Microbial urea-nitrogen recycling in arctic ground squirrels: the effect of ambient temperature of hibernation.

Energy conservation associated with hibernation is maximized at the intersection of low body temperature (T), long torpor bouts, and few interbout arousals. In the arctic ground squirrel (Urocitellus parryii), energy conservation during hibernation is best achieved at ambient temperatures (T) around 0 °C; however, they spend the majority of hibernation at considerably lower T. Because arctic ground squirrels switch to mixed fuel metabolism, including protein catabolism, at extreme low T of hibernation, we sought to investigate how microbial urea-nitrogen recycling is used under different thermal conditions.

Stable isotope analysis of CO in breath indicates metabolic fuel shifts in torpid arctic ground squirrels.

Stable carbon isotope ratios (δC) in breath show promise as an indicator of immediate metabolic fuel utilization in animals because tissue lipids have a lower δC value than carbohydrates and proteins. Metabolic fuel consumption is often estimated using the respiratory exchange ratio (RER), which has lipid and carbohydrate boundaries, but does not differentiate between protein and mixed fuel catabolism at intermediate values. Because lipids have relatively low δC values, measurements of stable carbon isotopes in breath may help distinguish between catabolism of protein and mixed fuel that includes lipid.

A test of alternative models for increased tissue nitrogen isotope ratios during fasting in hibernating arctic ground squirrels.

We describe two models explaining the increase in tissue nitrogen isotope ratios (δ(15)N) that occurs during fasting in animals. The catabolic model posits that protein breakdown selectively removes the lighter isotope of nitrogen ((14)N) from catabolized tissues, causing an increase in the proportion of heavy nitrogen isotope ((15)N). The anabolic model posits that protein synthesis during fasting results in elevated δ(15)N values, as the unreplaced loss of (14)N to urea results in a higher proportion of (15)N in plasma amino acids used for protein synthesis.

Estimating lean mass over a wide range of body composition: a calibration of deuterium dilution in the arctic ground squirrel.

Calculating body water through isotope dilution has become a useful way to nondestructively estimate body composition in many species. The most accurate estimates using this method require calibration against proximate chemical analysis of body composition for individual species, but no studies to our knowledge have calibrated this method on a hibernating mammal that seasonally undergoes dramatic changes in body composition. We use deuterium oxide to estimate total body water in captive arctic ground squirrels, Urocitellus parryii, and compare two approaches of calculating lean mass from total body water, both calibrated against lean mass based on proximate analysis.

Hibernating above the permafrost: effects of ambient temperature and season on expression of metabolic genes in liver and brown adipose tissue of arctic ground squirrels.

Hibernating arctic ground squirrels (Urocitellus parryii), overwintering in frozen soils, maintain large gradients between ambient temperature (T(a)) and body temperature (T(b)) by substantially increasing metabolic rate during torpor while maintaining a subzero T(b). We used quantitative reverse-transcription PCR (qRT-PCR) to determine how the expression of 56 metabolic genes was affected by season (active in summer vs hibernating), metabolic load during torpor (imposed by differences in T(a): +2 vs -10°C) and hibernation state (torpid vs after arousal). Compared with active ground squirrels sampled in summer, liver from hibernators showed increased expression of genes associated with fatty acid catabolism (CPT1A, FABP1 and ACAT1), ketogenesis (HMGCS2) and gluconeogenesis (PCK1) and decreased expression of genes associated with fatty acid synthesis (ACACB, SCD and ELOVL6), amino acid metabolism, the urea cycle (PAH, BCKDHA and OTC), glycolysis (PDK1 and PFKM) and lipid metabolism (ACAT2).

Phenological variation in annual timing of hibernation and breeding in nearby populations of Arctic ground squirrels.

Ecologists need an empirical understanding of physiological and behavioural adjustments that animals can make in response to seasonal and long-term variations in environmental conditions. Because many species experience trade-offs between timing and duration of one seasonal event versus another and because interacting species may also shift phenologies at different rates, it is possible that, in aggregate, phenological shifts could result in mismatches that disrupt ecological communities. We investigated the timing of seasonal events over 14 years in two Arctic ground squirrel populations living 20 km apart in Northern Alaska.