Polyspecific associations occur when species overlap in their environment by chance, converge at common resources, or in response to predation pressure. However, because larger groups may themselves attract the attention of predators, species forming associations must balance the costs and benefits of comingling. Experimental and observational research suggests red colobus (Piliocolobus badius) associate with Diana monkeys (Cercopithecus diana) in Taï National Park, Côte d’Ivoire in response to seasonally shifting predation pressure from chimpanzees (Pan troglodytes verus). Research on this system has focused on immediate behavioral effects of comingling by red colobus and Diana monkeys. We expand on these analyses and explore longer-term changes in Diana monkey behavior. We use scan samples (N = 7025) collected on three Diana monkey groups over 5 years (N = 380 days) to assess differences in diet, activity budget, and strata use in relation to associating with red colobus and chimpanzee hunting seasonality and test for interaction effects between these two variables. We found limited evidence showing Diana monkeys make more than ephemeral behavioral changes in response to comingling with red colobus during chimpanzees’ hunting season (September–November). Synergistic effects of association and hunting season include expanded use of the main canopy, decreased fruit consumption, and increased invertebrate consumption. We find little evidence indicating Diana monkeys minimize behaviors that increase risk of predation from chimpanzees while associated with red colobus during the season when chimpanzees hunt most often. We conclude that when there are few member costs to forming mixed groups, participating species may tolerate associations even without accruing any benefits.