Differential Reproductive Success in a Peak Population of the Grey-Sided Vole Clethrionomys rufocanus

Individual variation in reproduction of females in a peak population of Clethrionomys rufocanus (Sund.) was studied in an attempt to evaluate the premises for rapid evolutionary changes as proposed by Chitty. There seemed to be sufficient individual variation in reproductive success to cause rapid selection. Reproductive output of individual females was based on number of litters produced during the breeding season rather on litter size, which showed inconsistent variation. Differential onset of breeding caused great variation in the number of litters produced by overwintered females. An early start to breeding increased the maximum number of litters an overwintered female could produce. Reproductive success was related to habitat quality which thus seemed to be an ecological factor acting as a selective force. Females in possession of high quality territories have an early onset of breeding and consequently high reproductive success. Body weight was positively correlated with reproductive success and may possibly be an individual trait which is selected for at peak densities. It is concluded that further progress can not be attained before the relationship between body size and reproductive success is further illuminated and the genetical component of this and other important individual traits is known.