Responses of Small Mammals to Live-traps and Weather Conditions

Abstract

A comparison was made between the responses to livetraps of marked and unmarked individuals of Microtus pennsylvanicus and Peromyscus leucopus. Previously captured individuals of M. pennsylvanicus were more readily captured than were the unmarked ones, even after an interval of a month. There was no difference in the rates of capture of marked and unmarked P. leucopus. Comparisons of a given live-trapping technique utilized in two different habitats (marsh and old field) indicated no significant difference in its effectiveness after the second day of trapping. M. pennsylvanicus, in an old field, was found to abandon diurnal activity in favor of nocturnal and/or crepuscular activity when the temperature rose above 200 C. During the winter, when the temperature dropped below 00 C, activity was much less at all times. Except for less activity during the days the temperature rose above 200 C, such responses were not observed in the marsh. Blarina brevicauda was found to be more active on cloudy days than on those that were sunny or rainy. No correlations between activity and temperature variations was observed for this species. Estimates of population densities of small mammals are normally derived from trapping data. The results depend upon a variety of factors in addition to the actual abundance of the mammals. Variations in responses to traps, positioning of the traps, home range size, amount of movement within the population, and weather conditions influence the estimates of population densities (Sealander and James, 1958; Goodnight and Koestner, 1942; Hayne, 1950; Calhoun and Casby, 1958; Getz, 1961a). Responses to traps are particularly important. A method widely used in determining population densities from trapping data is the proportional method (Lincoln Index or modifications thereof; Hayne, 1949). A basic assumption involved in this method is that the probability of capture of the marked animals is the same as that of the unmarked animals. Morris (1955) and Chitty and Kempson (1949) found that marked individuals entered traps more readily than did those that had not been captured previously. Tanaka (1956) found the same to be true of certain species, while in others the probability of capture decreased after the initial capture. He further found that some may be isoresponsive. Leslie (1952) assumed the marked and unmarked individuals to be captured with equal facility, although prebaiting was utilized to overcome any initial reluctance of individuals to enter traps (this initial reluctance has been discussed by Sealander, et al., 1958). Responses to weather conditions also influence estimates of population densities, particularly those sampled by means of snap-back traps. Burt (1940), Gentry and Odum (1957), Pearson (1959, 1960)