And some of the words can have a suffix.
The same team of researchers had previously shown that males used a suffix "oo" to duplicate the size of his vocal repertory (which allowed them to produce the sounds Hok and Krak as well as Hok-oo and Krak-oo). In this new study, the ethologists explain some of the rules that govern the semantic combinations of calls. For example, Campbell's monkeys can add a particular type of call to an existing sequence in order to make the message more precise or to alter it. They can also combine sequences relaying different messages in order to convey a third message.They are not only able to tell each other what kind of danger they face, or what kind of predator, but to gather up before moving to a new site, or to tell of some other Campbell's monkeys coming onto our turf.
This is the most complicated grammar found so far in apes, but it is only mildly surprising, since every few months it seems we learn of some new language or tool-making or social ability of apes. This is this month's mild surprise. It certainly is further evidence that human language evolved out of ape precursers. The researchers suggest the reason may be that they live among dense foliage, so sound is their only way of communicating to the group. You can't just wave your arms to get attention or point. I guess it's a reasonable conjecture.
*"Words" and "sentences" are my words. The researchers would never be so anthropomorphic.