Abstract
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 (FIC) 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.

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