SEASONAL ADAPTATIONS OF SIBERIAN HAMSTERS .2. PATTERN OF CHANGE IN DAY LENGTH CONTROLS ANNUAL TESTICULAR AND BODY-WEIGHT RHYTHMS

Studies of rodent photoperiodism almost without exception have employe d fixed day lengths (DLs) and abrupt transitions from long to short DL s. Because the natural progression of changes in DL carries predictive information and may have physiological consequences, we determined se asonal fluctuations in testis size and body weight in Siberian hamster s maintained in a year-long pattern of increasing and decreasing DLs. A cycle of gonadal development, regression, and recrudescence, and cor responding changes in body weight, were observed in hamsters maintaine d in separate simulated natural photoperiods (SNPs) in which DLs neith er 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 dev elopment influenced the adult incidence of responsiveness to short DLs . Photorefractoriness to short DLs occurred earlier in hamsters kept i n static 10-h than in 12-h DLs. Increasing DLs in winter had little im pact on the rate of gonadal recrudescence and weight gain, These data extend earlier investigations in showing that photoperiodic history de termines gonadal responses over a broad range of DLs and influences go nadal responsiveness to short DLs and the triggering of the interval t imer underlying recrudescence.