ABSTRACT
Microtus arvalis is widely found in Europe where it usually lives in grassland. It extends to the upper limit of the alpine meadows (3000 m in France). For unknown reasons, it is not found in lower Brittany or in some part of the Southwest. However, it is abundant in western France. It has an annual cycle of sexual activity reaching a peak from March to June. Animals older than 9 months are rarely captured (age estimated according to the crystalline lens; 1 , 2 Their chances of survival depends on their birth month. 3
