costs to a level approaching the resting metabolic rate of normothermic hamsters (24% below ADMR) and helps compensate for the energetic costs of locomotor activity. Individuals with a high tendency to enter torpor also save substantial amounts of energy by lowering average metabolic rates during the normothermic phase of the day. Calculations based on these data show that daily torpor in Djungarian hamsters may serve to allow the continuation of foraging at low $T_{a}'s$ during winter in the Siberian steppe.
Article
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In Phodopus sungorus spontaneous shallow daily torpor occurred only during winter. Frequency of torpor was not affected by low ambient temperature but the seasonal cueing seems primarily dependent on photoperiodic control. Maximum torpor frequency was found in January with 30% of all hamsters living inside or outside being torpid at a time. It is calculated that torpor will reduce long term energy requirements of Phodopus by only 5%. Therefore it is concluded that torpor is not primarily aimed to reduce energy requirements but to guarantee survival of a fraction of a population during short periods of extreme cold load or inaccessability of food.
Chapter
The significance of biological rhythms can be discussed under at least two aspects. They serve, on the one hand, to attain an optimal temporal arrangement of animal behaviour within the cycles of the environment, as in the four “circa-clocks” (Aschoff 1981). On the other hand, this external adaptation results in internal temporal order which in itself may have selective value. In addition, there are many rhythmic processes within the organism, not related to any environmental periodicity, which in various ways contribute to the maintenance of functional integrity of the internal milieu (Aschoff and Wever 1961). In focussing on how circadian rhythms contribute to survival, we do well to consider them, first, as part of a spectrum of rhythms and to evaluate their possible intrinsic function regardless of the environmental day-night cycle. We then will proceed to a discussion of possible benefits to be derived from the adjustment to the periodic environment.
Article
1.1. Tb in lab-born Peromyscus maniculatus and P. leucopus was monitored via surgically implanted transmitters during exposure to 10°C and 9L:15D.2.2. With food available ad lib, no significant interspecific differences in the incidence, frequency, and patterns of spontaneous daily torpor were found.3.3. Restriction of food to 75% daily ration increased the incidence of torpor in both species, and induced longer, deeper torpor bouts.4.4. A significantly greater proportion of P. maniculatus responded to rationing by becoming torpid. Furthermore, induced torpor boults in this species were more profound than those in P. leucopus.5.5. This differential response is consistent with differences in these species respective environments.
Article
Physiological limits to energy budgets were estimated in Djungarian hamsters (Phodopus sungorus) using food balance and respirometric methods. The summer acclimatized, reproductively inactive hamsters could balance their energy budget at-2 C, assimilating 91.1 kJanimal-1 day-1 after gradual cold acclimation, whereas non-acclimated hamsters showed negative energy balance assimilating only 54.4 kJanimal-1day-1. At the same ambient temperature, multiparous females (although neither pregnant nor lactating at the time) maintained positive energy balance assimilating 81.6 kJanimal-1day-1. Hamsters are capable of rapid adjustments of their maximum assimilation rates to meet their current energy demands, but only up to the value of about 3.5xBMR. It is concluded, that the actual energy budgets of small mammals keep, all the time, fairly near the upper physiological limit, with body reserves ready to buffer short-term oscillations.