MARTIN, T.J.*; BARNES, B.M.: Natural entrainment of circadian rhythms in the Arctic ground squirrel, Spermophilus parryii

Light-sensitive radio collars and abdominally implanted temperature loggers were used in the field to measure daily activity and temperature patterns of the Arctic ground squirrel (Spermophilus parryii) throughout the active season, which includes periods with 24 hours of daylight. Activity and temperature patterns show that animals remain entrained throughout the active season to a regular activity and body temperature cycle. Telemetry data and behavioral observations show that squirrels are not above ground to observe significant celestial light-dark transitions, but expose themselves to approximately 12L:12D by using their burrows. Five days of photoperiodic phase shift experiments with animals held captive had no effect on their entrainment when re-released in the field. Therefore, the classical phase response model for circadian entrainment by light-dark pulses cannot explain how squirrels maintain regular entrainment. The role of other aspects of light, such as light intensity and spectral composition as potential zeitgebers, are presently being investigated.