'Rigid' internal timing in the circadian rhythm of flight activity in a tropical bat

The timing of the emergence activity of a colony of the bat Taphozous melanopogon occupying a cave environment in Madurai (lat. 9°58' N, long. 78°10' E) and consisting of approximately 150-180 animals of both sexes was studies at 10-day intervals for one year. Even though the time of sunset during the eyar varied over a range of 41 min, the bats displayed an astonishingly 'rigid' time of emergence that was restricted to a narrow 'gate' of 16 min from 18:25 to 18:41 h. The timing of the termination of the activity was less precise in all seasons. Light/dark cycles are clearly the Zeitgeber. Even so, the bats began flying when it was very dark (0.1 lx) during shorter days, and they flew out as the sun was disappearing in the evening horizon (50 or more lx) during long days. Evidently there is no invariant, 'fixed' lower threshold intensity that uniformly elicits the onset of activity throughout the year as reported earlier by other authors (Aschoff, 1969; Erkert, 1974). It is proposed that the bats undergo a systematic seasonal shift in the threshold sensitivity to light in response to the photic Zeitgeber of the environment. The result is a remarkable precision in terms of the hours of the civil/calendar day in the emergence of Taphozous melanopogon.