Mirror imaging phase response curves obtained for the circadian rhythm of a bat with single steps of light and darkness∗

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Abstract
The circadian rhythm in the flight activity of a tropical microchiropteran bat Taphozous melanopogon responds at all phases with delay phase shifts to single light‐on steps (DD/LL transfers). The circadian rhythm responds at all phases with advance phase shifts to single light‐off steps (LL/DD transfers). Phase shifts were measured from the delays or advances of the onsets of flight activity on days following DD/LL and LL/DD transfers relative to the temporal course of the onsets of activity in controls. The magnitude of the phase shifts was a function of the phases in which the transfers were made. The On‐PRC and Off‐PRC plotted from such data are mirror‐images in their time‐course and wave‐form. The phase shifts of the circadian rhythm in either direction were accompanied by changes in period (for the duration of our recordings after die transfer). The period lengthened following a delay shift and it shortened following an advance shift. The phase shifts are abrupt and discernible in the first cycle after perturbation. There are no transients.