Photoperiodic time measurement in Djungarian hamsters evaluated from T-cycle studies.

We investigated the photoperiodic response to T-cycles (0.5 h of light at intervals ranging from 23.0 h to 25.3 h) of two phenotypes of Djungarian hamsters that either exhibit or lack physiological short-day adjustments under a photoperiod of 9 h light:15 h darkness. Illumination of the same circadian time caused a similar photoperiodic response in both phenotypes. Thus hamsters found to be insensitive under a full short-day photoperiod can exhibit short-day adjustments after exposure to certain T-cycles. Given these results we conclude that the absence of photoperiodic adjustments normally found in short-day-insensitive hamsters results from their atypical entrainment under a full short-day photoperiod. We further suggest that the photoperiodic phenomena seen in Djungarian hamsters cannot be adequately explained by an external coincidence model of photoperiodic time measurement. As a more suitable model, we propose one form of internal coincidence where duration of daily motor activity reflects the phasing of multiple, endogenous oscillators. This conclusion is supported by the close relationship between duration of activity and the photoperiodic response as well as by the observation that a light pulse modulates duration of activity in a phase-dependent manner.