Reproduction of Different Female Age Categories and Dynamics in a Non-Cyclic Field Vole, Microtus agrestis, Population

Abstract

Reproductive effort, an index of number of litters produced, for three age categories of females, was examined in a non-cyclic field vole population during the 1984 and 1985 breeding seasons using mark-recapture data. The three categories of females recognized were: overwintered and young of the year born early and late in the season. Females in the two last mentioned categories bred the year they were born. The reproductive effort of overwintered females was higher in 1984 than in 1985, whereas the reproductive effort of the young of the year (YY-females) was higher in 1985 than in 1984. The low reproductive effort by YY-females in 1984 was connected with low recruitment and a decline in population density over the breeding season. The following year breeding began earlier, juvenile recruitment was much higher, and population density increased markedly. Thus, the annual pattern of dynamics in this non-cyclic population appeared to be shaped primarily by the breeding performance of YY-females. The great between-year variation observed was possibly due to differences in food conditions. The strong correlation between reproductive effort of YY-females and dynamics of this non-cyclic population parallels observations on