Mechanisms for delayed density-dependent reproductive traits in field voles, Microtus agrestis: the importance of inherited environmental effects

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 vol es (Microtus agrestis) from two cyclic out-of-phase populations (increase a nd peak phases) were sampled in early spring and bred in the laboratory for two generations under standardised conditions with ambient light and tempe rature. Monitoring of the source populations by capture-mark-recapture show ed large differences in reproductive performance. In the increase area, rep roduction started six weeks earlier, the probability of maturation of young -of-the-year was more than ten times higher during mid-summer, and reproduc tion 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 d istribution of eye lens masses from autopsy samples). Although the populati on differences in reproductive traits were to some degree also present amon g the overwintered animals in the laboratory, we found no difference in rep roductive traits in the laboratory-born generations. There was a strongly d eclining seasonal trend in probability of sexual maturation both in the fie ld and in the laboratory under ambient light conditions. However, in the fi eld there were large population differences in the steepness of the seasona l 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 th e decline is caused by a direct response to the environment and not due to variation in any intrinsic state of the individuals reflecting the environm ent experienced by the previous generation(s).