Interactions Between the Cotton Rats, Sigmodon fulviventer and S. hispidus

Abstract

Interactions between two sympatric species of cotton rat, Sigmodon fulviventer and S. hispidus, were studied for 1 year. Both species had similar ecological requirements and occurred in bunchgrass (Sporobolus wrightii) on the central plateau in Durango, Mexico. Demographic changes in S. hispidus populations following experimental removal of S. fulviventer, the numerically dominant animal, included: (1) higher population levels; (2) slightly higher survival rates; (3) slightly greater life expectancy; (4) occupation of areas formerly held by S. fulviventer; and (5) greater distances captured from individual centers of activity. These changes were attributed to S. hispidus immigrating into an unoccupied space in habitat common to both species and less opportunity for interspecific interactions following removal of S. fulviventer. A postexperimental trapping study 1 year later revealed that S. fulviventer had increased its populations to near preremoval levels and had reinvaded and reoccupied areas held by S. hispidus during the removal experiment. Correspondingly, the S. hispidus population had decreased and been displaced by S. fulviventer. It was concluded that both species were interacting in unmanipulated populations, but that the interactions were of a passive (avoidance) nature. These interspecific interactions may influence population regulation, habitat selection, geographical range-extending ability and evolution in both species. Although competitive interactions were not demonstrated, certain data suggested the possibility of some form of interspecific competition. At present, S. fulviventer appears to be the numerically and interactively dominant Sigmodon. INTRODUCTION Intersp-ecific interactions between closely related small mammals arem difficult to demonstrate and evaluate in field situations. Baker (1969) observed that when two species of grass-eating, runway-making rodents of the genus Sigmodon (cotton rats) live in the same area, one is generally abundant and the other less abundant. Such a situation existed in the desert-grassland ecotone of E-Central Durango, Mexico (240 38' N lat and 1030 42' W long). S. fulviventer Allen, usually abundant, and S. hispidus Say and Ord, usually uncommon, were resident in bunchgrass (Sporobolus wrightii) habitat. Hence, this area offered an excellent opportunity to study population dynamics and interactions of both species. Adults of S. fulviventer and S. hispidus in this part of Mexico have similar body weights and measurements but are easily distinguished by different hair colors and other characteristics (Baker, 1969). This study attempts to describe the numerical and spatial relationships between these cotton rats, and to demonstrate certain demographic changes which occur when sympatric populations of these 1 Present address: Department of Zoology and Entomology, Iowa State University, Ames 50010.