Circadian activity of the red squirrel, Tamiasciurus hudsonicus, in continuous darkness and continuous illumination

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Department of Kinesiology and Integrative Physiology; Department of Biological Sciences


In order to examine the effects of constant conditions on wheelrunning activity, 58 red squirrels (Tamiasciurus hudsonicus ) were taken directly from the field and placed in either total darkness (DD) or continuous illumination (LL) 500 lux, for up to 70 days. The squirrels were sampled at various times of the year so that seasonal changes in the endogenous rhythm could be examined. The initial phasing of activity correlated with the photoperiod in the field and had a free-running period (τ) close to 24 hours. However, τ increased in a nearly linear fashion, averaging about 0.19 min/day for squirrels in DD and about 0.87 min/day for squirrels in LL. All animals in LL had rhythms which eventually became dissociated into two or more components. On the other hand, squirrels kept in DD had running patterns which usually remained intact. After being in constant conditions for between 20 and 70 days, 28 squirrels were subjected to step transitions in illumination from DD to LL (500, 350 or 200 lux) or vice versa. An increase in illumination level generally resulted in a phase advance and a decreased τ. A decrease in illumination level generally resulted in a phase delay and increased τ. The maximum amount of activity occurred at some critical intensity between 200 and 350 lux. The free-running period was directly correlated with the entrainment phase-angle difference. This relationship was compared to the red squirrel's light-pulse response curve. These data support Pittendrigh's (1965) entrainment model.

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© 1975 Swets & Zeitlinger B.V. Publisher’s version of record: https://doi.org/10.1007/BF01451034

Publication Title

International Journal of Biometeorology