JOURNAL OF BIOLOGICAL RHYTHMS / October 2000Goldman et al. / PH TOPERIOD RESPONSIVENESS IN HAMSTERS Genetic and Environmental Influences on Short-Day Responsiveness in Siberian

Abstract Siberian hamsters are photoperiodic rodents that typically exhibit sev-eral physiological changes when exposed to a short-day photoperiod. However, development of the winter phenotype in short days is largely conditional on prior photoperiod history: Hamsters that have been reared in an exceptionally long day length (18 L) do not usually exhibit the winter phenotype after transfer to short days, whereas animals reared under “moderately ” long days (16 L) are more variable in responsiveness to subsequent short-day exposure, with 20 % to 30 % generally failing to exhibit winter-type responses. Hamsters reared exclu-sively in an “intermediate ” day length (14 L) are almost uniformly responsive to short photoperiod. In the present study, the authors examine the influence of photoperiod history on short-day responsiveness in a breeding line of hamsters that has been subjected to artificial selection for resistance to the effects of short days. The results demonstrate that photoperiod history is an important determi-nant of short-day responsiveness in both random-bred (UNS) hamsters and ani-mals artificially selected and bred for nonresponsiveness to short photoperio