Delayed maturation in female bank voles: optimal decision or social constraint?

Abstract

1. Density, maturation and survival of female bank vole ( Clethrionomys glareolus ) in the northern taiga of Finnish Lapland were studied using long‐term capture–mark–recapture data from two large grids, one food‐addition grid and one control grid, in 1982–94. 2. The density on the food grid was consistently higher than the density on the control grid. 3. Females born early in the breeding season usually matured, except at very high densities. Those born later in the summer season commonly delayed maturation to the following spring. 4. Winter survival of sub‐adult (having delayed maturation) females was significantly higher than survival of adult (breeding) females. However, empirical values of sub‐adult and adult survival, as well as difference between them, were not consistent with survival values assumed in theoretical models on optimal deferred breeding. 5. There was a density‐dependent relationship between the maturation rate of young voles and the density of already established breeding females (both bank voles and all Clethrionomys together; C. rutilus and C. rufocanus occasionally occurred on the study grids). This density dependence was different for the two grids (weaker on the food‐addition grid). 6. These findings are discussed within an evolutionary context: we have, on the basis of these findings, no evidence suggesting that the observed delayed maturation represents an evolutionary optimal strategy. Rather, there is evidence suggesting that the delay is due to social constraints.