How flexible are bonobo calls? Whereas functionally fixed or “functionally referential” calls are elicited consistently by the same stimuli, functionally flexible calls can be used in a range of different contexts. The bonobo “peep” call type is one such call, being used in various activities and across a positive-neutral-negative range. Researchers collected audio recordings of wild bonobos at LuiKotale, DRC, and compared spectrograms of the peeps in different contexts. They selected 8 individuals with 8 positive peeps (food related), 8 neutral peeps (non-food -> rest/travel related), and 4 negative peeps (predator or agonistic), which had fewer because they are rarer. There was no acoustic difference between positive and neutral peeps, but both were different from negative peeps. That peeps in positive and neutral contexts were similar shows that the same vocalisation can be flexibly used across contexts. This could be an intermediate state between functionally fixed animal calls and incredibly flexible human language.
Watson, S. K., Townsend, S. W., Schel, A. M., Wilke, C., Wallace, E. K., Cheng, L., … Slocombe, K. E. (2015). Vocal Learning in the Functionally Referential Food Grunts of Chimpanzees. Current Biology, 25(4), 495–499.
Zuberbühler, K. (2015). Linguistic capacity of non-human animals. Wiley Interdisciplinary Reviews: Cognitive Science, 6(3), 313–321.
Human language deviates from primate communication in three ways: "compositionality, audience awareness, and vocal control". Some primates use call sequences and combine calls with gestures, showing ability for syntactic comprehension, but don't come close to human language composition. The author suggests that this is less a reflection on their physical ability to combine calls and more on a lack of complex mental concepts. There is very little physiological difference between the vocal tracts of humans and other primates, but (at a neurological level) humans have greater control over vocal folds, larynx, and respiratory muscles. Great apes actually do show a sophisticated sense of how their communication affects others and it simply remains to be seen how deep this intentionality goes.