Reproduction of the Bank Vole (Evotomys glareolus, Schreber). II. Seasonal Changes in the Reproductive Organs of the Male

The present paper deals with the male Evotomys and is concerned with two main problems; the duration of the breeding and the non-breeding seasons, and the structure and growth of the reproductive organs. It is, in consequence, complementary to the preceding paper on the female. The only previous work we are aware of on the male of the British species is that of Baker (1930) on a series of 359 males trapped near Oxford over a period of three consecutive years. Baker, aking the arbitrary criterion of 14 gm body-weight or over, found that 260 of these were adult. If spermatozoa were abundant in the tail of the epididymis, as determined from teased preparations, and also if the seminal vesicles were found to weigh 100 mg or over, the animal was said to be fecund. Baker found that none of the 13 adult males obtained during the winter, October to February inclusive, of 1925-26 was fecund, while 16% of the 45 obtained in the following winter and 52% of the 27 obtained in the winter of 1927-28 were fecund. Thus he concluded that in one of the three winters concerned there was complete sterility.