Hibernation: Endotherms

The main function of hibernation and daily torpor in heterothermic mammals and birds (i.e. species capable of expressing torpor) is to conserve energy and water and thus to survive during adverse environmental conditions or periods of food shortage no matter if they live in the arctic or the tropics. However, the reduced energy requirements also permit survival of bad weather during reproduction to prolong gestation into more favourable periods, conservation of nutrients for growth during development, and overall result in reduced foraging needs and thus exposure to predators, which appear major contributing reasons why heterotherms are often long lived and have lower extinction rates than strictly homeothermic species that cannot use torpor. Known heterothermic mammals and birds are diverse with about 2/3 of mammalian orders and 1/3 of avian orders containing heterothermic species, and their number continues to grow

Heterothermy in a Small Passerine : Eastern Yellow Robins Use Nocturnal Torpor in Winter

Torpor is a controlled reduction of metabolism and body temperature, and its appropriate use allows small birds to adapt to and survive challenging conditions. However, despite its great energy conservation potential, torpor use by passerine birds is understudied although they are small and comprise over half of extant bird species. Here, we first determined whether a free-living, small ~20 g Australian passerine, the eastern yellow robin (Eopsaltria australis), expresses torpor by measuring skin temperature (Ts) as a proxy for body temperature. Second, we tested if skin temperature fluctuated in relation to ambient temperature (Ta). We found that the Ts of eastern yellow robins fluctuated during winter by 9.1 ± 3.9°C on average (average minimum Ts 30.1 ± 2.3°C), providing the first evidence of torpor expression in this species. Daily minimum Ts decreased with Ta, reducing the estimated metabolic rate by as much as 32%. We hope that our results will encourage further studies to expand our knowledge on the use of torpor in wild passerines. The implications of such studies are important because species with highly flexible energy requirements may have an advantage over strict homeotherms during the current increasing frequency of extreme and unpredictable weather events, driven by changing climate

Seasonal Control of Mammalian Energy Balance : Recent advances in the understanding of daily torpor and hibernation

Funding This work was supported in part by BBSRC grant (BB/M001504/1) to PB, DFG Emmy-Noether HE6383 to AH, German Center for Diabetes Research (DZD) to MJ.Peer reviewedPostprin

Geographical variation in the standard physiology of brushtail possums (Trichosurus): implications for conservation translocations

Identifying spatial patterns in the variation of physiological traits that occur within and between species is a fundamental goal of comparative physiology. There has been a focus on identifying and explaining this variation at broad taxonomic scales, but more recently attention has shifted to examining patterns of intra-specific physiological variation. Here we examine geographic variation in the physiology of brushtail possums (Trichosurus), widely distributed Australian marsupials, and discuss how pertinent intra-specific variation may be to conservation physiology. We found significant geographical patterns in metabolism, body temperature, evaporative water loss and relative water economy. These patterns suggest that possums from warmer, drier habitats have more frugal energy and water use and increased capacity for heat loss at high ambient temperatures. Our results are consistent with environmental correlates for broad-scale macro-physiological studies, and most intra-generic and intra-specific studies of marsupials and other mammals. Most translocations of brushtail possums occur into Australia\u27s arid zone, where the distribution and abundance of possums and other native mammals have declined since European settlement, leading to reintroduction programmes aiming to re-establish functional mammal communities. We suggest that the sub-species T. vulpecula hypoleucus from Western Australia would be the most physiologically appropriate for translocation to these arid habitats, having physiological traits most favourable for the extreme Ta, low and variable water availability and low productivity that characterize arid environments. Our findings demonstrate that geographically widespread populations can differ physiologically, and as a consequence some populations are more suitable for translocation to particular habitats than others. Consideration of these differences will likely improve the success and welfare outcomes of translocation, reintroduction and management programmes

Seasonal Control of Mammalian Energy Balance : Recent advances in the understanding of daily torpor and hibernation

Funding This work was supported in part by BBSRC grant (BB/M001504/1) to PB, DFG Emmy-Noether HE6383 to AH, German Center for Diabetes Research (DZD) to MJ.Peer reviewedPostprin

Ecology, energetics and thermal biology of sugar gliders

'Petaurus breviceps' inhabit tropical to cool-temperate regions within Australia and New Guinea. Despite their small body size (115-160 g) populations persist even in areas such as the New England region, where ambient temperature (Ta) frequently falls below 0°C over winter. Small mammals encounter a variety of energetic stresses at low Ta as a result or high thermal conductance requiring high metabolic rates (MR) for normothermic thermoregulation. Additionally insectivorous and nectarivorous species, such as sugar gliders, are confronted with seasonal reductions to food resources over winter. In order to survive and reproduce under these conditions, sugar gliders must employ a variety of behavioural and physiological strategies that include huddling and daily torpor. Although these strategies appear pivotal to their survival,almost all available information on this species is derived from captive animals and little is known on the seasonal adjustments of wild sugar gliders in terms of their thermal biology and behaviour. Moreover, little is known about the extent to which these adjustments are governed by reduced food availability and/or detrimental environmental conditions

Precocious Torpor in an Altricial Mammal and the Functional Implications of Heterothermy During Development

Most mammals and birds are altricial, small and naked at birth/hatching. They attain endothermic thermoregulation at a fraction of their adult size at a vulnerable stage with high heat loss when many could profit from using torpor for energy conservation. Nevertheless, detailed data on the interrelations between torpor expression and development of endothermic thermoregulation are currently restricted to <0.1% of extant endotherms. We investigated at what age and body mass (BM) desert hamsters (Phodopus roborovskii), wild-caught in Inner Mongolia and born in autumn/early winter when environmental temperatures in the wild begin to decrease, are able to defend their body temperature (Tb) at an ambient temperature (Ta) of ∼21°C and how soon thereafter they could express torpor. Measurements of surface temperatures via infrared thermometer and thermal camera show that although neonate hamsters (BM 0.9 ± 0.1 g) cooled rapidly to near Ta, already on day 15 (BM 5.5 ± 0.2 g) they could defend a high and constant Tb. As soon as day 16 (BM 5.8 ± 0.2 g), when their maximum activity metabolism (measured as oxygen consumption) approached maxima measured in vertebrates, animals were able to enter torpor for several hours with a reduction of metabolism by >90%, followed by endothermic arousal. Over the next weeks, torpor depth and duration decreased together with a reduction in resting metabolic rate at Ta 30–32°C. Our data show that development of endothermy and torpor expression in this altricial hamster is extremely fast. The results suggest that precocious torpor by juvenile hamsters in autumn and winter is an important survival tool in their vast and harsh Asian desert habitats, but likely also for many other small mammals and birds worldwide

Behavioural ecology and thermal physiology of Australian Owlet-Nightjars ('Aegotheles cristatus')

The Australian owlet-nightjar ('Aegothelidae cristatus') is a sedentary, nocturnal, avian insectivore that uses a variety of adaptations to balance its energy budget. This has enabled this species to successfully inhabit diverse habitats throughout Australia. Owlet-nightjars are Caprimulgiformes, a group that typically have low metabolic rates and are capable of entering torpor to conserve energy. However, unlike many species of Caprimulgiformes, owlet-nightjars do not migrate and therefore must cope metabolically and behaviourally with seasonal variations in ambient temperature (Ta) and food resources. They are also unique amongst the Caprimulgiformes in that they are obligate cavity users year-round. I studied the behavioural and physiological qualities that enable this species to subsist in two dissimilar habitats, the semi-arid desert of central Australia and the comparatively cold, mesic, eucalypt woodlands atop the Northern Tablelands of NSW. I used radiotelemetry to locate diurnal roost sites, to track birds to determine home range, and to quantify body temperature (Tb) fluctuations in relation to roost and ambient thermal conditions. I compared characteristics of cavity roosts with randomly selected unoccupied cavities and used an information theoretic approach to assess variables which may be important for roost selection. Faecal samples collected from birds and cavity roosts at both locations were used to compare the diet and inferred foraging tactics between the two habitats in relation to arthropod availability. Finally, I measured the metabolic rate and thermal conductance of individual birds in the laboratory during both summer and winter using open-flow respirometry in the laboratory

Non-invasive evaluation of stress hormone responses in a captive population of sugar gliders (Petaurus breviceps)

Faecal hormone monitoring offers a robust tool to non-invasively determine the physiological stress experienced by an individual when faced with natural or human-driven stressors. Although already quantified for several species, the method needs to be validated for each new species to ensure reliable quantification of the respective glucocorticoids. Here we investigated whether measurement of faecal glucocorticoid metabolite (fGCM) provides a feasible and non-invasive way to assess the physiological state of sugar gliders (Petaurus breviceps), an arboreal marsupial native to Australia, by using both a biological and physiological validation. Our analysis confirmed that the cortisol enzyme immunoassay (EIA) was the most appropriate assay for monitoring fGCM concentrations in sugar gliders. Comparing the fGCM response to the physiological and the biological validation, we found that while the administration of ACTH led to a significant increase in fGCM concentration in all individuals, only six of eight individuals showed a considerable fGCM response following the biological validation. Our study identified the most appropriate immunoassay for monitoring fGCM concentrations as an indicator of physiological stress in sugar gliders, but also supports recent suggestions that, if possible, both biological and physiological stressors should be used when testing the suitability of an EIA for a species.Grants from the German Academic Exchange Service (DAAD) and the A.F.W. Schimper Stiftung für ökologische Forschung to J.N. and by the Australian Research Council and the University of New England to F.G.http://www.publish.csiro.au/amhj2019Anatomy and PhysiologyMammal Research Institut

Phoenix from the Ashes: Fire, Torpor, and the Evolution of Mammalian Endothermy

The evolution of endothermy in mammals and birds has been widely debated. Endothermy is characterized by high endogenous heat production via combustion of metabolic fuels. This differs from ectothermy in most living organisms, which generally do not produce substantial amounts of internal heat for thermoregulation (Tattersall et al., 2012; Withers et al., 2016). Endogenous heat production is energetically very costly. In comparison to ectothermic terrestrial vertebrates, namely the amphibians and reptiles, the minimum metabolic rate (MR) of normothermic or homeothermic (high constant body temperature, Tb) animals at rest is about 4–8-fold higher in the endotherms. This difference is even more pronounced at low ambient temperatures (Ta) at which the Tb of ectotherms follows Ta, and the MR decreases to even lower levels. In contrast, the Tb of homeothermic endotherms remains high and constant over a wide range of Ta. Therefore, to compensate for increased heat loss at low Ta, MR of especially small mammals and birds must increase substantially and can be 100-fold or more of that in ectotherms (Bartholomew, 1982). Of course this high MR requires a substantial uptake of food and in endotherms much of this chemical energy is simply converted into heat for thermoregulation rather than growth or reproduction as in ectotherms