EN
Development of sexual maturity and reproductive activity in Arvicola sapidus Miller, 1908, was studied. The sample analyzed consisted of 374 specimens (208 males and 166 females), captured in various localities in southern Navarre (Spain). Maturity and sexual activity of the males were evaluated both from the cytological analysis of testicular and epididymal tissues and by the testicular and vesicular size. Maturity and sexual activity of the females were determined as a function of the vulva condition, size and vascularization of the uterus, presence of embryos and placental scars, histological analysis of the ovary and development of the mammary glands. The results show that sexual maturity development is related to age, season and size of both the individual and its reproductive organs. In the studied population there are males with spermatozoa present throughout the entire year. However, the size of testes and seminal vesicles of the adult males varied yearly, reaching lowest values at the end of autumn and winter. The mean number of embryos per litter is 3.7 ± 1.4 (ft = 51, range; 1-7), being closely related to the mother's weight. Intrauterine mortality affects at leats 5.1% of the embryos and 17.6% of the litters, while 53.7% of the females show placental scars. Females show high sexual activity between March and October (46.1-90.0% are pregnant or nursing) and are less active in the remaining months. In southern Navarre, A. sapidus breeds throughout the year with two well differentiated periods, one of high reproductive intensity (March-October, with maximum between April and June), and another of lesser intensity (November-February).