Krüll Frans
Institut für Zoologie der Universität Erlangen-Nürnberg, Lehrstuhl II, Bismarckstr. 10, D-8520, Erlangen, Federal Republic of Germany.
Oecologia. 1976 Jun;24(2):141-148. doi: 10.1007/BF00572755.
The synchronizing effect of the azimuth-position of the sun on the period of locomotor activity of animals in high arctic summer was studied. Different species of finches served as test animals. In a test room a bulb with reflector was turned around a cage once in 24 h. Just like the sun in high arctic summer over the skyline, this bulb stayed all the time at the same height over the floor. The light beam was casted downwards at a slant angle in the direction of the cage. With this "circling sun" it was mostly possible to synchronize locomotor activity of the finches with the rotation of the earth. At definite times of the year however several birds had a free running activity period, despite of the circling of the "sun". It is likely that endogenous annual periodicity (oscillations of sex hormone level) is responsable for the changing effectiveness of this weak Zeitgeber.
