Sex Ratios of Some Minnesota Rodents

Abstract

An understanding of the dynamics of a population is best obtained by following all of the activities of the individuals of that population. Since this is normally impossible when working with wild species we must depend upon sampling to obtain data-e.g., sex ratios-from which the structure and status of the population can be estimated. It has been suggested by various workers that such factors as season of the year, age of the animals, and method of collecting may cause the observed sex ratios to deviate from the true sex ratio of the population. The following discussion of these points is based upon data gathered from some of Minnesota's more common native rodents. The collecting was done between 1950 and 1955 on areas adjacent to Basswood Lake in Lake County, Minnesota, and in 1954 and 1955 at the Rosemount Agricultural Experiment Station in Dakota County, Minnesota. The work at Basswood Lake was partially supported by the Quetico-Superior Wilderness Research Center. IDuring the period of study, data adequate for analysis were gathered fromn populations of the woodland deer mouse (Peromyscus maniculatus gracilis), prairie deer mouse (P. m. bairdii), red-backed vole (Clethrionomys gapperi), mneadow vole (Microtus pennsylvanicus), and the jumping mouse (Zapus bhdsonius). All of the woodland deer mice and red-backed voles and part of the meadow voles and jumping mice were taken from the vicinity of Basswood Lake. The rest of the meadow voles and jumping mice and all of the prairie deer mice were taken at Rosemount. The collecting on the Basswood Lake mainland was based on lines of 50 traps set sitigly at 30-foot intervals. On the small islands not less than seven traps were used and these islan'ds were covered to the extent that we felt that traps were available to all of the small mammals present. The traps were baited with peanut butter and were then left set for three days. All of the trapping in this area was carried out between June 21 and July 23. The lines at Rosemount were set with threetraps at each of 67 stations. These stations were approximately 15 feet apart. Samples were taken at two week intervals between July 15, 1954 and August 15, 1955. The lines were left set for three days. There was no indication that these two methods, of sampling the mainland populations produced significantly different results.