Campbells mona monkeys also generate alarm calls, but in a different way than vervet monkeys. Instead of having discrete calls for each predator, Campbell monkeys have two distinct types of calls which contain different calls which consist in an acoustic continuum of affixes which change meaning. It has been suggested that this is a homology to human morphology. Similarly, the cotton-top tamarin is able to use a limited vocal range of alarm calls to distinguish between aerial and land predators. Both the Campbell monkey and the cotton-top tamarin have demonstrated abilities similar to vervet monkeys ability to distinguish likely direction of predation and appropriate responses.
That these three species use vocalizations to warn others of danger has been called by some proof of proto-language in primates. However, there is some evidence that this behavior does not refer to the predators themselves but to threat, distinguishing calls from words.