Periodicity of recurrent hypothermia during hibernation in the pocket mouse,Perognathus longimembris

Summary

The timing of bouts of torpor was continuously monitored in 36 pocket mice,Perognathus longimembris, for as long as 11 months. Animals were kept at 8 °C or 18 °C, with light cycles of LD 12∶12, 10∶14, 15∶9, or continuous darkness. At 8 °C, torpor duration had a polymodal frequency distribution, and the near absence of bouts of torpor lasting exactly 24, 48, 72, and 96 h divided the bouts into discrete classes (one-day, two-day, etc.). The maximum duration of torpor was 2.5 times greater at 8 °C than 18 °C.

A circadian rhythmicity in the timing of all classes of torpor bouts was indicated because, when entrained to a 24-h photocycle, the intervals between successive arousals were multiples of 24 h. This rhythmicity persisted in continuous darkness, demonstrating its endogenous origin.

There were two different free-running periods (τ) for the torpor rhythm of each animal in constant darkness at 8 °C. In general, series of multi-day torpor bouts had a period less than 24 h, while series of one-day bouts showed a period near to or greater than 24 h. The transition between the two frequencies of the torpor rhythm commonly coincided with a phase shift involving one or more transient cycles.

The general circadian oscillator model of Aschoff and Wever was modified to describe the timing and the changes in duration of torpor. The data may be viewed as arising from an asymmetrical oscillation which is synchronized by the lights-on transition of the photocycle, and whose shape is correlated with the period of the rhythm. The different classes of torpor (one-day, two-day, etc.) were also timed differently with respect to the entraining conditions of a light-dark cycle, and these differences in timing are consistent with oscillator theory.