Circannual Rhythmicity and Entrainment of Surface Activity in the Hibernator, Perognathus longimembris

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Abstract
Long-term patterns of surface activity were monitored in 15 pocket mice, Perognathus longimembris, which constructed burrow systems in dirt-filled terrariums kept at 16°c and 31°c on 12-hour photoperiods. a rhythm, with an average period of about 10 months, was evident in the behavior of the mice at 16°c. During each cycle, these animals spent several months continuously underground even though food was always available on the surface. The transition between the underground and surface active phases of the cycle was usually gradual. However, the mice immediately resumed nightly foraging if the upper layers of the soil were heated each day during this transition time. They did not respond to this stimulus during the middle of the underground phase of the annual cycle. I suggest that in nature the vertical uniformity of soil temperatures in the spring may cue the synchronization of the circannual rhythm with the seasons. The absence of surface activity is apparently unrelated to torpor because mice at 31°c, where torpor does not occur, also spent several months continuously underground. However, the regular 10-month periodicity of the behavioral cycle was disrupted in this hot environment. Torpor was also monitored for up to 20 months in 25 mice at 8°c and 18°c under 9-hour and 15-hour photoperiods, but circannual rhythms of torpor were not evident under these experimental conditions.