Adaptive Aspects of Activity Rhythms in Bats

Bats living under laboratory or field conditions manifested precise nocturnal activity rhythms. Light-sampling at the light-to-dark transition was apparently the chief means of synchronizing an endogenous, non-24-hour activity rhythm to the daily light cycle.1. Two horseshoe bats, free-living in separate, small recording rooms, readily adjusted the time of activity to correspond to an L:D schedule.2. Three horseshoe bats, roosting in a darkened tunnel with no direct view of the 12L:12D schedule of the outside room, anticipated the light-dark change by flying out regularly during the hour before the lights were turned out, then were active for about 8 hours.3. A colony of Myotis myotis exited from a church loft at an average light intensity of 0.05 lux and returned at dawn. Attempted darkening of the church did not result in earlier light-sampling. Illumination of the flight path at the normal exit time resulted in intense sampling but prevented the actual departure of most of the bats.