Entrainment by different environmental stimuli in the frugivorous bats from the Lonar crater

The timings of emergence at dusk and return at dawn of a colony of frugivorous bats Rousettus leschenaulti, roosting in a temple ruin of the Lonar crater (19.97°N, 76.52°E), were investigated at 10-day intervals for one year. The onset of emergence occurred about 18 min after sunset throughout the year irrespective of the prevailing light intensity or temperature but the end of returning activity occurred at a fixed light intensity of about 4 lux irrespective of the time of sunrise or prevailing temperature. Apparently, the evening oscillator controlling the onset of emergence was set by the sunset, i.e. the lights-off stimulus of the natural light - dark cycles, while the morning oscillator controlling the end of activity was set by a certain invariant threshold intensity of the dawn twilight.