Daily Torpor in the Laboratory Mouse, Mus musculus Var. Albino

Abstract

The body temperature (Tre) of Mus musculus fed ad libitum was labile and varied between 36.0 ± 0.3 and 39.3 ± 0.3 C at ambient temperatures (Ta) ranging from 2.5 to 38.0 C. The thermal neutral oxygen consumption was 1.47 ml O2/g·h, which occurred between the lower critical temperature of 31.0 C and the upper critical temperature of 35.0 C. The thermal conductance decreased from a high of 0.26 ml O2/g·h·C to a low of 0.15 ml O2/g·h·C as the Ta decreased from 31.0 to 2.5 C. The conductance increased from the lower to the upper limits of the thermal neutral zone, reaching a maximum of 0.43 ml O2/g·h·C at a Ta of 35.0 C. The highest conductance was 0.74 ml O2/g·h·C at a Ta of 38.0 C. The lability of Tre increased with a restriction of the daily food ration, averaging 34.5 ± 0.3 C and ranged between 22.0 and 37.0 C at Ta's of 19–20 C. Five of 14 animals on ad libitum food allotments became torpid within 24 h after food was withheld. Heat production appeared to be suppressed during entry into torpor, which was attributed to a reduction in shivering. The "critical Tre" of torpid mice was 16–19 C. Oxygen consumption was directly related to Tre at a Tre of 31 C and above, but it remained relatively constant at Tre's between 31 and 16 C. The Tre – Ta gradient and oxygen consumption of torpid mice increased at T's below 16 C. The length of the torpor period was inversely related to Ta between 23 and 16 C but was frequently interrupted with spontaneous arousals at a Ta below 16 C. During arousal, the oxygen consumption at a particular Tre was greater than at the corresponding Tre during entry into torpor. Arousal was frequently accompanied by a metabolic peak, followed by a decrease of Tre and a decline in oxygen consumption. The arousal rate ranged between 0.11 C and 0.25 C/min and appeared to be independent of the Ta at which arousal took place. Torpid animals could not right themselves, stand, and shiver until their Tre reached 18 C. They could gather and ingest seeds at a Tre of 24 C and behaved similarly to normothermic animals when the Tre was above 26 C.