Between-group antagonism or territoriality in primates may serve two different but compatible functions: resource defense or mate defense. Females are expected to be involved more strongly in the first, males in the second. The resource defense hypothesis predicts that home range overlap should decrease as defensibility and population density increase, and that females should be involved in hostile between-group interactions. The mate defense hypothesis predicts that between-group relations should be hostile and that males should take the primary role in antagonistic encounters. In a comparative study of 12 populations of 6 Presbytis species in Southeast Asia, we found support for the mate defense hypothesis; only males produce loud calls, between-group antagonism is entirely a male affair, and neither defensibility nor population density determine spatial exclusivity or the level of antagonism. We discuss the differences between our findings and traditional interpretations of territorial behavior.