LIGHT IRRADIANCE AND WAVELENGTH AS SEASONAL CUES FOR DJUNGARIAN HAMSTERS

To examine the effects of different wavelengths of light on the circad ian clock, Djungarian hamsters were maintained under long photoperiod 16 h light and 8 h dark (LD 16:8) and were exposed to white, blue and red light of different irradiances for 5 weeks; one week in constant l ight (LL) to determine the endogenous period (tau) and the activity ph ase (alpha) and 4 weeks in a LD 16:8 photoperiod to look for long-term effects on reproduction. In white and blue light the hamsters followe d ''Aschoff's Rule,'' i.e., the higher the irradiance the longer tau ( Aschoff, 1964). In red light tau remained unchanged up to an intensity of 36 mW/m2 and then increased abruptly to values of 25.2 h. The acti vity phase also changed during LL. Low light intensities decompressed alpha. With increasing light intensity alpha compressed which is equiv alent to shortening the subjective night length. Blue light was most e ffective in compressing alpha, followed by white and red light. Some h amsters became arrhythmic after LL photoperiod, the higher the light i ntensity used, the more often this occurred. Melatonin and serotonin i n the pineal glands were lowered in arrhythmic hamsters in the middle of their dark phase. During four weeks in LD 16:8 photoperiod the test es decreased to 80% of initial size in low light intensities (0.5 mW/m (2)) of white and blue light, and in all irradiances (up to 70 mW/m(2) ) of red light tested. The results show that changes oflight spectrum and intensity may be potent cues enabling Djungarian hamsters to adapt to different seasons.