"If the Zuberbühler team’s observations are correct, the Campbell’s monkeys can both vary the meaning of specific calls by adding suffixes and combine calls to generate a different meaning. Their call system, the researchers write, “may be the most complex example of ‘proto-syntax’ in animal communication known to date.”"
From Zuberbühler's previous work (e.g. Zuberbühler 2002, Zuberbühler 2006) it was already clear that some monkey species have very limited degree of flexibility in combining the vocalizations, but the fact that the combinatorics seem to be more complex than thought before seems quite interesting.
P.S.: Ed Yong also has a nice write-up