Circadian Rhythm of Metabolic Rate in Pocket Mice

Perognathus longimembris, under a variety of conditions, demonstrated essentially a 24-hr rhythm in oxygen consumption. The amplitude, correlation with light cycle, and duration of periods of high and low metabolism varied. In September, mice provided with food and held at 22–24° C had a rhythm closely synchronized with a 12-hr light cycle (high in dark); mice were hypometabolic 12.5% of the days. In May, mice were more often out of synchrony and were hypometabolic 32.5% of the days. Without food at 22° C, mice became torpid every day; the first period of torpor began in the dark, and the rhythm of oxygen consumption was thereafter reversed. At 10° C without food, mice failed to arouse to a normal metabolic rate every day; sometimes they were deeply hypometabolic for 48–56 hr. A circadian rhythm of reduced amplitude continued, however, and arousals to normal rate occurred approximately on multiples of 24 hr. Arousals continued to occur in mice without food at 10° C despite exposure to or state of animals under the following conditions: constant darkness with or without sound, air with 4.5% CO2, obesity, 9 months' prior conditioning to 10° C, 1,400 R acute whole-body gamma radiation. Mice acclimatized to 10° C had the shortest periods of normal metabolism and obese mice the longest. In P. longimembris (mean wt 10.7 g), P. inornatus (14.6 g) and P. formosus (20.5 g) at 10° C without food, frequency and extent of hypometabolism varied inversely with body weight.