Small-scale altitudinal effects on reproduction in bank voles

Elevation gradients and corresponding changes in environment have the potential to affect seasonal reproduction and mortality and hence the seasonal age structure of animal populations. This has been frequently shown in mammals and birds on larger geographic scales, but evidence for small-scale gradient effects is scanty. We studied such small scale altitudinal effects on the proportions of juveniles in a population of bank voles (Myodes glareolus) in a forested habitat along an elevation gradient of 355 m. This was done by regular live trapping in different altitudes over a two-year period. We also measured altitudinal differences in ambient temperatures and vegetational parameters in order to shed light on the underlying mechanisms of such purported differences in age structure. As could be expected for a seasonal breeder, the proportion of juveniles showed an initial increase in spring, an optimum in summer and a subsequent regression in early autumn, reflecting the seasonal pattern of reproduction. In addition, there was evidence for altitudinal effects on the occurrence of juveniles, which was decreased in higher altitudes, assumingly driven by altitudinal differences in onset and/or intensity of annual breeding activity. This altitudinal gradient in juvenile occurrence was partly explained by corresponding differences in ambient temperatures. Furthermore, there was good support that the higher abundance of herbs in lower altitudes played an important role in explaining the altitudinal differences in the proportion of juveniles. In conclusion, our study strongly supports the existence of altitudinal small-scale gradient effects on reproductive parameters in a small rodent, most probably due to the action of altitude-related factors such as microclimate and vegetational parameters.