Basking hamsters reduce resting metabolism, body temperature and energy costs during rewarming from torpor

Atypical for northern ungulates, energy metabolism is lowest during summer in female wild boars (Sus scrofa)

Typically, large ungulates show a single seasonal peak of heart rate, a proxy of energy expenditure, in early summer. Different to other large ungulates, wild boar females had peak heart rates early in the year (at ~ April, 1), which likely indicates high costs of reproduction. This peak was followed by a trough over summer and a secondary summit in autumn/early winter, which coincided with the mast seeding of oak trees and the mating season. Wild boars counteracted the effects of cold temperatures by decreasing subcutaneous body temperature by peripheral vasoconstriction. They also passively gained solar radiation energy by basking in the sun. However, the shape of the seasonal rhythm in HR indicates that it was apparently not primarily caused by thermoregulatory costs but by the costs of reproduction. Wild boar farrow early in the year, visible in high HRs and sudden changes in intraperitoneal body temperature of females. Arguably, a prerequisite for this early reproduction as well as for high energy metabolism over winter is the broad variety of food consumed by this species, i.e., the omnivorous lifestyle. Extremely warm and dry summers, as experienced during the study years (2017, 2018), may increasingly become a bottleneck for food intake of wild boar.

Surviving winter on the Qinghai-Tibetan Plateau: Pikas suppress energy demands and exploit yak feces to survive winter

The Qinghai-Tibetan Plateau, with low precipitation, low oxygen partial pressure, and temperatures routinely dropping below -30 °C in winter, presents several physiological challenges to its fauna. Yet it is home to many endemic mammalian species, including the plateau pika (Ochotona curzoniae). How these small animals that are incapable of hibernation survive the winter is an enigma. Measurements of daily energy expenditure (DEE) using the doubly labeled water method show that pikas suppress their DEE during winter. At the same body weight, pikas in winter expend 29.7% less than in summer, despite ambient temperatures being approximately 25 °C lower. Combined with resting metabolic rates (RMRs), this gives them an exceptionally low metabolic scope in winter (DEE/RMRt = 1.60 ± 0.30; RMRt is resting metabolic rate at thermoneutrality). Using implanted body temperature loggers and filming in the wild, we show that this is achieved by reducing body temperature and physical activity. Thyroid hormone (T₃ and T₄) measurements indicate this metabolic suppression is probably mediated via the thyroid axis. Winter activity was lower at sites where domestic yak (Bos grunniens) densities were higher. Pikas supplement their food intake at these sites by eating yak feces, demonstrated by direct observation, identification of yak DNA in pika stomach contents, and greater convergence in the yak/pika microbiotas in winter. This interspecific coprophagy allows pikas to thrive where yak are abundant and partially explains why pika densities are higher where domestic yak, their supposed direct competitors for food, are more abundant.

Huddling Conserves Energy, Decreases Core Body Temperature, but Increases Activity in Brandt's Voles (Lasiopodomys brandtii)

Huddling as social thermoregulatory behavior is commonly used by small mammals to reduce heat loss and energy expenditure in the cold. Our study aimed to determine the effect of huddling behavior on energy conservation, thermogenesis, core body temperature (T_b) regulation and body composition in Brandt's voles (Lasiopodomys brandtii). Adult captive-bred female Brandt's voles (n = 124) (~50 g) in 31 cages with 4 individuals each were exposed to cool (23 ± 1°C) and cold (4 ± 1°C) ambient temperatures (T_a) and were allowed to huddle or were physically separated. The cold huddling (Cold-H) groups significantly reduced food intake by 29% and saved digestible energy 156.99 kJ/day compared with cold separated groups (Cold-S); in cool huddling groups (Cool-H) the reduction in food intake was 26% and digestible energy was saved by 105.19 kJ/day in comparison to the separated groups (Cool-S). Resting metabolic rate (RMR) of huddling groups was 35.7 and 37.2% lower than in separated groups at cold and cool T_as, respectively. Maximum non-shivering thermogenesis (NSTmax) of huddling voles was not affected by T_a, but in Cold-S voles it was significantly increased in comparison to Cool-S. Huddling groups decreased wet thermal conductance by 39% compared with separated groups in the cold, but not in the cool T_a. Unexpectedly, huddling voles significantly decreased T_b by 0.25 - 0.50°C at each T_a. Nevertheless, activity of Cold-H voles was higher than in Cold-S voles. Thus, huddling is energetically highly effective because of reduced metabolic rate, thermogenic capacity and relaxed T_b regulation despite the increase of activity. Therefore, Brandt's voles can remain active and maintain their body condition without increased energetic costs during cold exposure. This study highlights the ecological significance of huddling behavior for maintenance of individual fitness at low costs, and thus survival of population during severe winter in small mammals.

2-Deoxy-D-glucose, not mercaptoacetate, induces a reversible reduction of body temperature in male desert hamsters (Phodopus roborovskii)

The initiation of torpor is supposed to be related to the availability of metabolic fuels. Studies on metabolic fuel inhibition of glucose by using 2-deoxy-D-glucose (2DG) or fatty acid by mercaptoacetate (MA) in heterothermic mammals produced mixed outcomes. To examine the roles of availability of glucose and fatty acid in the initiation of torpor in desert hamsters (Phodopus roborovskii), we intraperitoneally administrated 2DG and MA to summer-acclimated male hamsters while body temperature (T_b), metabolic rate (MR) and respiratory quotient (RQ) were simultaneously recorded to monitor their thermoregulatory response. 2DG induced a reversible reduction of T_b in desert hamsters both at ambient temperature (T_a) of 23°C and 5°C. At T_a of 23°C, T_b, MR and RQ decreased in a dose-dependent manner with a large T_b-T_a differential (> 6.5°C) and a lowest T_b of 28.0°C which were comparable to those in fasted hamsters. At T_a of 5°C, 2DG-treated hamsters also decreased T_b to the same level as at T_a 23°C, but MR was significantly higher than that at T_a of 23°C at each dose, suggesting doses of 2DG directly affected the hypothalamic T_b set-point. Different from fasted hamsters which maintain normothermic at T_a of 5°C, 2DG-treated hamsters showed a substantial reduction of T_b at T_a 5°C, indic_ating an overwhelming effect on the thermoregulatory system regardless of T_a. Furthermore, the rapid decrease of T_b and outstretched body posture in 2DG-treated hamsters suggest that the effects of 2DG were not simply mimicking the torpor pathways but that other mechanisms are involved. Interestingly, MA failed to induce a torpor-like state in male desert hamsters. Our results suggest that availability of glucose rather than fatty acid plays an important role for initiation of torpor in desert hamsters.

The role of basking in the development of endothermy and torpor in a marsupial

Marsupials have a slow rate of development and this allows a detailed examination of thermoregulatory developmental changes and stages. We quantified the cooling rates of marsupial dunnarts (Sminthopsis crassicaudata) at 40-56 days (d) old, and torpor and basking behaviour in animals given the option to bask in four age groups from 60 to 150 d. The development of thermoregulation was a continuum, but was characterised by three major thermoregulatory stages: (1) at 40 d, animals were unable to maintain a constant high body temperature during short-term cold exposure; (2) at 60 d, animals could maintain a high T _b for the first part of the night at an ambient temperature of 15.0 ± 0.7 °C; later in the night, they entered an apparent torpor bout but could only rewarm passively when basking under a heat lamp; (3) from ~90 d, they expressed prolonged torpor bouts and were able to rewarm endogenously. Young newly weaned 60 d animals were able to avoid hypothermia by basking. In this case, basking was not an optional behavioural method of reducing the cost of rewarming from torpor, but was essential for thermoregulation independent of the nest temperature. Results from our study suggest that basking is a crucial behavioural trait that permits young marsupials and perhaps other juvenile altricial mammals to overcome the developmental stage between poikilothermy early in development and full endothermy later in life.

The flexible clock: predictive and reactive homeostasis, energy balance and the circadian regulation of sleep–wake timing

The Darwinian fitness of mammals living in a rhythmic environment depends on endogenous daily (circadian) rhythms in behavior and physiology. Here, we discuss the mechanisms underlying the circadian regulation of physiology and behavior in mammals. We also review recent efforts to understand circadian flexibility, such as how the phase of activity and rest is altered depending on the encountered environment. We explain why shifting activity to the day is an adaptive strategy to cope with energetic challenges and show how this can reduce thermoregulatory costs. A framework is provided to make predictions about the optimal timing of activity and rest of non-model species for a wide range of habitats. This Review illustrates how the timing of daily rhythms is reciprocally linked to energy homeostasis, and it highlights the importance of this link in understanding daily rhythms in physiology and behavior.

The functional requirements of mammalian hair: a compromise between crypsis and thermoregulation?

Mammalian fur often shows agouti banding with a proximal dark band near the skin and a lighter distal band. We examined the function of both bands in relation to camouflage, thermal properties of pelts, and thermal energetics of dunnarts (Sminthopsis crassicaudata), which are known to use torpor and basking. Although the distal band of dunnart fur darkened with increasing latitude, which is important for camouflage, it did not affect the thermal properties and the length of the dark band and total hair length were not correlated. In contrast, the length of the proximal dark band of preserved pelts exposed to sunlight was positively correlated (r (2) = 0.59) with the temperature underneath the pelt (T pelt). All dunnarts offered radiant heat basked by exposing the dark band of the hair during both rest and torpor. Basking dunnarts with longer dark bands had lower resting metabolism (r (2) = 0.69), warmed faster from torpor (r (2) = 0.77), required less energy to do so (r (2) = 0.32), and reached a higher subcutaneous temperature (T sub) at the end of rewarming (r (2) = 0.75). We provide the first experimental evidence on the possible dual function of the color banding of mammalian fur. The distal colored band appears to be important for camouflage, whereas the length of the dark proximal hair band facilitates heat gain for energy conservation and allows animals to rewarm quickly and economically from torpor.