Seasonal changes in the precision of the circadian clock of a tropical bat under natural photoperiod.

The emergence and returning activity patterns of a colony of microchiropteran bats Hipposideros speoris, under natural light-dark cycles keep pace with the timings of sunset and sunrise respectively. The onset of emergence flight occurred at different environmental twilight intensities which vary over the seasons. The seasonal changes of phase relationship between the onset and end of flight activity to sunset and sunrise respectively are discussed. As a result ψ-onset, ψ-end, and ψ-midpoint all undergo marked seasonal variations and the values obtained are well in accordance with the seasonal rule of Aschoff. The changes in the timing of onset and end of activity reflect the changes in the duration of activity time of the colony. The activity time is positively correlated with the duration of night time. The possible involvement of the 'non-parametric' action on the entrainment of this colony is briefly discussed.