Diel pattern of locomotor activity in populations of root voles Microtus oeconomus

Surface locomotor activity of root voles (Microtus oeconomus) during summer and autumn was recorded with passage-counters in four experimental populations in 0.5 ha outdoor enclosures. Populations differed considerably with respect to density, geographical origin and social behavior of animals, and year of observation. The general characteristics of diel activity patterns, however, were very similar among populations. Root voles were generally more active during daytime than during the night, and high activity always occurred around sunrise and sunset. The most prominent feature of the activity patterns was a clear-cut ultradian rhythm at both day and night, resulting in seven activity bouts per 24 h. Pattern adjustment to changing daylength was achieved by an interaction between endogenous ultradian rhythmicity and resetting of the ultradian rhythm at both sunrise and sunset. The daytime rhythm had a period length of 3.0 to 3.5 h, while the nighttime rhythm had a significantly longer period length of 3.5 to 4.0 h. Data in the literature suggest that this mechanism of triggering activity may also occur in other microtine species.