Cost-free vigilance during feeding in folivorous primates? Examining the effect of predation risk, scramble competition, and infanticide threat on vigilance in ursine colobus monkeys (Colobus vellerosus)

Abstract

Vigilance often decreases with increasing group size, due to lower predation risk or greater scramble competition for food. A group size effect on vigilance is seldom seen in primates, perhaps because scanning and feeding often occur simultaneously or because the distinction between routine and induced vigilance has not been investigated. We analyzed feeding and resting observations separately while distinguishing between routine and induced scans in four groups of wild ursine colobus monkeys (Colobus vellerosus) experiencing scramble competition for food and infanticide risk. We used linear mixed-effect models to test the effect of group size, age–sex class, number of neighbors, number of adult male neighbors, and height in the canopy on scanning rates (vigilance) with and without evident conspecific threat. Food type was also examined in the feeding models. Perceived predation risk affected vigilance more than scramble competition for food and infanticide risk. Routine and induced vigilance were greatest at lower canopy heights during feeding and resting and increased when individuals had fewer neighbors while resting. A group size effect was found on induced vigilance while resting, but scanning increased with group size, which probably indicates visual monitoring of conspecifics. Scanning rates decreased while feeding on foods that required extensive manipulation. This supports the idea that vigilance is relatively cost free for upright feeders when eating food that requires little manipulation, a common feature of folivore diets. In the presence of threatening conspecific males, close proximity to resident males decreased individual vigilance, demonstrating the defensive role of these males in the group.