An alleged negative interaction between Microtus ochrogaster and Sigmodon hispidus was examined in the laboratory. Sigmodon proved to be the dominant animal, and excluded Microtus from confined areas except when dense cover was provided. Movements of Microtus were reduced by a free-ranging Sigmodon but not by a confined one. Frequency of interspecific contact apparently was responsible for negative interactions. A model that emphasizes frequency of interspecific contact (FIe) explains the probable relationship between Microtus ochrogaster and Sigmodon hispidus; a variation of this model might explain the phenomenon of competitive exclusion in all myomorph rodents.The prairie vole, Microtus ochrogaster, and the hispid cotton rat, Sigmodon hispidus, occur sympatrically near the peripheries of their respective ranges in central Kansas. Microtus ochrogaster has long inhabited the central Great Plains (Hibbard, 1970), whereas Sigmodon hispidus is an invader from the south that first appeared in central Kansas about 40 years ago (Cockrum, 1948). Northward dispersal of S. hispidus onto the Great Plains apparently has coincided with periodic declines in population density of M. ochrogaster; Martin (1956), Frydendall (1969), and Terman (1973 gave examples of this, and suggested that there is a negative interaction between the two species in the field. However, their studies were conducted under natural conditions and lacked precise information derived from systematically manipulated variables. The following hypotheses were suggested by those field results and by the observations and comments of other investigators.If interspecific aggression between Microtus and Sigmodon is the mechanism favoring cotton rats over voles (Baker, 1971;Martin, 1956), then Sigmodon should be aggressive toward Microtus and should be dominant in most interspecific encounters. Therefore, I hypothesized that Sigmodon would actively attack Microtus and would be aggressively dominant in interspecific pairings.If Sigmodon limits the spatial distribution of Microtus (Baker, 1969;Fleharty and Olson, 1969;Terman and Johnson, 1971), this phenomenon should be observable in a laboratory arena. Therefore, I hypothesized that voles would reduce their use of areas occupied by Sigmodon. However, it is possible that the amount of habitat cover (complexity) might affect this relationship (Crombie, 1946; Krebs et ai., 1971;MacArthur, 1972); thus, I hypothesized that dense cover would enable Microtus and Sigmodon to occupy the same area.If the Microtus-Sigmodon interaction lacks interspecific recognition and avoidance (Baker, 1971;Brown, 1966; Calhoun, 1963;Jameson, 1947), cotton rats should affect the· movement of voles only when actual physical contact is involved. Therefore, I hypothesized that movements of voles would not be 705
