Response to Lieberman on 'Monkey vocal tracts are speech-ready'

We thank P. Lieberman for his technical comment, and we are pleased that he accepts our data, methods, and results and agrees with our main conclusion: that a macaque’s vocal tract would be able to produce speech sounds if macaques had the required neural control. However, we cannot agree that our findings, which expand the phonetic potential of macaques eightfold relative to that reported in his seminal 1969 paper, in any sense constitute a “replication” of that study or demonstrate the correctness of his earlier conclusions. To recap, both studies used measurements of macaque monkey vocal tracts to create a computer model, which was then queried to determine what vocalizations it could potentially produce: a space representing the “phonetic potential” of that vocal tract. The key difference between the two studies is that our vocal tract measurements were derived from x-rays of living monkeys vocalizing and communicating ( 1 ), whereas the measurements of Lieberman et al . [( 2 ), p. 1186] were derived from a single cast of a dead monkey, with possible perturbations “estimated” by “manipulating … an anesthetized monkey.” We believe that this difference in the quality of the input data is responsible for the key difference in our results: an eightfold increase in the macaque phonetic potential as estimated by our model [see our Fig. 3 in ( 1 )]. Going beyond Lieberman’s original study, we also generated five “monkey vowels” that optimally partitioned this enlarged acoustic space. Perceptual experiments then showed that humans readily discriminate between these five vowels. Five vowels were chosen because that is the modal number of vowels in human languages around the world, although the specific vowels vary, of course, from language to language ( 3 , 4 ). Given that nonhuman primate formant perception is very similar to that of humans ( 5 , 6 ), this finding …