Persistent free-running circannual reproductive cycles during prolonged exposure to a constant 12L:12D photoperiod in laboratory woodchucks (Marmota monax).

Serum levels of gonadal steroid were assayed at approximately 3-month intervals in groups of 5 to 8 male or female woodchucks which were exposed to a natural photoperiod for 1 year as yearlings or 3 years as adults (Study 1), or a constant photoperiod of 12L:12D from birth for 4.5 years (Study 2). After 4.5 years of 12L:12D, food intake was measured in November and compared with that in natural photoperiod animals (Study 3). Other groups of 11 males and 3 females were housed in 12L:12D for 2.5 years after capture at 2 months of age, and gonadal structure and serum steroid levels in November were compared with those of animals at selected times in the normal annual cycle (Study 4). All animals were provided food and water ad libitum and were not induced to hibernate. In Study 1, normal circannual breeding season elevations in testosterone in males and in progesterone in females were detected in most animals maintained in natural photoperiod. In Study 2, similar cycles persisted for 4.5 years in animals exposed to 12L:12D. However, based on quarterly blood samples, obvious asynchrony relative to natural light animals appeared to develop after 2, 3, or 4 years, with apparent free-running intervals of about 10 to 11 months. In Study 3, mean daily food consumption in late autumn for woodchucks in the 12L:12D group was 72% greater than animals in the natural photoperiod. In Study 4, some woodchucks exposed to 12L:12D for only 2.5 years had prematurely increased spermatogenic activity, Leydig tissue development, and elevated serum testosterone levels in November. They were similar in November to those in natural photoperiod animals in March, and significantly greater than those in natural photoperiod animals in November when normal regression and repair of the testis was complete. Likewise, females in the 12L:12D group had luteinized follicles and elevated progesterone in November which were not noted in natural photoperiod animals and which were similar to those observed during the spring in unbred females under normal conditions. The results suggest that circannual cycles of metabolic and reproductive activity in woodchucks persist in the absence of normal changes in photoperiod, are entrained to seasonal changes in the natural photoperiod, and can recede to a periodicity of less than 12 months within 2.5 to 4 years of laboratory maintenance in 12L:12D.