Seasonal adaptations of Siberian hamsters. I. Accelerated gonadal and somatic development in increasing versus static long day lengths.

Studies of rodent photoperiodism have almost exclusively employed fixed photoperiods with abrupt shifts in day length (DL) of 4-8 h effected in a single day. Because DLs in nature change continuously and incrementally, rates of gonadal and somatic development were compared in the present study in hamsters exposed to gradually increasing vs. fixed DLs. Male hamsters gestated in 12 h light/day (12L) and exposed to DLs that increased gradually from 12L to 15.5L (experiment 1) or from 12L to 16L (experiment 2) weighed more and had larger gonads than did hamsters gestated and maintained from birth in a DL of absolutely greater duration (16L). Gradual increases in DL from 10L to 12L also resulted in more rapid somatic development than did gestation and maintenance in a static 12L photoperiod. The pattern of gradual increases in DL was not sufficient to accelerate development at all DLs but was effective in the range of 12L to 19L. Siberian hamsters are responsive to the pattern of photoperiodic change; the maximally stimulatory photoperiodic stimulus is not a fixed long day, but is instead increasing DLs, even those with absolutely shorter photophases; short gestational DLs potentiate the effects of longer postnatal DLs. The relations heretofore elaborated between fixed DLs and physiological responses differ from those obtained with more natural photoperiodic transitions.