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%.