Effect of the Zeitgeber pattern on the resynchronization behaviour of dark-active mammals

The activity of 12 Egyptian fruit bats, Rousettus aegyptiacus, was synchronized by 24-h illumination cycles varying in light/dark (L/D) ratios, and the effect of these ratios on the resynchronization following phase shifts of the Zeitgeber signal (+8 h and -8 h) was observed. The resynchronization time in L/D:16/8 was the same as that in L/D:12/12, but that in L/D:8/16 was significantly increased. After an advance-shift, some of the bats (2 of 6 in L/D:16/8 and 6 of 12 in L/D:8/16) resynchronized 'antidromically', by lengthening the activity period. The tendency toward antidromic resynchronization appears to increase with the age of the animal. Two ways of interpreting these results are: (1) as a consequence of a reduction in the strength of the Zeitgeber signal due to the altered L/D ratio; and (2) in terms of a modification of the phase response. Among the implications for human circadian periodicity is the possibility that resynchronization following long west-east flights at high latitudes might be affected by the time of year.