Circadian activity patterns, photoperiodic responses and population cycles in voles : I. Long-term variations in circadian activity patterns.

Patterns of above-ground activity were recorded in two free-ranging populations of Microtus agrestis (L.) and one population of M. arvalis (Pall.) over several years by means of passage counters. Long-term variations of the circadian patterns were observed, but did not repeat themselves in a 12-month cycle. Variations in all three populations could be described by a sinusoidal function of an 18-month period. Maxima and minima of the sine-function were connected to distinct photoperiods (equinoxes and solstices). In a cyclic North Swedish population, the sine-function was superimposed by a second function which shows saturation behaviour. The relevance of these findings is discussed with respect to vole population cycles.