[How the circannual sexual cycle of the malefield vole is determined (author's transl)].

Experiments were carried out on male field voles which have been captured in January. The resting sexual state of the animals at this time was determined by examination of the testis and of the genital tract during unilateral castration or by exploratory laparatomy. They were subsequently bred for 30 days in a climatic room, during which time they were fed on winter or spring grass, which has been preserved by freezing. 10 It was observed that a complete return to full sexual activity (from the winter resting state) could be produced by ad libitum winter feeding, provided that the minimum daily intake for each animal was 25 g of fresh forage (80% water content). This effect occured equally well, at constant temperature, in darkness and under natural lighting. The testicular development thus obtained was as important as the one which was observed in wild animals captured in the fields in March. 20 Neither the quality of the forages, nor variation in temperature or photoperiodism had any direct effect on the seasonal sexual cycle at critical periods winter and spring. 30 A caloric restriction of 25 to 35% in the ration inhibited the sexual development. 40 It is suggested that the circannual reproductive cycle of the vole is determined by the decrease in the quantity of food which is available in the fields in winter, and by its increase in spring. The water content of grass influences the intake.