Annual plasma testosterone and thyroxine cycles in relation to hibernation in the edible dormouse Glis glis.

Twelve male edible dormice, captured in the autumn of 1979, were studied for 2 years under natural lighting in an animal enclosure with permanently open windows, at Montpellier, in Southern France (latitude 43 degrees 50' N). Ambient temperature fluctuated annually between +4 and 27 degrees, and the photoperiod between 10L:14D and 16L:8D. Another group of 26 animals, captured in the autumn of 1980, was observed under the same conditions for 1 year. Body weight and plasma testosterone and thyroxine concentrations were measured at monthly intervals under standardized conditions. Hibernation generally extended from October to February but lasted till May in a few cases, and the critical temperature for dormancy varied between 13 and 16 degrees in November and 23 degrees in May. Body weight fluctuated annually according to a biphasic pattern, with two maxima, respectively, in spring and autumn (prehibernation), and two minima, in summer and winter. However, plasma testosterone and thyroxine concentrations displayed clearly monophasic annual cycles, closely correlated, both to each other and to the ambient temperature and photoperiod. Both the testosterone and thyroxine cycles culminated in June-July. Interestingly, the annual ascending phase for both hormonal cycles never preceded the terminal arousal of the animal. Further, whatever the season, 30 degrees was clearly the critical minimal body temperature required for a seasonal increase to occur in plasma testosterone. These annual cycles are discussed in the light of the data acquired recently for hibernators other than the dormouse.