Journal of Language Evolution, 2017, 20–36 doi: 10.1093/jole/lzx015 Research article Divergent acoustic properties of gelada and baboon vocalizations and their implications for the evolution of human speech Morgan L. Gustison1,* and Thore J. Bergman1,2 1Department of Psychology, University of Michigan and 2Department of Ecology and Evolutionary Biology, University of Michigan *Corresponding author:
[email protected] Abstract Human speech has many complex spectral and temporal features traditionally thought to be absent in the vocalizations of other primates. Recent explorations of the vocal capabilities of non-human pri- mates are challenging this view. Here, we continue this trend by exploring the spectro-temporal prop- erties of gelada (Theropithecus gelada) vocalizations. First, we made cross-species comparisons of geladas, chacma baboons, and human vowel space area. We found that adult male and female gelada exhaled grunts–a call type shared with baboons—have formant profiles that overlap more with human vowel space than do baboon grunts. These gelada grunts also contained more modulation of fundamental and formant frequencies than did baboon grunts. Second, we compared formant profiles and modulation of exhaled grunts to the derived call types (those not shared with baboons) produced by gelada males. These derived calls contained divergent formant profiles, and a subset of them, not- ably wobbles and vocalized yawns, were more modulated than grunts. Third, we investigated the rhythmic patterns of wobbles, a call type shown previously to contain cycles that match the 3–8 Hz tempo of speech. We use a larger dataset to show that the wobble rhythm overlaps more with speech rhythm than previously thought.