CIRCADIAN ACTIVITY RHYTHMS IN CHAFFINCHES (FRINGILLA COELEBS) UNDER CONSTANT CONDITIONS

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Abstract
Under constant conditions of temperature and light, with food and water always available and without perturbation by periodic noises, individually caged chaffinches (Fringilla coelebs) show a clear circadian rhythm of locomotor activity. The period [tau] is divided in activity-time [alpha] and rest-time [rho]. Period and activity-time as well as the average amount of activity/ 24 hr. depend on intensity of illumination. With an increase in light intensity from about 0.2 lux to about 5.0 lux, the period becomes shorter and the activity-time longer. The amount of activity increases more than proportionally to the lengthening of activity-time. These results are in agreement with the circadian rule for light-active animals. They are interpreted in view of a special model. This model describes the discontinuous rhythm of activity and rest by a continuous basic oscillation which passes twice during each period through a threshold. The bird is active as long as the oscillation is above the threshold. Changes in the [alpha] : p-ratio, in combination with the changes in amount of activity, are explained as changes in the position of the level of the oscillation relative to the position of the threshold. Level and frequency of the circadian oscillation are positively correlated to each other and to the intensity of illumination.

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