Abstract Predation of tropical arboreal mammals is rarely observed. Here, we report the first observation of predation of a member of the genus Cacajao, and add to the knowledge of eagle predation in the genus Cebus. We compare responses to predation by Cacajao ouakary and Cebus albifrons with previous studies of congenerics and with other close relatives, and compare the alarm vocalizations of C. ouakary with those of other pitheciines. Reactions to non-predating raptors are reported. Constraints on primate anti-predator responses in flooded and non-flooded habitats are considered.