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

1. Density, maturation and survival of female bank vole (Clethrionomys glar eolus) in the northern taiga of Finnish Lapland were studied using long-ter m 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 ver y high densities. Those born later in the summer season commonly delayed ma turation to the following spring. 4. Winter survival of sub-adult (having delayed maturation) females was sig nificantly higher than survival of adult (breeding) females. However, empir ical values of subadult and adult survival, as well as difference between t hem, 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 o f young voles and the density of already established breeding females (both bank voles and all Clethrionomys together; C. rutilus and C. rufocanus occ asionally occurred on the study grids). This density dependence was differe nt 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 dela yed maturation represents an evolutionary optimal strategy. Rather, there i s evidence suggesting that the delay is due to social constraints.