Time patterns and metabolic rates of natural torpor in the Richardson's ground squirrel

Abstract

Using radiotelemetry for body-temperature sensing in field animals, torpor season in the Richardson's ground squirrel (Spermophilus richardsonii) was found to commence from mid-July in adults and mid-September in juveniles and to terminate for both groups in mid-March. The duration of torpor averaged 3 and 4.3 days in July and August and gradually lengthened to 19.1 days in January, after which it shortened to 14.2 and 6.0 days in February and March. The duration of intertorpor homeothermy ranged from 300 to 1500 min. The metabolic costs associated with entry into torpor, during torpor, arousal from torpor, and intertorpor were quantified in the laboratory at ambient temperatures comparable with those observed in burrow. Applying these metabolic data to the time budget for torpor in field animals, the estimated seasonal energetic costs for entry into torpor averaged 12.8% of total, deep torpor 16.6%, arousal from torpor 19.0%, and intertorpor 51.6%. The monthly energy savings by utilization of torpor ranged from 38.8% in July and 51.2% in March to between 81.6 and 96.0% between August and February. For the whole torpor season, which extended 8 months, the energy savings amounted to 87.8%.

Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels

“…Despite their substantial energetic cost (9,35,66), periodic arousals are ubiquitous among mammalian hibernators, and numerous hypotheses have been advanced to explain their functional significance (71). In most instances, arousals are viewed as emerging from constraints imposed on animals that sustain low T b s for extended intervals; the periodic normothermia is viewed as necessary for removal of this, as yet to be identified, constraint.…”

“…They sustain elevated T b s for Ͻ1 day, after which they return to hibernation (21,53,71). Up to 80% of a hibernating mammal's winter energy budget is consumed during these periodic arousals (9,35,66), and the consequent depletion of energy stores may contribute to overwinter mortality (71). The functional significance of periodic arousals has proven enigmatic: hypotheses that arousals are required to clear metabolic waste (16), replenish blood glucose (20), restore cellular electrolyte balance (17), regenerate the gonads (1), prevent muscular atrophy (68), or eliminate sleep debt (10,56) have been proposed.…”

Are long chain acyl CoAs responsible for suppression of mitochondrial metabolism in hibernating 13-lined ground squirrels?

“…In this way hibernators are able to conserve ~90% of the energy expended by their homeothermic counterparts (Wang, 1979). During torpor, metabolic rate (MR, approximated by whole animal oxygen consumption) is reduced to ~5% of basal metabolic rate (BMR ;Geiser, 2004), and T b subsequently falls, and is maintained a few degrees above ambient temperature.…”

A simple molecular mathematical model of mammalian hibernation

“…A striking feature of IBAs is that they often decrease in frequency until a steady, near-periodic pattern of torpor and arousal is attained (for example, see Boyer and Barnes (1999)); often the frequency of IBAs increases near the end of hibernation as well (Boyer and Barnes, 1999;Epperson and Martin, 2002;Hut et al, 2002a). IBAs are very energetically expensive, accounting for about 70% of the energy used during a hibernation season (Wang, 1979).…”