The New York Times is reporting (here) that a study of Campbell's Monkey cries in the Ivory Coast has found the use of suffixes to modify the meaning of a call. The monkeys have 6 basic calls which they can modify. For example, krak is the call made in response to seeing a leopard; krak-oo appears to be a modification of the call that indicates the caller only thinks or hears that a leopard is nearby.
boom is an invitational sound (i.e., come to me), but when monkeys combine it with krak-oo to get boom-krak-oo the call signals a falling tree.
For those who stress syntax as the unique element of language, a few dance steps may be needed to explain this one. It sounds like the sort of story we are going to be hearing about for years.