Early outflying and late homeflying in the Indian pygmy bat under natural conditions

The outflying and homeflying activity pattern of a colony of the Indian pygmy bat Pipistrellus mimus occupying a tunnel was studied under natural conditions. Before leaving the tunnel for foraging, the bats made circling flights outside to sample the environmental light conditions. The onset and end of activity was related to the times of sunset and sunrise respectively. The onset of flight during evening commenced after sunset from mid-September to mid-April, before sunset during the rest of the year. Onset of activity occurred at higher light intensities compared to light intensities prevailing during the return flight to the roost in the morning. The duration of activity time showed a curvilinear relationship with the duration of the night. The phase relationship between onset and end of activity, and sunset and sunrise, showed marked seasonal variations in the values of Psi(onset), Psi(end), and Psi(midpoint). However, such changes in the phase angle properties did not obey Aschoff's seasonal rule. Based upon the data obtained on the onset and end of activity patterns for five species of bats, including the data from this study, it is reported that P. mimus is the earliest to set out for foraging in the evening and among the last to return home in the morning. This might be due to dusk and dawn peaks in activity of the prey insects.