When a Richardson's ground squirrel hibernates and enters the physiological state of torpor, body temperature declines and heart rate, respiration and metabolism slow dramatically, allowing the ground squirrel to conserve energy.
Ground squirrels spend 85-92% of hibernation in the physiological state of torpor; the remainder is spent in brief periods of energetically expensive euthermy when body temerature is at 37°C. Torpor bouts last about 4-6 days in July and August, then steadily become longer as soil temperature declines. By early January, Richardson’s ground squirrels spend 20-25 consecutive days continuously in the state of torpor. Torpor bouts are interrupted by brief periods of rewarming back to normal mammalian body temperature of 37°C. Arousals last less than 24 hours each, and consist of a 2- to 3-hour period of rewarming followed by 12-15 hours of inactivity at normal body temperature. Ground squirrels then cool back down to ambient soil temperature as they enter another torpor bout. data As a general trend, the colder the animal is the longer the torpor bout will be. data
Richardson's ground squirrels survive during hibernation by metabolizing fat reserves built up during the summer. Adult ground squirrels lose more weight during hibernation than juveniles, and juvenile females lose more weight than juvenile males. This pattern corresponds to the duration of hibernation for each cohort. (see Annual Weight Cycle)
The time of emergence in spring tends
to be correlated with periods when soil and air temperatures start to regularly
stay above freezing. Males emerge above ground about 2 weeks before
females. Often these males have terminated torpor a week before they appear
above ground, whereas females terminate torpor only 1 day before they appear
above ground. Thus males terminate torpor in early February when
deep soil temperatures are still very cold. Only males with food
cached in the hibernaculum can afford the energetic expense of staying
warm while sequestered in a cold hibernaculum. The presumed advantage
of this early termination of torpor is that males can undergo testicular
recrudescence and build up a fat reserve prior to emergence and the subsequent
mating season. (see Reproductive Physiology)