Monophasic and diphasic patterns of the circadian caecotrophy rhythm of rabbits

The circadian caecotrophy rhythm was synchronized with the light-dark cycle of 12 : 12 h. During this the rabbits practised caecotrophy regularly during the light period. While most rabbits manifested 1 caecotrophy per 24 h (monophasic caecotrophy), some had an additional caecotrophy during the dark period (diphasic caecotrophy). During continuous light the circadian caecotrophy rhythm ran free monophasically, even in those rabbits which were diphasic under the preceding 12 : 12 regime. The average period length amounted to 24.7 +/- 0.3 h. Following restoration of the 12 : 12 routine animals reestablished their original caecotrophy pattern. In a further test the caecotrophy pattern remained constant during a constant 12 : 12 regime, but changed in 7 of 16 animals when the photoperiod was reduced first to 60 min and then to 2 x 60 min light every 24 h. The reduction of the lit time resulted in an increased occurrence of diphasic animals. Details of synchronization of the caecotrophy rhythm with the different light-dark schedules are given. These results accord with data obtained in nocturnal rodent species.