Vigilance in socially foraging animals (e.g., elk) may serve to detect predators (i.e., reduce predation risk) or to monitor the behavior of conspecifics (i.e., reduce conspecific risk and/or increase reproductive benefits). These potential benefits and costs of vigilance may not be equal among different genders and age classes foraging together. We observed male and female elk from 3 age classes (yearlings, nonreproductive, reproductive) during 2 seasons (calving season, breeding season) that vary in social interactions (i.e., conspecific risk) and in 2 regions of Yellowstone National Park that varied in predation risk due to density of wolves. This study was designed to determine the potential functional benefit of vigilance across a range of herd sizes. If vigilance serves to monitor the behavior of conspecifics, we expected it to increase in the fall breeding season along with aggressive behaviors, regardless of changes in predation risk. If vigilance serves to detect predators, we expected it to increase in regions with wolves regardless of changes in conspecific risk. Adult male vigilance and aggression increased during the fall breeding season, but yearling males had significantly lower levels of vigilance. Adult female vigilance increased during the calving season, with increasing predator encounter risk, decreasing herd size, and edge position within the herd. Yearling female vigilance decreased with herd size but was lower than adult females. We conclude that the primary benefit of vigilance for male elk is to monitor conspecifics, but the primary benefit of vigilance for female elk is to detect potential predators.