Group Hibernation Does Not Reduce Energetic Costs of Young Yellow‐Bellied Marmots

iMarmot: an integrative platform for comparative and functional genomics of marmots

Background

Marmots are large Holarctic rodents with unique biological features, making them potential animal models in various research fields. Due to the rapid accumulation of the genetic data in marmots, a highly integrative database is urgent needed.

Description

iMarmot is freely available on the web at http://www.marmotdb.org/ and currently contains the biological information of 14 marmots, genomic sequence of 6 marmots, syntenic relationship and orthologs among 3 marmots, and expression profiles of several hibernators and plague hosts. To assist with the genomic and transcriptomic analysis, we also integrated a set of analysis and visualization tools, such as KEGG or GO enrichment analysis, PCA, Blast, Muscle, GeneWise, Lastz, and JBrowse. Particularly, one DEGs (differentially expressed genes) module has been implemented in this database to visualize the gene expression changes in hibernators and plague hosts.

Conclusion

This database will provide comprehensive information and analysis platform for researchers interested in understanding the biological features of marmots.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Hibernation strategies and patterns in sympatric arctic species, the Alaska marmot and the arctic ground squirrel

We compared patterns of core body temperature ( Tb ) change, including inter-individual synchrony, in 2 free-living arctic hibernators that differ in size and sociality, the Alaska marmot ( Marmota broweri ) and the arctic ground squirrel ( Urocitellus parryii ). We report overwinter Tb changes from 3 to 4 marmots from the same hibernaculum in each of 3 years and from 7 ground squirrels that hibernated at 2 nearby burrow sites in 1 year. Very close synchrony in the timing of torpor and arousal cycles in Alaska marmots indicates social hibernation and thermoregulation, while lack of synchrony in arctic ground squirrels further confirms solitary hibernation. The mean duration between the first and last marmot measured within the group to initiate an arousal was 3.7±2.5h and to recool to 30°C during torpor entrance was 5.7±3.7h. The minimum Tb recorded in marmots was 0.6°C and in ground squirrels was −2.0°C. Marmots entering torpor displayed an interrupted pattern of Tb change defined by 2 distinct rates of cooling, early and late during entry, that differed by 21-fold. Ground squirrels cooled in a continuous pattern, initially 3-fold slower than marmots during rapid cooling but 4-fold faster during slow cooling. Both species must minimize energy expenditure to survive long arctic winters; our results suggest that Alaska marmots do this through social thermoregulation, while arctic ground squirrels decrease Tb below freezing to minimize the difference between body and ambient temperatures.

Winter weather versus group thermoregulation: what determines survival in hibernating mammals?

For socially hibernating mammals, the effectiveness of huddling as a means of energy conservation should increase with group size. However, group size has only been linked to increased survival in a few hibernating species, and the relative importance of social structure versus winter conditions during hibernation remains uncertain. We studied the influence of winter weather conditions, social group composition, age-structure, and other environmental factors and individual attributes on the overwinter survival of hoary marmots (Marmota caligata) in the Yukon Territory, Canada. Juvenile hoary marmot survival was negatively correlated with the mean winter (November to May) Pacific Decadal Oscillation (PDO) index. Survival in older age-classes was negatively correlated with PDO lagged by 1 year. Social group size and structure were weakly correlated with survival in comparison to PDO. The relationship between winter PDO and survival was most likely due to the importance of snowpack as insulation during hibernation. The apparent response of hoary marmots to changing winter conditions contrasted sharply with those of other marmot species and other mammalian alpine herbivores. In conclusion, the severity of winter weather may constrain the effectiveness of group thermoregulation in socially hibernating mammals.

Social thermoregulation and torpor in the Siberian hamster

Social thermoregulation and huddling bring about energy benefits to animals sharing a nest because of the smaller surface-to-volume ratio of a huddle and the higher local temperature in the nest. We tested whether living in groups and huddling affect daily torpor, metabolic rate and seasonal changes in the body mass of a small heterothermic rodent, the Siberian hamster (Phodopus sungorus), housed under semi-natural conditions both singly and in groups of four litter-mates. We predicted that in hamsters housed in groups: (1) synchronized torpor bouts would be longer and deeper than non-synchronized ones but shallower than in solitary hamsters, (2) seasonal variations in metabolic rate would be lower than in solitary hamsters, and (3) the winter decrease in body mass would be smaller in grouped than in singly housed hamsters. We found that group housing led to a smaller decrease in body mass in winter, and affected the length and depth of daily torpor. In group-living hamsters more than 50% of all torpor episodes were synchronized and torpid animals were often found in huddles formed of all cage-mates. The longest and deepest torpor bouts in groups were recorded when all animals in a group entered torpor simultaneously. Although the minimum body temperature during torpor was higher, torpor duration was slightly longer than in solitary hamsters. We did not record significant differences in the body mass-adjusted rate of oxygen consumption between solitary and grouped animals, either in the cold or at the lower critical temperature. We conclude that social thermoregulation enables maintenance of a larger body mass, and thus a larger body fat content, which can ensure better body condition at the beginning of the reproductive season.

One for all and all for one: the energetic benefits of huddling in endotherms

Huddling can be defined as "an active and close aggregation of animals". It is a cooperative group behaviour, permitting individuals involved in social thermoregulation to minimize heat loss and thereby lower their energy expenditure, and possibly allowing them to reallocate the saved energy to other functions such as growth or reproduction. Huddling is especially important in the case of animals faced with high heat loss due to a high surface-to-volume ratio, poor insulation, or living in cold environments. Although numerous experimental studies have focused on the huddling behaviour of a wide range of species, to our knowledge, this is the first attempt to review the various implications of this widely used behavioural strategy. Huddling allows individuals to maximise energy savings by (1) decreasing their cold-exposed body surface area, (2) reducing their heat loss through warming of ambient temperatures surrounding the group, and (3) eventually lowering their body temperature through physiological processes. Huddling provides substantial energy savings and is estimated to reduce energy expenditure by between 6 and 53%. Broad variations in the energetic benefits of huddling depend on the number of individuals and species involved in huddles, the ambient temperatures to which individuals are exposed and the density of the aggregations. It has been shown that huddling individuals have increased survival, a lower food intake, a decreased body mass loss, increased growth rate, reduced water loss, and/or a more constant body temperature together with a significant reduction in metabolic rate. Though huddling has been studied widely, this review reveals the intricacies of this adaptive behaviour.

Social structure and facultative mating systems of hoary marmots (Marmota caligata)

Mate-choice theory predicts different optimal mating systems depending on resource availability and habitat stability. Regions with limited resources are thought to promote monogamy. We tested predictions of monogamy in a social rodent, the hoary marmot (Marmota caligata), at the northern climatic extreme of its distribution. Mating systems, social structure and genetic relationships were investigated within and among neighbouring colonies of marmots within a 4 km(2) valley near Kluane National Park, Yukon, Canada, using 21 microsatellite loci. While both monogamous and polygynous populations of hoary marmots have been observed in the southern reaches of this species' range; northern populations of this species are thought to be predominantly monogamous. Contrary to previous studies, we did not find northern hoary marmot social groups to be predominantly monogamous; rather, the mating system seemed to be facultative, varying between monogamy and polygyny within, as well as among, social groups. These findings reveal that the mating systems within colonies of this species are more flexible than previously thought, potentially reflecting local variation in resource availability.

Energetic Consequences and Ecological Significance of Heterothermy and Social Thermoregulation in Striped Skunks (Mephitis mephitis)

We assessed patterns and energetic consequences of different overwintering strategies, torpor, and social thermoregulation in the striped skunk (Mephitis mephitis) under natural ambient temperature and photoperiod. Striped skunks entered spontaneous daily torpor, with the lowest torpid body temperature (T(b)) reaching 26.0 degrees C, the lowest recorded T(b) for a carnivore. Patterns of daily torpor differed between solitary and grouped skunks: all solitary skunks regularly entered daily torpor, but only some individuals in communal dens employed torpor. When they did, it was shallow and infrequent. Solitary skunks entered torpor on average 50 times (in 120 d) compared with 6 times for grouped skunks. During torpor, solitary skunks had average minimum T(b) of 26.8 degrees C and bout duration of 7.8 h, whereas grouped skunks had average minimum T(b) of 30.9 degrees C and bout duration of 5.4 h. Torpor by solitary skunks occurred during their activity phase, but grouped skunks' shallow torpor bouts were restricted to their diurnal resting phase. On average, grouped skunks experienced lower percent daily fat loss, and they emerged in spring with higher percent body fat of 25.5%. In contrast, solitary skunks emerged in spring with only 9.3% body fat. In conclusion, the use of daily torpor and social thermoregulation in northern populations of striped skunks represent two strikingly different mechanisms to minimize energetic costs and increase individual fitness in response to unfavorable environmental conditions.

Annual lipid cycles in hibernators: integration of physiology and behavior

Mammalian hibernation is a temporary suspension of euthermia allowing endotherms to undergo reversible hypothermia and generate a marked savings in energy expenditure. In most fat-storing hibernator species, seasonal changes in food intake, triacylglycerol deposition, metabolism, and reproductive development are controlled by a circannual clock. In ground-dwelling sciurid rodents (ground squirrels and marmots), for example, energy intake increases during a summer body mass gain phase, and toward the end of this phase metabolic rate also begins to decrease, resulting in a profound increase in lipid deposition as fat. Increased activity of lipogenic hormones and enzymes correspond with this increase. The hibernation mass loss phase begins after the body mass peak in the fall and ends in spring. During this phase, stored lipids are slowly utilized in a programmed manner by undergoing deep torpor or hibernation during which the hypothalamic setpoint for body temperature is typically reduced to just above 0 degrees C. Throughout the hibernation season, bouts of deep torpor are punctuated by periodic arousals in which brown adipose tissue thermogenesis plays a critical role. Lipid oxidation nearly exclusively fuels deep torpor and most of the rewarming process. The fatty acid composition of stored lipids can affect the depth and duration of deep torpor, and saturated fatty acids may be preferentially used during hibernation, whereas polyunsaturated fatty acids may be preferentially retained. Female and underweight male hibernators terminate hibernation in spring when aboveground food becomes available; in contrast, heavier males with sufficient lipid reserves spontaneously terminate hibernation several weeks before females and independent of food availability. Mating occurs shortly after emergence from hibernation, and the lipid cycle begins again with the completion of reproduction. Lipid deposition and mobilization, temperature regulation, reproduction, and circannual timing are intimately interdependent. The unique manner in which they are controlled during the annual cycle, especially lipid reserves, makes hibernators valuable and promising models for research into the mechanisms underlying these processes in all mammals.