Obligate Hibernators Research Articles

Effects of the anti-inflammatory drug budesonide on the gut microbiota and cytokine production of 13-lined ground squirrels during prehibernation fattening.

The gut microbiome is essential for maintaining organismal health. Gut microbiota may be disrupted through external factors like dietary change, which can lead to gut inflammation, resulting in obesity. Hibernating mammals develop low-grade gut inflammation when they accumulate fat deposits in preparation for hibernation, making them useful models for studying the relationship between the microbiome, inflammation, and weight gain. Nonsteroidal anti-inflammatory drugs and steroids are commonly used in humans to target gut inflammation, but how these drugs affect the gut microbiome and its stability is unclear. We investigated the effect of the glucocorticoid drug budesonide on the gut microbiome and cytokine levels of an obligate hibernator, the 13-lined ground squirrel, during the fattening season. We used 16S rRNA gene sequencing to characterize bacterial communities in the lumen and mucosa of the cecum and colon and measured proinflammatory [tumor necrosis factor-α (TNF-α)/interleukin 6 (IL-6)] and anti-inflammatory (IL-10) cytokine levels. Budesonide affected the microbiome only in the cecum lumen, where bacterial diversity was higher in the control group, and communities significantly differed between treatments. Across gut sections, Marvinbryantia and Enterococcus were significantly higher in the budesonide group, whereas Sarcina was higher in the control group. TNF-α and IL-6 levels were higher in control squirrels compared with the budesonide group, but there was no difference in IL-10 levels. Overall, budesonide treatment affected the microbial community and diversity of 13-lined ground squirrels in the cecum lumen. Our study presents another step toward developing ground squirrels as a model for studying the interaction between the microbiota and host inflammation.NEW & NOTEWORTHY Disruptions of gut microbiota can lead to inflammation, resulting in weight gain. Inflammation can be treated with budesonide, but how budesonide affects gut microbiota is unclear. Thirteen-lined ground squirrels experience low-grade gut inflammation during prehibernation fattening, which compares with human inflammation-weight gain mechanisms. We showed that budesonide treatment decreased microbiome diversity and lead to a shift in community in the cecum lumen. Our study supports developing ground squirrels as a model for studying microbiome-inflammation interactions.

Paternal behavior in captive fat-tailed dwarf lemurs (Cheirogaleus medius) is preserved under socially relevant conditions.

Fat-tailed dwarf lemurs (Cheirogaleus medius), primates endemic to Madagascar, are obligate hibernators that form stable, lifelong pairs in the wild. Given the temporal constraints imposed by seasonal hibernation, infant dwarf lemurs must grow, develop, and wean within the first two months of life. Maternal as well as paternal infant care, observed in the wild, has been deemed critical for infant survival. Given the importance of fathers' involvement in early infant care, we expect this behavior to persist even under captive conditions. At the Duke Lemur Center, in Durham NC, we observed two families of fat-tailed dwarf lemurs and focused on the behavior of adult males within the first two months of the infants' lives. We report evidence of paternal involvement, including babysitting, co-feeding, grooming, accompanying, and leading infants, consistent with observations from the wild. As expected, paternal babysitting decreased as infants gained independence, while co-feeding increased. Supplemental anecdotes, video recorded by observers, also highlight clear cases of involvement by both parents, and even older siblings, in safeguarding and socializing new infants. We argue that maintaining captive fat-tailed dwarf lemur populations under socially and ecologically relevant conditions facilitates the full expression of physiological and behavioral repertoires. Most importantly, it also allows dwarf lemurs to realize their species' potential and become robust proxies of their wild kin.

Primate hibernation: The past, present, and promise of captive dwarf lemurs.

The dwarf lemurs (Cheirogaleus spp.) of Madagascar are the only obligate hibernators among primates. Despite century-old field accounts of seasonal lethargy, and more recent evidence of hibernation in the western fat-tailed dwarf lemur (Cheirogaleus medius), inducing hibernation in captivity remained elusive for decades. This included the Duke Lemur Center (DLC), which maintains fat-tailed dwarf lemurs and has produced sporadic research on reproduction and metabolism. With cumulative knowledge from the field, a newly robust colony, and better infrastructure, we recently induced hibernation in DLC dwarf lemurs. We describe two follow-up experiments in subsequent years. First, we show that dwarf lemurs under stable cold conditions (13°C) with available food continued to eat daily, expressed shallower and shorter torpor bouts, and had a modified gut microbiome compared to peers without food. Second, we demonstrate that dwarf lemurs under fluctuating temperatures (12-30°C) can passively rewarm daily, which was associated with altered patterns of fat depletion and reduced oxidative stress. Despite the limitations of working with endangered primates, we highlight the promise of studying hibernation in captive dwarf lemurs. Follow-up studies on genomics and epigenetics, metabolism, and endocrinology could have relevance across multidisciplinary fields, from biomedicine to evolutionary biology, and conservation.

Mitochondrial microRNA profiles are altered in thirteen-lined ground squirrels (Ictidomys tridecemlineatus) during hibernation.

Thirteen-lined ground squirrels (TLGSs) are obligate hibernators that cycle between torpor (low metabolic rate and body temperature) and interbout euthermia (IBE; typical euthermic body temperature and metabolism) from late autumn to spring. Many physiological changes occur throughout hibernation, including a reduction in liver mitochondrial metabolism during torpor, which is reversed during arousal to interbout euthermia. Nuclear-encoded microRNA (miRNA, small posttranscriptional regulator molecules) differ in abundance throughout TLGS hibernation and have been shown to regulate mitochondrial gene expression in mammalian cell culture (where they are referred to as mitomiRs). This study characterized differences in mitomiR profiles from TLGS liver mitochondria isolated during summer, torpor, and IBE, and predicted their mitochondrial targets. Using small RNA sequencing, differentially abundant mitomiRs were identified between hibernation states, and using quantitative PCR analysis, we quantified the expression of predicted mitochondrial mRNA targets. Most differences in mitomiR abundances were seasonal (i.e., between summer and winter) with only one mitomiR differentially abundant between IBE and torpor. Multiple factor analysis (MFA) revealed three clusters divided by hibernation states, where clustering was predominantly driven by mitomiR abundances. Nine of these differentially abundant mitomiRs had predicted mitochondrial RNA targets, including subunits of electron transfer system complexes I and IV, 12S rRNA, and two tRNAs. Overall, mitomiRs were predicted to suppress the expression of their mitochondrial targets and may have some involvement in regulating protein translation in mitochondria. This study found differences in mitomiR abundances between seasons and hibernation states of TLGS and suggests potential mechanisms for regulating the mitochondrial electron transfer system.NEW & NOTEWORTHY During the hibernation season, thirteen-lined ground squirrels periodically increase metabolism remarkably between torpor and interbout euthermia (IBE). This process involves rapid reactivation of mitochondrial respiration. We predicted that mitochondrial microRNA (mitomiRs) might be altered during this response. We found that the abundance of 38 liver mitomiRs differs based on hibernation state (summer, IBE, and torpor). Small RNA sequencing identified mitomiR profiles, including some mitomiRs that are predicted to bind to mitochondrial RNAs.

Resource availability alters breeding strategies in a small mammal community.

Following a resource pulse, animals may finance reproduction by consuming concurrently available resources (income breeding) or by storing resources for future reproduction (capital breeding). Understanding how these reproductive strategies are used is important for determining the ecological mechanisms that structure timing of reproduction and that drive interannual population fluctuations in animals. We gathered a reproductive dataset for five small mammal species over a 12-year period in Northeastern USA during which six masting events of American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis) occurred. Masting created alternate years where seeds were either available late (masting year) or early (cached from the previous year) in the breeding season. The small mammal species differed in reliance on seeds and overwintering strategies. We quantified the diet using stable isotopes and recorded reproduction timing, proportion breeding and litter size in females and testes size in males. Timing of seed availability minimally affected litter size but strongly affected proportion breeding and timing of reproduction. During masting years (late seed availability), a higher proportion of females reproduced, with breeding taking place later in the season (lactation timed with peak seed availability), although the delay was restricted in Napaeozapus insignis, an obligate hibernator. After a fall mast, cached seeds were used as capital in the following spring (early seed availability) to support a litter that, depending on the species, occurred 24-79 days sooner than a mast year. No late-season reproduction occurred in years with early seed availability except for Myodes gapperi which produced a second litter, likely financed by fungal consumption. Males also showed strong responses to seed availability, mirroring female reproduction with testes size staying constant in years with late seed availability and sharply decreasing over the breeding season in years with early seed availability. Our results highlight that although photoperiod and temperature broadly set bounds of the breeding season in temperate environments, resource availability influences the reproductive strategies that species use, which in turn alters reproductive timing and can drive large inter-annual population fluctuations. Differences in overwintering strategies and diet may further modulate reproductive timing and output relative to resource pulses.

Mammal hibernation as a strategy for adaptation to unfavorable environmental conditions

To analyse the literature data on the survival pathways of heterothermic endotherms in unfavorable environmental conditions, during periods of low availability of food resources.The article provides data on the differences between daily and seasonal heterothermy. The features of preparation for hibernation in facultative and obligate hibernators are highlighted. Hypotheses of the origin and evolution of heterothermy are considered. The most probable causes of periodic awakenings of animals from hibernation during the hibernation period are summarised. Considerable attention is paid to the restructuring of energy metabolism during hibernation – the transition from carbohydrate to lipid metabolism. Data have been analysed indicating the importance of fatty acids obtained from food during the active summer period, both for the synthesis of reserve fats and in the regulation of hibernation. Based on data on the accumulation of monoenoic fatty acids in tissues during hibernation, it has been suggested that they have an adaptive significance aimed at limiting oxidative stress and preserving vital cell functions.The data presented can be used both for conducting fundamental research on the adaptive mechanisms of interaction of an organism with its environment, and for solving practical problems, especially when choosing models of calorie restriction or intermittent fasting, as well as studying tissue tolerance to oxidative stress and resistance to the damaging effects of ischemia – reperfusion.

Inferring condition in wild mammals: body condition indices confer no benefit over measuring body mass across ecological contexts.

Many studies assume that it is beneficial for individuals of a species to be heavier, or have a higher body condition index (BCI), without accounting for the physiological relevance of variation in the composition of different body tissues. We hypothesized that the relationship between BCI and masses of physiologically important tissues (fat and lean) would be conditional on annual patterns of energy acquisition and expenditure. We studied three species with contrasting ecologies in their respective natural ranges: an obligate hibernator (Columbian ground squirrel, Urocitellus columbianus), a facultative hibernator (black-tailed prairie dog, Cynomys ludovicianus), and a food-caching non-hibernator (North American red squirrel, Tamiasciurus hudsonicus). We measured fat and lean mass in adults of both sexes using quantitative magnetic resonance (QMR). We measured body mass and two measures of skeletal structure (zygomatic width and right hind foot length) to develop sex- and species-specific BCIs, and tested the utility of BCI to predict body composition in each species. Body condition indices were more consistently, and more strongly correlated, with lean mass than fat mass. The indices were most positively correlated with fat when fat was expected to be very high (pre-hibernation prairie dogs). In all cases, however, BCI was never better than body mass alone in predicting fat or lean mass. While the accuracy of BCI in estimating fat varied across the natural histories and annual energetic patterns of the species considered, measuring body mass alone was as effective, or superior in capturing sufficient variation in fat and lean in most cases.

Firmicutes and Bacteroidetes contribute to mass gain variation in female obligate hibernators

Abstract Obtaining body condition is an important life history challenge that directly impacts individual fitness and is particularly important for hibernating animals, whose maintenance of adequate body fat and mass is essential for survival. It is well-documented that host-associated microorganisms play a vital role in animal physiology and behavior. Recent work demonstrates that gut microbes are associated with fat accumulation and obesity, particularly the phyla Firmicutes and Bacteroidetes. The focus of most microbiome studies has been on human health or involved lab-reared animals used as a model system. However, these microbes likely are important for individual fitness in wild populations and provide potential mechanistic insights into the adaptability and survival of wildlife. Here we tested whether symbiotic microorganisms within the phyla of Firmicutes and Bacteroidetes were associated with summer mass gain in an exceptionally well-studied wild population of yellow-bellied marmots (Marmota flaviventer) by analyzing 207 fecal samples collected over 5 summer active seasons. Results showed that marmots with higher mass gain rates had a greater relative abundance of Firmicutes. In contrast, a higher relative abundance of Bacteroidetes was associated with lower mass gain rates, but only for marmots living in harsher environments. Similar patterns were found at the family level where Ruminococcaceae, a member of Firmicutes, was associated with higher mass gain rates, and Muribaculaceae, a member of Bacteroidetes, was associated with lower mass gain rates in harsher environments. Although correlative, these results highlight the potential importance of symbiotic gut microbiota to mass gain in the wild—a trait associated with survival and fitness in many taxonomic groups.

Seasonal variation in glucose and insulin is modulated by food and temperature conditions in a hibernating primate.

Feast-fast cycles allow animals to live in seasonal environments by promoting fat storage when food is plentiful and lipolysis when food is scarce. Fat-storing hibernators have mastered this cycle over a circannual schedule, by undergoing extreme fattening to stockpile fuel for the ensuing hibernation season. Insulin is intrinsic to carbohydrate and lipid metabolism and is central to regulating feast-fast cycles in mammalian hibernators. Here, we examine glucose and insulin dynamics across the feast-fast cycle in fat-tailed dwarf lemurs, the only obligate hibernator among primates. Unlike cold-adapted hibernators, dwarf lemurs inhabit tropical forests in Madagascar and hibernate under various temperature conditions. Using the captive colony at the Duke Lemur Center, we determined fasting glucose and insulin, and glucose tolerance, in dwarf lemurs across seasons. During the lean season, we maintained dwarf lemurs under stable warm, stable cold, or fluctuating ambient temperatures that variably included food provisioning or deprivation. Overall, we find that dwarf lemurs can show signatures of reversible, lean-season insulin resistance. During the fattening season prior to hibernation, dwarf lemurs had low glucose, insulin, and HOMA-IR despite consuming high-sugar diets. In the active season after hibernation, glucose, insulin, HOMA-IR, and glucose tolerance all increased, highlighting the metabolic processes at play during periods of weight gain versus weight loss. During the lean season, glucose remained low, but insulin and HOMA-IR increased, particularly in animals kept under warm conditions with daily food. Moreover, these lemurs had the greatest glucose intolerance in our study and had average HOMA-IR values consistent with insulin resistance (5.49), while those without food under cold (1.95) or fluctuating (1.17) temperatures did not. Remarkably low insulin in dwarf lemurs under fluctuating temperatures raises new questions about lipid metabolism when animals can passively warm and cool rather than undergo sporadic arousals. Our results underscore that seasonal changes in insulin and glucose tolerance are likely hallmarks of hibernating mammals. Because dwarf lemurs can hibernate under a range of conditions in captivity, they are an emerging model for primate metabolic flexibility with implications for human health.

Food availability positively affects the survival and somatic maintenance of hibernating garden dormice (Eliomys quercinus)

BackgroundTorpor is an energy saving strategy achieved by substantial reductions of metabolic rate and body temperature that enables animals to survive periods of low resource availability. During hibernation (multiday torpor), the frequency of periodic rewarming—characterised by high levels of oxidative stress—is associated with shortening of telomeres, a marker of somatic maintenance.ObjectivesIn this study, we determined the impact of ambient temperature on feeding behaviour and telomere dynamics in hibernating garden dormice (Eliomys quercinus) over winter. This obligate hibernator prepares for hibernation by accumulating fat stores but can also feed during hibernation.MethodologyFood intake, torpor pattern, changes in telomere length, and body mass change were assessed in animals housed at experimentally controlled temperatures of either 14 °C (i.e., a mild winter) or 3 °C (i.e., a cold winter) over 6 months.ResultsWhen hibernating at 14 °C, dormice experienced 1.7-fold more frequent and 2.4-fold longer inter-bout euthermia, and spent significantly less time torpid, compared to animals hibernating at 3 °C. Higher food intake enabled individuals to compensate for increased energetic costs when hibernating at milder temperatures (14 °C vs. 3 °C), to buffer body mass loss and thus increase winter survival. Interestingly, we observed a significant increase of telomere length over the entire hibernation period, irrespective of temperature treatment.ConclusionWe conclude that higher temperatures during winter, if associated with sufficient food availability, can have a positive effect on the individual’s energy balance and somatic maintenance. These results suggest that winter food availability might be a crucial determinant for the survival of the garden dormouse in the context of ever-increasing environmental temperatures.

Variation in gut microbiome structure across the annual hibernation cycle in a wild primate.

The gut microbiome can mediate host metabolism, including facilitating energy-saving strategies like hibernation. The dwarf lemurs of Madagascar (Cheirogaleus spp.) are the only obligate hibernators among primates. They also hibernate in the subtropics, and unlike temperate hibernators, fatten by converting fruit sugars to lipid deposits, torpor at relatively warm temperatures, and forage for a generalized diet after emergence. Despite these ecological differences, we might expect hibernation to shape the gut microbiome in similar ways across mammals. We, therefore, compare gut microbiome profiles, determined by amplicon sequencing of rectal swabs, in wild furry-eared dwarf lemurs (C. crossleyi) during fattening, hibernation, and after emergence. The dwarf lemurs exhibited reduced gut microbial diversity during fattening, intermediate diversity and increased community homogenization during hibernation, and greatest diversity after emergence. The Mycoplasma genus was enriched during fattening, whereas the Aerococcaceae and Actinomycetaceae families, and not Akkermansia, bloomed during hibernation. As expected, the dwarf lemurs showed seasonal reconfigurations of the gut microbiome; however, the patterns of microbial diversity diverged from temperate hibernators, and better resembled the shifts associated with dietary fruits and sugars in primates and model organisms. Our results thus highlight the potential for dwarf lemurs to probe microbiome-mediated metabolism in primates under contrasting conditions.

Translating PUFA omega 6:3 ratios from wild to captive hibernators (Urocitellus parryii) enhances sex-dependent mass-gain without increasing physiological stress indicators

Omega 3 polyunsaturated fatty acids (PUFAs) are well-documented for their influence on health and weight loss. Recent studies indicate omega 3 PUFAs may exert a negative impact on cellular stress and physiology in some hibernators. We asked if physiological stress indicators, lipid peroxidation and mass gain in Arctic Ground Squirrels (AGS) were negatively influenced by naturally occurring dietary omega 3 PUFA levels compared to omega 3 PUFA levels found in common laboratory diets. We found plasma fatty acid profiles of free-ranging AGS to be high in omega 3 PUFAs with balanced omega 6:3 ratios, while standard laboratory diets and plasma of captive AGS are high in omega 6 and low in omega 3 PUFAs with higher omega 6:3 ratios. Subsequently, we designed a diet to mimick free-range AGS omega 6:3 ratios in captive AGS. Groups of wild-caught juvenile AGS were either fed: (1) Mazuri Rodent Chow (Standard Rodent chow, 4.95 omega 6:3 ratio), or (2) balanced omega 6:3 chow (Balanced Diet, 1.38 omega 6:3). AGS fed the Balanced Diet had plasma omega 6:3 ratios that mimicked plasma profiles of wild AGS. Balanced Diet increased female body mass before hibernation, but did not influence levels of cortisol in plasma or levels of the lipid peroxidation product 4-HNE in brown adipose tissue. Overall, as the mass gain is critical during pre-hibernation for obligate hibernators, the results show that mimicking a fatty acid profile of wild AGS facilitates sex-dependent mass accumulation without increasing stress indicators.

Characterization of captive and wild 13-lined ground squirrel cecal microbiotas using Illumina-based sequencing

BackgroundHibernating animals experience extreme changes in diet that make them useful systems for understanding host-microbial symbioses. However, most of our current knowledge about the hibernator gut microbiota is derived from studies using captive animals. Given that there are substantial differences between captive and wild environments, conclusions drawn from studies with captive hibernators may not reflect the gut microbiota’s role in the physiology of wild animals. To address this, we used Illumina-based sequencing of the 16S rRNA gene to compare the bacterial cecal microbiotas of captive and wild 13-lined ground squirrels (TLGS) in the summer. As the first study to use Illumina-based technology to compare the microbiotas of an obligate rodent hibernator across the year, we also reported changes in captive TLGS microbiotas in summer, winter, and spring.ResultsWild TLGS microbiotas had greater richness and phylogenetic diversity with less variation in beta diversity when compared to captive microbiotas. Taxa identified as core operational taxonomic units (OTUs) and found to significantly contribute to differences in beta diversity were primarily in the families Lachnospiraceae and Ruminococcaceae. Captive TLGS microbiotas shared phyla and core OTUs across the year, but active season (summer and spring) microbiotas had different alpha and beta diversities than winter season microbiotas.ConclusionsThis is the first study to compare the microbiotas of captive and wild rodent hibernators. Our findings suggest that data from captive and wild ground squirrels should be interpreted separately due to their distinct microbiotas. Additionally, as the first study to compare seasonal microbiotas of obligate rodent hibernators using Illumina-based 16S rRNA sequencing, we reported changes in captive TLGS microbiotas that are consistent with previous work. Taken together, this study provides foundational information for improving the reproducibility and experimental design of future hibernation microbiota studies.

Gut microbiome is affected by gut region but robust to host physiological changes in captive active-season ground squirrels

BackgroundThirteen-lined ground squirrels (Ictidomys tridecemlineatus) are obligate hibernators and are only active 4–5 months annually. During this period, squirrels rapidly acquire fat for use during hibernation. We investigated how the gut microbiome changed over the active season in the mucosa and lumen of two gut sections: the cecum and ileum. We sequenced the 16S rRNA gene to assess diversity and composition of the squirrel gut microbiome and used differential abundance and network analyses to identify relationships among gut sections.ResultsMicrobial composition significantly differed between the cecum and ileum, and within the ileum between the mucosa and lumen. Cecum mucosa and lumen samples did not differ in alpha diversity and composition, and clustered by individual squirrel. Ileum mucosa and lumen samples differed in community composition, which can likely be attributed to the transient nature of food-associated bacteria in the lumen. We did not detect a shift in microbiome diversity and overall composition over the duration of the active season, indicating that the squirrel microbiome may be relatively robust to changes in physiology.ConclusionsOverall, we found that the 13-lined ground squirrel microbiome is shaped by microenvironment during the active season. Our results provide baseline data for new avenues of research, such as investigating potential differences in microbial function among these physiologically unique gut environments.

Development of metabolic inflammation during pre-hibernation fattening in 13-lined ground squirrels (Ictidomys tridecemlineatus).

Obesity is a worldwide pandemic with significant comorbidities. It is often accompanied by mild inflammation of the intestine followed by inflammation of metabolic tissues such as liver, adipose tissue, and skeletal muscle. Several laboratory models of obesity exist, but seasonal models like hibernators may be valuable for understanding the pathogenesis of obesity independent of genetic or high-fat diet-induced changes. As part of their annual cycle, obligate hibernators, like the 13-lined ground squirrel (Ictidomys tridecemlineatus), undergo a rapid shift from a lean to an obese state to store energy in the form of fat for their prolonged winter fast. Here, we show that ground squirrels gained mass steadily throughout the active season despite a drop in energy intake starting around 9weeks post-hibernation. Glucose tolerance tests revealed a significant decrease in tolerance late in the active season. In visceral adipose, we found increases in adipocyte size, tumor necrosis factor (TNF)-α and interleukin (IL)-6 levels. IL-6 levels also increased in liver and muscle and TNF-α increased in the ileum late in the active season. Levels of the anti-inflammatory cytokine, IL-10, decreased in visceral adipose and colon tissues around the same time. These data suggest metabolic inflammation develops along with adiposity late in the squirrels' active season.

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.

Elevated ambient temperature accelerates aspects of torpor phenology in an obligate hibernator.

The thirteen-lined ground squirrel (Ictidomys tridecemlineatus) is assumed to be an obligate hibernator - commencing and terminating hibernation on a circannual rhythm, regardless of environmental conditions - but, until now, this assumption had never been fully tested. We housed three groups of captive-born ground squirrels from Aug. 2017 to Aug. 2018 under constant photoperiod (12hL:12h D) at 5, 16 or 25°C, and monitored hibernation using body temperature loggers. At 5 and 16°C all animals hibernated from autumn to spring with no differences in date of first/last torpor or duration of interbout euthermic periods (IBE), but torpor bout duration was 25% shorter at 16°C. One of 4 animals housed at 25°C did not hibernate. For the other three 25°C animals, the first torpor date did not differ from the other groups, but the last torpor bout (5 Feb.) occurred almost 8 weeks earlier. These animals aroused from torpor more frequently and IBE lasted significantly longer, so the total time spent torpid was less than 50% of the other groups. Unlike the 5 or 16°C animals, 25°C animals re-entered torpor in late spring 2018. Taken together these data suggest that this species is an obligate hibernator, but that high ambient temperatures can accelerate the endogenous circannual hibernation rhythm.

Age and location influence the costs of compensatory and accelerated growth in a hibernating mammal

Abstract The increase of structural growth rates to compensate for a poor initial body condition, defined as compensatory growth, may have physiological costs, but little is known about its effects on individual fitness in the wild. Yellow-bellied marmots (Marmota flaviventer) are obligate hibernators and depend on fat accumulation acquired during an approximately 4-month summer to survive overwinter. We investigated the costs of survival and longevity of rapid growth in a wild population of yellow-bellied marmots. We used trapping data collected from 2002 to 2014 to calculate individual relative seasonal growth and assess its effects on longevity and annual survival of juveniles, yearlings, and adults. Sites were distributed in two main areas, down-valley and up-valley; the latter has a higher elevation and is an overall harsher environment. We found that relative seasonal growth had no effect on individual longevity or on juvenile and adult annual survival. For yearlings, the effect of relative seasonal growth on survival depended on the location: yearlings with high relative seasonal growth had lower survival if located up-valley, and higher survival if located down-valley. In conclusion, juveniles and adults do not appear to have detectable costs of rapid growth, although there are costs to yearling survival depending on environmental conditions. Substantial structural growth occurs when marmots are yearlings and our results are likely driven by the high conflicting demands of somatic growth versus maintenance at this stage. Thus, the costs of rapid growth are age and site dependent and may be seen in the short term for species facing temporal constraints on growth.

Environmental temperature effects on adipose tissue growth in a hibernator.

Obligate hibernators express circannual patterns of body mass and hibernation, which persist under constant laboratory conditions. Brown adipose tissue (BAT) is important for thermogenesis during arousals from hibernation, whereas white adipose tissue (WAT) serves as energy storage and thermal insulation. The goal of this study was to investigate the effects of environmental temperature on BAT and WAT. We hypothesized that changes to environmental temperature would not influence the pattern of mass gain or BAT and WAT volume in the thirteen-lined ground squirrel (Ictidomys tridecemlineatus). To test this, we housed animals at thermoneutral 25°C (warm-housed) or 5°C (cold-housed), with the same photoperiod (12 h light:12 h dark) over an entire year. Throughout the year we measured the volume and water:fat ratio of WAT and BAT using magnetic resonance imaging (MRI). We found no evidence of torpor in the warm-housed animals, indicating that this species might not be an obligate hibernator, as previously assumed. Regardless of ambient temperature, BAT volume increased prior to winter, then decreased in late winter with no change in water:fat ratio. By contrast, both body mass and WAT volume of cold-housed animals declined throughout the winter and recovered after hibernation, but thermoneutral housing produced no circannual pattern in body mass, even though WAT volume declined in late winter. Cold exposure appears to be a primary regulator for WAT but BAT may exhibit an endogenous circannual rhythm in terms of depot volume.

Perineuronal Net Expression in the Brain of an Obligate Hibernator

The brains of mammalian hibernators are naturally protected. Hibernating thirteen‐lined ground squirrels undergo rapid and extreme changes in body temperature and brain perfusion as they cycle between lengthy torpor bouts and brief periods of euthermia called interbout arousals (IBAs). Arousal from torpor to IBA occurs rapidly, but there is no evidence of brain injury accompanying this transition, despite the rapid and extreme change in physiology. Previous work investigating seasonal gene expression in the hypothalamus and cerebral cortex provided evidence for substantial regional differences in brain structural and functional regulation during hibernation. However, due to the extreme nature of the hibernation phenotype, protection must be a main priority for all brain regions. Interestingly, the hypothalamus showed a significant increase in aggrecan mRNA expression during hibernation, which is a main component of complex extracellular matrix structures called perineuronal nets (PNNs), which are known to be both protective. Hypothalamic function is important for hibernation and PNNs could help promote both protection and continued function during this time. Aggrecan expression was low in the cerebral cortex and did not show a seasonal change, suggesting that this change was specific to the hypothalamus. Here, we examined direct expression of PNNs in the thirteen‐lined ground squirrel brain using fluorescent Wisteria floribunda lectin histochemistry. Expression of PNNs was not found globally throughout the brain, but was found in discrete regions, including areas of the hypothalamus. However, PNN expression was also found throughout the cerebral cortex and other parts of the brain, including areas of the thalamus, epithalamus, and septum, indicating that PNNs are not specific to the hypothalamus. Additionally, PNNs were found in both hibernating (Torpor and IBA) and non‐hibernating (summer) animals, indicating that these extracellular matrix structures are present throughout the year. However, it is currently unknown if the structural components of the PNNs are the same in all brain regions or if these change seasonally. Overall, this work supports the idea that the extracellular matrix is contributing to neuroprotection during hibernation in the brain, but does not support the idea that the presence of PNNs is specific and unique to the hypothalamus during hibernation.Support or Funding InformationThis work is supported by UWL startup funding and a UWL Summer Faculty Research Grant to C.S.