Alteration of growth, advancement of puberty, and season-appropriate circannual breeding during 28 months of photoperiod reversal in woodchucks (Marmota monax).

Abstract

In woodchucks (Marmota monax), as in other marmotine rodents, endogenous circannual metabolic and reproductive cycles persist and free-run at intervals of 10-12 mo for up to 4 yr in a neutral photoperiod of 12L:12D. The annual metabolic cycle is not altered by exposure to constant long days or short days for several months. In the wild, changes in food intake cause body weight to peak in late summer and reach nadirs in early spring, and there is a single annual breeding season in late winter. To determine whether normal changes in photoperiod entrain the endogenous circannual cycle, aspects of reproduction, food intake, and body weight were studied for 28 mo in eight groups of woodchucks housed in light-controlled rooms. These included four groups of 7-10 males and four groups of 7-10 females exposed to automated and continuous daily changes in photoperiod. The imposed changes in photoperiod resembled native Northern Hemisphere (boreal) changes or Southern Hemisphere (austral) changes in photoperiod. The treatments started at the summer solstice shortly after capture as 2.5-mo-old juveniles or 14.5-mo-old yearling adults. In austral vs. boreal woodchucks, started as juveniles, annual increases in testosterone and testis descent at 5, 14, and 23 mo of study were advanced by 2, 5, and 6 mo, respectively. Annual increases in progesterone at 7, 16, and 25 mo of study were advanced by 4, 5, and 6 mo, respectively. The older woodchucks were significantly but less dramatically affected by austral photoperiod for the first 6 mo of study, and by 8-12 mo were advanced the same as the younger animals. At 16-17 and 26-27 mo of photoperiod reversal, 16 austral females had normal pregnancy rates (78%) and produced normal-sized litters (4.3 +/- 0.2) that were advanced by 5 and 6 mo, respectively, relative to those of boreal controls. Body weights in austral vs. boreal juveniles were reduced at 3-5 mo of study but not in older woodchucks. At 1 and then 2 yr of austral lighting, peaks in body weight were advanced by 4 and then 5 mo relative to boreal controls in young animals, and by 2 and then 5 mo in older animals. Between 1 and 2 yr of austral lighting in both age groups the nadirs in food intake (45 g per day) were about 16-25% of peak food intake 6-8 mo earlier, as in boreals, but advanced by 5-6 mo relative to boreals.(ABSTRACT TRUNCATED AT 400 WORDS)