TEMPERATURE CYCLES AS ZEITGEBER FOR THE CIRCADIAN CLOCK OF 2 BURROWING RODENTS, THE NORMOTHERMIC ANTELOPE GROUND-SQUIRREL AND THE HETEROTHERMIC SYRIAN-HAMSTER

The circadian systems of two burrowing rodents, the normothermic diurn al antelope ground squirrel (Ammospermophilus leucurus) and the hetero thermic nocturnal Syrian hamster (Mesocricetus auratus) were compared with respect to entrainment by temperature cycles. Both species were s ubjected to the same ambient temperature (Ta) cycles with amplitudes b etween 4 and 12 degrees C at constant illuminations (100 and 0.05 lux in squirrels; 1.0 lux in hamsters). Wheel running activity was continu ously measured. There was considerable interindividual variation in th e daily pattern of wheel-running activity and in the ability to entrai n to Ta cycle of the same amplitude in both species. The activity rhyt hms of about 33 to 67% of the animals of th two species entrained to T a cycles with amplitudes of 6 to 12 degrees C. One of six squirrels an d one of nine hamsters even entrained to Ta cycles of 4 degrees C. In the antelope ground squirrels, activity occurred predominantly in the cooler phase of the Ta cycle, whereas hamsters were mainly active duri ng the warmer phase. In some squirrels, the activity rhythms were spli t in two main components which were both entrained to the cooler fract ion of the Ta cycle, sometimes with additional (masking) activity duri ng the warmer fraction (above 30 degrees C). The results do not suppor t the earlier view that temperature cycles affect the circadian system s of heterothermic mammals, including hibernators, more strongly than those of normothermic species. It is suggested that behavioral and phy siological adjustments to the environmental conditions play an importa nt role for mammalian circadian systems to respond to temperature chan ges as a zeitgeber.