Common tenrecs (Tenrec ecaudatus) reduce oxygen consumption in hypoxia and in hypercapnia without concordant changes to body temperature or heart rate

Common tenrecs (Tenrec ecaudatus) are fossorial mammals that use burrows during both active and hibernating seasons in Madagascar and its neighboring islands. Prevailing thought was that tenrecs hibernate for 8-9 months individually, but 13 tenrecs were removed from the same sealed burrow 1 m deep from the surface. Such group hibernation in sealed burrows presumably creates a hypoxic and/or hypercapnic environment and suggests that this placental mammal may have an increased tolerance to hypoxia and hypercapnia. Higher tolerances to hypoxia and hypercapnia have been documented for other mammals capable of hibernation and to determine if this is the case for tenrecs, we exposed them to acute hypoxia (4 h of 16 or 7% O2), progressive hypoxia (2 h of 16, 10 and 4% O2), or progressive hypercapnia (2 h of 2, 5 and 10% CO2) at cold (16 °C) or warm (28 °C) ambient temperatures (Ta). Oxygen equilibrium curves were also constructed on the whole blood of tenrecs at 10, 25, and 37 °C to determine if hemoglobin (Hb)-O2 affinity contributes to hypoxia tolerance. In animals held at 16 °C, normoxic and normocapnic levels of oxygen consumption rate (V ˙ O 2 ), body temperature (Tb), and heart rate (HR) were highly variable between individuals. This inter-individual variation was greatly reduced in animals held at 28 °C for oxygen consumption rate and body temperature. Both hypoxia (acute and progressive) and progressive hypercapnia led to decreases inV ˙ O 2 as well as the variation inV ˙ O 2 between animals held at 16 °C. The fall in oxygen consumption rate in 7% O2 independent of changes in body temperature in tenrecs held at 16 °C is unique and not consistent with the typical hypoxic metabolic response seen in other hibernating species that depends on concomitant falls in Tb. In animals held at 28 °C, exposure to O2 levels as low as 4% and CO2 levels as high as 10% had no significant effect onV ˙ O 2 , HR, or Tb, indicative of high tolerance to both hypoxia and hypercapnia. High variation in heart rate remained between individuals in all gas compositions and at all temperatures. Tenrec Hb-O2 affinity was similar to other homeothermic placental mammals and likely does not contribute to the increased hypoxia tolerance. Ultimately, our results suggest changes in Ta dictate physiological responses to hypoxia or hypercapnia in tenrecs, responses more characteristic of reptiles than of most placental mammals. Given that numerous anatomical and physiological characteristics of tenrecs suggest that they may be representative of an ancestral placental mammal, our findings suggest the typical hypoxic metabolic response evolved later in mammalian evolution.

Of fruits and fats: high-sugar diets restore fatty acid profiles in the white adipose tissue of captive dwarf lemurs

Fat-storing hibernators rely on fatty acids from white adipose tissue (WAT) as an energy source to sustain hibernation. Whereas arctic and temperate hibernators preferentially recruit dietary polyunsaturated fatty acids (PUFAs), tropical hibernators can rely on monounsaturated fatty acids that produce fewer lipid peroxides during oxidation. Nevertheless, compositional data on WAT from tropical hibernators are scant and questions remain regarding fat recruitment and metabolism under different environmental conditions. We analyse fatty acid profiles from the WAT of captive dwarf lemurs (Cheirogaleus medius) subjected to high-sugar or high-fat diets during fattening and cold or warm conditions during hibernation. Dwarf lemurs fed high-sugar (compared to high-fat) diets displayed WAT profiles more comparable to wild lemurs that fatten on fruits and better depleted their fat reserves during hibernation. One PUFA, linoleic acid, remained elevated before hibernation, potentially lingering from the diets provisioned prior to fattening. That dwarf lemurs preferentially recruit the PUFA linoleic acid from diets that are naturally low in availability could explain the discrepancy between captive and wild lemurs' WAT. While demonstrating that minor dietary changes can produce major changes in seasonal fat deposition and depletion, our results highlight the complex role for PUFA metabolism in the ecology of tropical hibernators.

Liver proteome response to torpor in a basoendothermic mammal, Tenrec ecaudatus, provides insights into the evolution of homeothermy

Many mammals use adaptive heterothermy (e.g., torpor, hibernation) to reduce metabolic demands of maintaining high body temperature (T_b). Torpor is typically characterized by coordinated declines in T_b and metabolic rate (MR) followed by active rewarming. Most hibernators experience periods of euthermy between bouts of torpor during which homeostatic processes are restored. In contrast, the common tenrec, a basoendothermic Afrotherian mammal, hibernates without interbout arousals and displays extreme flexibility in T_b and MR. We investigated the molecular basis of this plasticity in tenrecs by profiling the liver proteome of animals that were active or torpid with high and more stable T_b (∼32°C) or lower T_b (∼14°C). We identified 768 tenrec liver proteins, of which 50.9% were differentially abundant between torpid and active animals. Protein abundance was significantly more variable in active cold and torpid compared with active warm animals, suggesting poor control of proteostasis. Our data suggest that torpor in tenrecs may lead to mismatches in protein pools due to poor coordination of anabolic and catabolic processes. We propose that the evolution of endothermy leading to a more realized homeothermy of boreoeutherians likely led to greater coordination of homeostatic processes and reduced mismatches in thermal sensitivities of metabolic pathways.

Field Physiology: Studying Organismal Function in the Natural Environment

Continuous physiological measurements collected in field settings are essential to understand baseline, free-ranging physiology, physiological range and variability, and the physiological responses of organisms to disturbances. This article presents a current summary of the available technologies to continuously measure the direct physiological parameters in the field at high-resolution/instantaneous timescales from freely behaving animals. There is a particular focus on advantages versus disadvantages of available methods as well as emerging technologies "on the horizon" that may have been validated in captive or laboratory-based scenarios but have yet to be applied in the wild. Systems to record physiological variables from free-ranging animals are reviewed, including radio (VHF/UFH) telemetry, acoustic telemetry, and dataloggers. Physiological parameters that have been continuously measured in the field are addressed in seven sections including heart rate and electrocardiography (ECG); electromyography (EMG); electroencephalography (EEG); body temperature; respiratory, blood, and muscle oxygen; gastric pH and motility; and blood pressure and flow. The primary focal sections are heart rate and temperature as these can be, and have been, extensively studied in free-ranging organisms. Predicted aspects of future innovation in physiological monitoring are also discussed. The article concludes with an overview of best practices and points to consider regarding experimental designs, cautions, and effects on animals. © 2021 American Physiological Society. Compr Physiol 11:1979-2015, 2021.

On the modulation and maintenance of hibernation in captive dwarf lemurs

In nature, photoperiod signals environmental seasonality and is a strong selective "zeitgeber" that synchronizes biological rhythms. For animals facing seasonal environmental challenges and energetic bottlenecks, daily torpor and hibernation are two metabolic strategies that can save energy. In the wild, the dwarf lemurs of Madagascar are obligate hibernators, hibernating between 3 and 7 months a year. In captivity, however, dwarf lemurs generally express torpor for periods far shorter than the hibernation season in Madagascar. We investigated whether fat-tailed dwarf lemurs (Cheirogaleus medius) housed at the Duke Lemur Center (DLC) could hibernate, by subjecting 8 individuals to husbandry conditions more in accord with those in Madagascar, including alternating photoperiods, low ambient temperatures, and food restriction. All dwarf lemurs displayed daily and multiday torpor bouts, including bouts lasting ~ 11 days. Ambient temperature was the greatest predictor of torpor bout duration, and food ingestion and night length also played a role. Unlike their wild counterparts, who rarely leave their hibernacula and do not feed during hibernation, DLC dwarf lemurs sporadically moved and ate. While demonstrating that captive dwarf lemurs are physiologically capable of hibernation, we argue that facilitating their hibernation serves both husbandry and research goals: first, it enables lemurs to express the biphasic phenotypes (fattening and fat depletion) that are characteristic of their wild conspecifics; second, by "renaturalizing" dwarf lemurs in captivity, they will emerge a better model for understanding both metabolic extremes in primates generally and metabolic disorders in humans specifically.

Do endotherms have thermal performance curves?

Temperature is an important environmental factor governing the ability of organisms to grow, survive and reproduce. Thermal performance curves (TPCs), with some caveats, are useful for charting the relationship between body temperature and some measure of performance in ectotherms, and provide a standardized set of characteristics for interspecific comparisons. Endotherms, however, have a more complicated relationship with environmental temperature, as endothermy leads to a decoupling of body temperature from external temperature through use of metabolic heat production, large changes in insulation and variable rates of evaporative heat loss. This has impeded our ability to model endothermic performance in relation to environmental temperature as well as to readily compare performance between species. In this Commentary, we compare the strengths and weaknesses of potential TPC analogues (including other useful proxies for linking performance to temperature) in endotherms and suggest several ways forward in the comparative ecophysiology of endotherms. Our goal is to provide a common language with which ecologists and physiologists can evaluate the effects of temperature on performance. Key directions for improving our understanding of endotherm thermoregulatory physiology include a comparative approach to the study of the level and precision of body temperature, measuring performance directly over a range of body temperatures and building comprehensive mechanistic models of endotherm responses to environmental temperatures. We believe the answer to the question posed in the title could be 'yes', but only if 'performance' is well defined and understood in relation to body temperature variation, and the costs and benefits of endothermy are specifically modelled.

Tropical bats counter heat by combining torpor with adaptive hyperthermia

Many tropical mammals are vulnerable to heat because their water budget limits the use of evaporative cooling for heat compensation. Further increasing temperatures and aridity might consequently exceed their thermoregulatory capacities. Here, we describe two novel modes of torpor, a response usually associated with cold or resource bottlenecks, as efficient mechanisms to counter heat. We conducted a field study on the Malagasy bat Macronycteris commersoni resting in foliage during the hot season, unprotected from environmental extremes. On warm days, the bats alternated between remarkably short micro-torpor bouts and normal resting metabolism within a few minutes. On hot days, the bats extended their torpor bouts over the hottest time of the day while tolerating body temperatures up to 42.9°C. Adaptive hyperthermia combined with lowered metabolic heat production from torpor allows higher heat storage from the environment, negates the need for evaporative cooling and thus increases heat tolerance. However, it is a high-risk response as the torpid bats cannot defend body temperature if ambient temperature increases above a critical/lethal threshold. Torpor coupled with hyperthermia and micro-torpor bouts broaden our understanding of the basic principles of thermal physiology and demonstrate how mammals can perform near their upper thermal limits in an increasingly warmer world.

Ambient Temperature Cycles Affect Daily Torpor and Hibernation Patterns in Malagasy Tenrecs

Hibernation and daily torpor (heterothermy) allow endotherms to cope with demanding environmental conditions. The depth and duration of torpor bouts vary considerably between tropical and temperate climates, and tropical hibernators manage to cope with a wider spectrum of ambient temperature (T_a) regimes during heterothermy. As cycles in T_a can have profound effects on activity and torpor patterns as well as energy expenditure, we examined how these characteristics are affected by daily fluctuating versus constant T_a in a tropical hibernator, the lesser hedgehog tenrec (Echinops telfairi). Throughout the study, regardless of season, the tenrecs became torpid every day. In summer, E. telfairi used daily fluctuations in T_a to passively rewarm from daily torpor, which led to synchrony in the activity phases and torpor bouts between individuals and generally decreased energy expenditure. In contrast, animals housed at constant T_a showed considerable variation in timing and they had to invest more energy through endogenous heat production. During the hibernation season (winter) E. telfairi hibernated for several months in constant, as well as in fluctuating T_a and, as in summer, under fluctuating T_a arousals were much more uniform and showed less variation in timing compared to constant temperature regimes. The timing of torpor is not only important for its effective use, but synchronization of activity patterns could also be essential for social interactions, and successful foraging bouts. Our results highlight that T_a cycles can be an effective zeitgeber for activity and thermoregulatory rhythms throughout the year and that consideration should be given to the choice of temperature regime when studying heterothermy under laboratory conditions.

Induced pluripotent stem cells as a tool for comparative physiology: lessons from the thirteen-lined ground squirrel

Comparative physiologists are often interested in adaptive physiological phenomena found in unconventional model organisms; however, research on these species is frequently constrained by the limited availability of investigative tools. Here, we propose that induced pluripotent stem cells (iPSCs) from unconventional model organisms may retain certain species-specific features that can consequently be investigated in depth in vitro; we use hibernating mammals as an example. Many species (including ground squirrels, bats and bears) can enter a prolonged state of physiological dormancy known as hibernation to survive unfavorable seasonal conditions. Our understanding of the mechanisms underpinning the rapid transition and adaptation to a hypothermic, metabolically suppressed winter torpor state remains limited partially because of the lack of an easily accessible model. To address the fascinating unanswered questions underlying hibernation biology, we have developed a powerful model system: iPSCs from a hibernating species, the thirteen-lined ground squirrel (Ictidomys tridecemlineatus). These stem cells can potentially be differentiated into any cell type, and can be used for the analysis of cell-autonomous mechanisms that facilitate adaptation to hibernation and for comparisons with non-hibernators. Furthermore, we can manipulate candidate molecular and cellular pathways underlying relevant physiological phenomena by pharmacological or RNAi-based methods, and CRISPR/Cas9 gene editing. Moreover, iPSC strategies can be applied to other species (e.g. seals, naked mole rats, humming birds) for in vitro studies on adaptation to extreme physiological conditions. In this Commentary, we discuss factors to consider when attempting to generate iPSCs from unconventional model organisms, based on our experience with the thirteen-lined ground squirrel.

Waking the sleeping dragon: gene expression profiling reveals adaptive strategies of the hibernating reptile Pogona vitticeps

Background

Hibernation is a physiological state exploited by many animals exposed to prolonged adverse environmental conditions associated with winter. Large changes in metabolism and cellular function occur, with many stress response pathways modulated to tolerate physiological challenges that might otherwise be lethal. Many studies have sought to elucidate the molecular mechanisms of mammalian hibernation, but detailed analyses are lacking in reptiles. Here we examine gene expression in the Australian central bearded dragon (Pogona vitticeps) using mRNA-seq and label-free quantitative mass spectrometry in matched brain, heart and skeletal muscle samples from animals at late hibernation, 2 days post-arousal and 2 months post-arousal.

Results

We identified differentially expressed genes in all tissues between hibernation and post-arousal time points; with 4264 differentially expressed genes in brain, 5340 differentially expressed genes in heart, and 5587 differentially expressed genes in skeletal muscle. Furthermore, we identified 2482 differentially expressed genes across all tissues. Proteomic analysis identified 743 proteins (58 differentially expressed) in brain, 535 (57 differentially expressed) in heart, and 337 (36 differentially expressed) in skeletal muscle. Tissue-specific analyses revealed enrichment of protective mechanisms in all tissues, including neuroprotective pathways in brain, cardiac hypertrophic processes in heart, and atrophy protective pathways in skeletal muscle. In all tissues stress response pathways were induced during hibernation, as well as evidence for gene expression regulation at transcription, translation and post-translation.

Conclusions

These results reveal critical stress response pathways and protective mechanisms that allow for maintenance of both tissue-specific function, and survival during hibernation in the central bearded dragon. Furthermore, we provide evidence for multiple levels of gene expression regulation during hibernation, particularly enrichment of miRNA-mediated translational repression machinery; a process that would allow for rapid and energy efficient reactivation of translation from mature mRNA molecules at arousal. This study is the first molecular investigation of its kind in a hibernating reptile, and identifies strategies not yet observed in other hibernators to cope stress associated with this remarkable state of metabolic depression.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5750-x) contains supplementary material, which is available to authorized users.

Engineering Human Stasis for Long-Duration Spaceflight

Suspended animation for deep-space travelers is moving out of the realm of science fiction. Two approaches are considered: the first elaborates the current medical practice of therapeutic hypothermia; the second invokes the cascade of metabolic processes naturally employed by hibernators. We explore the basis and evidence behind each approach and argue that mimicry of natural hibernation will be critical to overcome the innate limitations of human physiology for long-duration space travel.

Molecular interactions underpinning the phenotype of hibernation in mammals

Mammals maintain a constant warm body temperature, facilitating a wide variety of metabolic reactions. Mammals that hibernate have the ability to slow their metabolism, which in turn reduces their body temperature and leads to a state of hypothermic torpor. For this metabolic rate reduction to occur on a whole-body scale, molecular interactions that change the physiology of cells, tissues and organs are required, resulting in a major departure from normal mammalian homeostasis. The aim of this Review is to cover recent advances in the molecular biology of mammalian hibernation, including the role of small molecules, seasonal changes in gene expression, cold-inducible RNA-binding proteins, the somatosensory system and emerging information on hibernating primates. To underscore the importance of differential gene expression across the hibernation cycle, mRNA levels for 14,261 ground squirrel genes during periods of activity and torpor are made available for several tissues via an interactive transcriptome browser. This Review also addresses recent findings on molecular interactions responsible for multi-day survival of near-freezing body temperatures, single-digit heart rates and a slowed metabolism that greatly reduces oxygen consumption. A better understanding of how natural hibernators survive these physiological extremes is beginning to lead to innovations in human medicine.