Is there any evidence for vocal learning in chimpanzee food calls?

In their study "Vocal Learning in the Functionally Referential Food Grunts of Chimpanzees", Watson et al.[1] claimed that they "provide the first evidence for vocal learning in a referential call in non-humans". We challenge this conclusion, on two counts. For one, we are not convinced that the authors controlled for arousal (or at least they did not report such data); furthermore, the vocal characteristics of the two groups largely overlapped already at the beginning of the study. Accordingly, we also question the authors' claim that their finding "sheds new light on the evolutionary history of human referential words".