Long-term entrainment of circannual reproductive and metabolic cycles by Northern and Southern Hemisphere photoperiods in woodchucks (Marmota monax)

Woodchucks were exposed to simulated Northern Hemisphere (boreal) or Southern Hemisphere (austral) natural photoperiods in groups of 17 males and 17-18 females at 20-23 degrees C for 69 mo and examined monthly. Photoentrainment of endogenous cycles was evaluated based on dates of peak body weight, peak testis volume, increased serum testosterone in males, and increased serum progesterone in females. Boreal photoperiods entrained and synchronized annual cycles in 15 of 17 males and in all 17 females; 2 males never entrained and free-ran at 9- to Ii-mo intervals. Austral photoperiods phase-advanced cycles by approximately 6 mo after 2.5 yr in 34 of 35 animals. Four entrained males became refractory after 4 yr, free-running at 6- to 10-mo intervals. Photoentrained boreal animals became phase-advanced by 1 mo during the first 2 yr, and then had 12-mo cycles for 4 yr. In Year 5, on average, boreal cycles included initial testosterone elevations in mid-January (vs. mid-July in australs), parturition in early March (vs. early September in australs), and peak body weight in mid-July (vs. late January in australs). The results confirm that endogenous circannual cycles of woodchucks can be entrained and synchronized for 6 yr by daily changes in photoperiod similar to those of midnorthern latitudes, and can be re-entrained and phase-advanced 6 mo by photoperiods of midsouthern latitudes.