Seasonal adaptations of Siberian hamsters. II. Pattern of change in daylength controls annual testicular and body weight rhythms.

Studies of rodent photoperiodism almost without exception have employed fixed day lengths (DLs) and abrupt transitions from long to short DLs. Because the natural progression of changes in DL carries predictive information and may have physiological consequences, we determined seasonal fluctuations in testis size and body weight in Siberian hamsters maintained in a year-long pattern of increasing and decreasing DLs. A cycle of gonadal development, regression, and recrudescence, and corresponding changes in body weight, were observed in hamsters maintained in separate simulated natural photoperiods (SNPs) in which DLs neither fell below nor exceeded the putative critical DL of 13 h. Gradually decreasing DLs as long as 15.3 h induced gonadal regression, and DLs as short as 12.3 h supported ponderal growth, depending on the hamster's prior photoperiodic history. DLs experienced by hamsters during development influenced the adult incidence of responsiveness to short DLs. Photorefractoriness to short DLs occurred earlier in hamsters kept in static 10-h than in 12-h DLs. Increasing DLs in winter had little impact on the rate of gonadal recrudescence and weight gain. These data extend earlier investigations in showing the photoperiodic history determines gonadal responses over a broad range of DLs and influences gonadal responsiveness to short DLs and the triggering of the interval timer underlying recrudescence.