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Reproductive traits of voles vary with the phases of the population density fluctuations. We sought to determine whether the source of this variation resides in the individuals or in their environment. Overwintering field voles (Microtus agrestis) from two cyclic out-of-phase populations (increase and peak phases) were sampled in early spring and bred in the laboratory for two generations under standardised conditions with ambient light and temperature. Monitoring of the source populations by capture-mark-recapture showed large differences in reproductive performance. In the increase area, reproduction started six weeks earlier, the probability of maturation of young-of-the-year was more than ten times higher during mid-summer, and reproduction continued nearly two months later in the autumn than in the peak area. These differences were not found to be associated with a difference in age structure of overwintered animals between the two areas (assessed by the distribution of eye lens masses from autopsy samples). Although the population differences in reproductive traits were to some degree also present among the overwintered animals in the laboratory, we found no difference in reproductive traits in the laboratory-born generations. There was a strongly declining seasonal trend in probability of sexual maturation both in the field and in the laboratory under ambient light conditions. However, in the field there were large population differences in the steepness of the seasonal decline that were not seen under the standardised laboratory conditions. We conclude that seasonal decline in maturation rates is governed by change in photoperiod, but that the population level variation in the shape of the decline is caused by a direct response to the environment and not due to variation in any intrinsic state of the individuals reflecting the environment experienced by the previous generation(s). |
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