INFLUENCE OF DENSITY ON THE SPATIAL BEHAVIOR OF FEMALE THIRTEEN-LINED GROUND SQUIRRELS, SPERMOPHILUS TRIDECEMLINEATUS

Abstract

We tested the hypotheses that intrasexual aggression among female thirteen-lined ground squirrels (Spermophilus tridecemlineatus) varies with female density and that females defend home burrows and surrounding areas. Independent of density, females used and maintained 1 home burrow preferentially. The frequencies of aggressive encounters and displays initiated by females in high-density populations were 6 and 4 times higher, respectively, than that initiated by females living at low density. In high-density populations, the average overlap of female home ranges was only 13% of home range area, suggesting female defense of these areas (territoriality). Furthermore, most aggressive encounters were initiated when females were within their territory borders. Females displaced opponents in 80.6% of these encounters, whereas they prevailed in only 26.3% of aggressive encounters initiated outside of territory borders. In experiments involving the temporary removal of individual females, 1 or more neighboring females began using the territory of the removed female, and the frequency of both encounters and displays increased during removals. Upon their return, removed females reestablished their original territories by displacing the responding neighbors. Our results indicate that female S. tridecemlineatus defend territories surrounding a home burrow site, and that intrasexual aggression relegates some females to less preferred areas.