Follicle growth during hibernation in the frog Rana ridibunda in Northern Greece

The seasonal ovarian dynamics from September to June in natural populations of the frog Rana ridibunda in Northern Greece have been studied. A continuous change in the percentage and diameter size of the ovarian follicles during the hibernating season was found. Early vitellogenic follicles reached their maximum percentage in April-May and were lowest in February. A small increase of their percentage during December and January compared to the three previous months indicated continuous growth of the ovarian follicles during hibernation. Advanced vitellogenic follicles reached their maximum percentage in September and were lowest in March. Postvitellogenic follicles appeared from September onwards and their percentage increased continuously until February, except for a small decrease in December and a stable level during January. A negative correlation between body length and percentage of postvitellogenic follicles was found in March.