The relative importance of site- and individual-specific aggression be
tween resident and nonresident adult female Microtus pennsylvanicus wa
s investigated using encounter trials in the field. Residents within t
heir home range exhibited significantly greater aggression when paired
with nonresident females and a trend towards greater aggression when
paired with resident females that were removed from their home range.
Nonresident females exhibited significantly more aggression than resid
ents when the trial was conducted at a neutral site. These results sug
gested a dual basis for female aggression with evidence for both site-
and individual-specificity.