WHEEL-RUNNING RHYTHMS IN ARVICANTHIS-NILOTICUS

Wheel-running behavior of the Nile grass rat, Arvicanthis niloticus, w as studied under a variety of lighting conditions to characterize circ adian rhythms in this species. A series of lighting schedules was used to determine the nature of entrainment, the rates of reentrainment af ter 6-h phase shifts, and the stability of free-running rhythms in con stant light (LL) and constant dark (DD). All 15 individuals showed pea ks of activity around dawn and dusk, 11 were more likely to run when l ights were on and 4 were more likely to run when the lights were off. Three of the latter animals were completely crepuscular, but 1 showed a distinctly different and relatively nocturnal pattern, remaining act ive well into the dark phase. All 7 animals that were exposed to 6-h p hase advances and delays reentrained to both shifts within 9 days. Sta ble and precise free-running rhythms were exhibited by all animals in LL and DD, and periods were longer in LL than no. The ratio of the act ive to the inactive phase was significantly higher in LL than in DD or LD conditions. We conclude that A. niloticus exhibit predominantly di urnal running rhythms, with peaks of activity occurring around dawn an d dusk, and that their rhythms are stable and precise and respond rapi dly and predictably to changes in lighting conditions. (C) 1997 Elsevi er Science Inc.