Testicular regression and recrudescence without subsequent photorefractoriness in Siberian hamsters.

Adult male Siberian hamsters transferred from 16 h light/day (16L) to 14 h light/day (14L) underwent gonadal regression and recrudescence over the course of 24 wk; the duration of reproductive quiescence was shorter by 10 wk than in hamsters transferred from 16L to 10L. A decrease in day length (DL) at week 12 from 14L to 10L greatly extended the duration of testicular involution, whereas transfer at week 12 from 10L to 8L was without effect. Hamsters that had manifested gonadal regression and recrudescence in 14L immediately initiated a second regression when challenged with 10L. Intermediate DLs induce gonadal regression and recrudescence without rendering hamsters refractory to shorter DLs. Intermediate DLs sufficient to initiate gonadal regression may be too long to trigger the interval timer that eventually induces refractoriness to shorter DLs and may be successively interpreted as short and long days, respectively, by Siberian hamsters.