As described in Chapter 5 a territory differs from a home range in that a territory represents a portion of the overall home range that is actively defended against encroachment by conspecifics from other groups (Burt 1943). By aggressively excluding neighbors, territorial animals maintain at least a portion of their range for their exclusive use. Discussions of the evolutionary benefits of defending a fixed territory typically focus on one of two purported functions: resource defense or mate defense (Cheney 1987). Although the two functions need not be mutually exclusive, the predictions of the two models differ in ways that can be addressed using observations from the field (van Schaik et al. 1992; Steenbeek 1999). Socioecological theory suggests that given the different levels of parental investment by males and females in most sexually reproducing animals, fitness in females is primarily limited by access to resources, whereas fitness in males is primarily limited by access to mates (Trivers 1972; Wrangham 1979). It follows, therefore, that if territoriality functions primarily as male mate defense, then (1) males should primarily be involved in range

defense and advertisement and (2) males should always be intolerant of each other when they meet. A corollary of the second prediction is that males should be most aggressive when females in their group are in estrus (Fashing 2001). Alternatively if territoriality functions primarily as female resource defense, then (1) females should primarily be involved in range defense and advertisement and (2) home-range overlap should be small. The resource-defense theory also differs in that it does not assume that all encounters between neighbors are hostile, especially when resources are not easily monopolized. A variant of the resource-defense model posits that males defend resources on behalf of females who share their range (Rutberg 1983). In this way the interests of males and female are met simultaneously (Rubenstein 1986).