Temporal variation of behavioural and demographical processes in cyclic Clethrionomys populations

Abstract

The social organization and behavioural ecology of Clethrionomys voles were studied in Konnevesi, Central Finland in the years 1982 - 88. The study consisted of long-term comparative work on an enclosed population of the bank vole Clethrionomys glareolus (Schreber) and island populations of the grey-sided vole C. rufocanus (Sund). Experimental studies, both in four enclosures and in the laboratory, were based on the experience of the dynamics of the local populations. Previously in the literature described social organization of Clethrionomys voles, based on the territoriality of breeding females, could be observed in traditional Clethrionomys habitats with Vaccinium field layer. In more productive habitats, or with supplementary feeding, more flexible social systems could be observed or induced; home ranges of breeding females overlapped more and daughters could even maturate within their mothers' home range. During the breeding season, the mutual familiarity existing between the founders of the breeding population of Clethrionomys glareolus affected the growth of the population significantly; during the course of a three months' experiment the populations of founder individuals familiar with each other grew faster and had better survival of the young. This resulted in population densities twice as high at the end of the breeding season as in the populations founded by individuals strange to each other. Thus, kin selection could be of importance for the growth of breeding Clethrionomys populations at least during some phases of the cycle. The activity pattern of the bank vole population, determined as the diel distribution of visits at the feeding station, was fairly uniform during the breeding season. This uniformity could be ascribed to the more flexible activity distribution of the young. On the whole, the activity pattern of a bank vole population seemed to be polyphasic and irregular. The normal way of social overwintering in Clethrionomys seems to be one of more or less tight social aggregation; in larger units or small groups consisting of a few females and at least one male. Territoriality during winter could not be induced, even with food distribution in the overwintering habitat, either. Habitat patchiness and food distribution, but especially the sufficiency of food seem to determine the social way of overwintering. When overwintering together with harvest mice Micromys minutus, both voles and mice avoided each others' ranges. This avoidance persisted during the first half of the breeding season, having a demographical effect in the vole population, too. The impact of the small mustelids on the study populations was observed throughout the local rodent cycles. Beside direct killing, delayed reproduction in the bank vole was observed during the winter and spring with numerous Mustelid predators. The suppressive impact of the presence of Mustela nivalis in the vicinity of breeding pairs of the bank vole on their breeding was positively shown in the laboratory, too. On the basis of my studies, I can conclude, that the social organization and behaviour in Clethrionomys voles seem to be flexible, and to a great extent phenotypically determined. During different phases of the cycle and in different habitats one can observe different kinds of social organization in Clethrionomys females. Social factors are obviously of importance especially in increasing and possibly in the peak phases of the cycle. The cause of the crash – and the general causes of cyclicity - must be sought on the basis of more diversified hypotheses; with great attention to predation, parasites and diseases. The individual vole adapts its behaviour to circumstances determined by the habitat and the phase of the cycle.