AGGRESSIVE-BEHAVIOR OF DIPODOMYS STEPHENSI, AN ENDANGERED SPECIES, AND DIPODOMYS AGILIS, A SYMPATRIC CONGENER

We studied aggression in the laboratory between two heteromyid rodents , Dipodomys agilis and Dipodomys stephensi, to determine if dominance behavior could produce nonoverlapping habitat associations. D. stephen si, which is heavier than its opponent, initiated more attacks and ret reated less frequently than did D. agilis. The average level of aggres sion shown by individual D. stephensi increased with body mass. The le vel of aggression shown by individual D. stephensi in a trial did not vary with the size or sex of the opponent, and individuals did not res pond differently to these attributes of the opponent. Because D. steph ensi is behaviorally dominant, we concluded that interspecific dominan ce could contribute to the observed association of D. agilis with coas tal-sage-scrub habitat, but could not, by itself, restrict D. stephens i to annual grassland habitat.