Male Reproductive Cycle of the European Adder, Vipera berus, and Its Relation to Annual Activity Periods

The spermatogenetic cycle of Vipera berus runs, in Sweden, as in other parts of the species' range, over one entire year with a short refractory period in May or June. The spermatocytogenesis is postnuptial, while the spermiogenesis is prenuptial. The spermiogenesis coincides with the short basking period, the testicular regression phase with the mating period. These two periods are separated by the spring molt of the males, which also seems to be the trigger for the mating activities. The testis weight and the diameter of the seminiferous tubule show maximum values during the winter and basking periods, while the vas deferens shows its maximum diameter during the mating period. The number of Leydig cells is at maximum during the basking and mating periods. The maximum height of the epithelium of the epididymis and renal sex segment are reached during these two periods. There is a tendency towards north, in western Europe, of a concentration of the spermatogenetic activities in the spring. This reflects a trend towards a shorter and more synchronized mating period in climatically extreme environments. The males in these northern populations have a shorter mating period and consequently can spend a comparatively longer period in the feeding areas.