Socioecological factors affecting range defensibility among howler monkeys

Range defensibility is defined as the ability of animals to efficiently move over an area to monitor and defend it. Therefore, range defensibility can help understand the spatial structure of animal territoriality. We used howler monkeys (Alouatta spp), a genus for which no agreement on the extent of their territoriality exists, to investigate the factors mediating range defensibility. We compared the defensibility index (D) across 63 groups of howler monkeys, representing 8 different species, based on a literature review. All species, except Alouatta palliata, were classified as potentially territorial according to D, although there was high variability within and among species. Group size had a positive effect on D, probably due to the greater ability of groups to defend a territory as they become larger. Study area had a negative effect on D, perhaps suggesting that unlike small areas, large areas allow groups to have territories that do not require significant defense from neighbours. However, population density was the factor with the strongest effect on D, with greater monitoring of home ranges under high levels of competition. Our results suggest that howler monkeys are theoretically capable of maintaining a territory and suggest that animals can show a gradient in territoriality, which can be mediated by the competitive context in which it occurs.