Predation is a major force structuring ecological communities, and it is well known that predation risk can exert strong effects on behavior and ecology of prey species. One of the potential effects of predation risk is the constraint on the activity patterns and activity budgets of prey. Island populations that evolve under reduced predation risk provide valuable opportunities to assess the effects of such risk on prey behavior. We tested whether diel activity patterns and anti-predator behavior differed between mammal populations living on 2 land-bridge islands of the Coiban Archipelago off the Pacific coast of Panama—where predators are nearly absent—and the adjacent mainland where a diverse community of predators persists. Using ground-based and arboreal camera trapping, we compared temporal activity kernel density curves, foraging and ranging behavior, and visit duration between 2 island and 4 mainland populations of 5 tropical forest mammal species. We also tested whether temporal overlap between prey competitor species differed between island and mainland populations. Overall, we found that activity levels during risky times were higher on islands than on the mainland for two species of agouti and the White-faced Capuchin monkey. Island prey populations showed far less anti-predator behavior and more foraging behavior than their mainland counterparts. They did not, however, show higher levels of nocturnal activity during darker nights nor longer visits, both considered as indicative of predation release. We also found lower spatiotemporal overlap between the white-faced capuchins and the mantled howler monkeys on islands than their counterparts on the mainland. This result suggests that when predation pressure is relaxed, there is a decrease in direct competitive interactions between these species, which may also be associated with changes in the use of forest strata. These findings provide partial support for the hypothesis that predation risk constrains activity patterns and influences spatiotemporal niche differentiation among competing mammalian prey species.