An Experimental Study of Some Factors Affecting the Spatial and Temporal Activity of Microtus pennsylvanicus

Abstract

Radioisotope tracing was used to follow the movements of Microtus pennsylvanicus in enclosures in the natural environment. Experiments were designed to determine the effects of increasing population density and other factors on various ecological and behavioral parameters. The process of establishing home ranges for animals alone in unfamiliar habitat was rapid. Discriminatory preferences for home range sites and return to previously occupied ranges were shown by the experimental animals. No evidence for territoriality among resident voles (with the possible exception of defense of nest sites) was observed. At least 50 per cent of the study population was active at any hour of the period over which observations were made. There was a tendency for the population to be more active during daylight hours. Individual voles showed a mean short-term activity cycle of 4.8 hours. Even though the percentage activity increased with density, the mean short-term activity cycle of the voles remained about 4.8 hours, a possible indication of some basic physiological phenomenon. The size and shape of a vole's home range and its temporal and spatial activity did not appear to be fixed individual characteristics. Dominance hierarchies existed among the study animals. Shifts in home range and increase of percentage weight appeared to be related to an individual's social status. Animals at higher densities gained significantly more weight than did voles at a density of one per pen. Nonresident individuals maintained a higher emigratory rate than did resident voles. Increasing population density caused an increase in the mean distance moved between successive points of location and an increase in the mean percentage activity of the study animals. These and related density-dependent phenomena could well be involved in the regulation of populations of Microtus.