An unusual association: co-use of mineral licks by howler and spider monkeys (Alouatta sara and Ateles chamek).

Mineral licks represent localized sites of high predation risk in western Amazonian forests, where arboreal primates must come to the ground to obtain mineral-rich soils. Polyspecific associations are one potential strategy to mitigate such risks, yet their occurrence at mineral licks has rarely been documented. We recorded 66 co-occurrence events between howler monkeys and spider monkeys based on 3467 trap days across 16 mineral licks in southeastern Peru. Co-occurrences between these two species were significantly more frequent than expected by chance. The vast majority of co-occurrence events showed no signs of aggression or competition, unlike reported interactions between these taxa at other resource sites, such as fruiting trees. We did, however, observe occasional cases in which spider monkeys, especially adult females with infants, displaced howlers from geophagy sites and appeared to display reduced tolerance for close proximity. These interactions were brief and non-aggressive, with howlers yielding space without conflict. Collectively, we extend evidence of howler–spider monkey associations at mineral licks to new species and regions, while underscoring the antipredator pressures and subtle competitive dynamics that structure primate behavior at these ecologically important sites.