Interspecific relationships among cricetid rodents in agrarian ecosystems were studied. The principal rodent species found in these habitats are Akodon azarae, Calomys laucha, Calomys musculinus and Oligoryzomys flavescens. Fieldwork was carried out in D. Gaynor, Buenos Aires Province (Argentina), from 1979 to 1982. Spatial and numerical relations in a two-habitat system comprising cropfields and their borders were analyzed. Asymmetrical interspecific competition for space was found mainly in summer with A. azarae being the dominant species. Individual differences in competitive ability were found. Females of A. azarae were more dominant than males, and juveniles of C. laucha were more segregated than adults. Coexistence mechanisms of different species in borders are discussed.Resume. -Les relations interspecifiques entre une serie de rongeurs Cricetides ont ete etudiees dans des ecosystemes agraires de la province de Buenos-Aires de 1979 ä 1982. Leurs relations spatiales et numeriques ont ete analysees dans un Systeme de double habitat comprenant les champs cultives et leurs bordures. Une competition interspecifique asymetrique pour Pespace a ete mise en evidence, surtout pendant Fete chez 1'espece dominante Akodon azarae^ et on a constate des differences individuelles dans 1'aptitude ä la competition. Les femelles de cette espece etaient plus dominantes que les males, et les juveniles de Calomys laucha etaient plus disperses que les adultes. Les mecanismes de coexistence d'especes differentes dans les bordures sont discutees.

. 1998. Seasonal variations in microhabitat use and feeding habits of the pampas mouse Akodon azarae in agroecosystems of central We studied microhabitat use and food habits of the pampas mouse Akodon azarae (Fischer, 1829), in agroecosystems of central Argentina. In summer (breeding season), A. azarae inhabited both cropfields and their weedy borders. Sexually active females were caught at microhabitats with 31% more green cover than inactive ones. The percentage of invertebrates in the stomach contents of females was 104% greater than in males. In winter (non-breeding season), individuals were restricted to the borders, selecting sites with high levels of forbs and green cover. There were no significant differences between the sexes, either in microhabitat use or in food habits. We propose that reproduction of females is influenced by green cover, and that a decrease of this resource may reduce breeding activity of females in the borders after most plants enter senescence. Females would feed on insects as a source of proteins, which are required for reproduction. In winter, when the population is sexually inactive, individuals of both sexes have similar requirements and occupy the most suitable sites available within a low-quality habitat. We conclude that seasonal variations in microhabitat use and food habits of A. azarae are mainly influenced by changes in resource availability and reproductive status of individuals.

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