Proportional effect of light on entrained circadian rhythms of birds and mammals

Summary Circadian rhythms of locomotor activity of two species of finches,Fringilla coelebs andCarduelis chloris, and four rodent species,Mesocricetus auratus, Glis glis, Eutamias sibiricus andFunambulus sp., were studied under the influence of 1212 h light-dark (LD) cycles with different levels of illumination during L and D. The LD intensity-ratio (Zeitgeber amplitude) was approximately constant at different mean levels of light intensity per cycle (Zeitgeber level). Theproportional effect of light, measured as the influence of mean light intensity per cycle (geometric mean) on the phase-angle differences between activity onset and light-on (in day-active species) or light-off (in night-active species) showed large interspecific variability. The variations—in extent and direction—were congruous with the effects of light on the free-running rhythms in the same species suggesting a functional relationship between effects of light on free-running and entrained circadian rhythms.