Word-level prosody in Balsas Nahuatl: The origin, development, and acoustic correlates of tone in a stress accent language

Here we investigate the historical origins and acoustic correlates of a hypothesized tonal development in subdialects of the Nahuatl spoken in the Balsas River valley of central Guerrero state in Mexico. We hypothesize that some subdialects have developed high tone on a syllable preceding a syllable with a breathy-voiced coda [ɦ] (< *h). In subdialects retaining [ɦ], coda [ɦ] was found to slightly lower F0 on the tautosyllabic vowel, creating a high–low F0 contour beginning on the preceding syllable. We propose that tone was developed as a reanalysis of the F0 contour as a phonological tonal specification. Through this tonal development, hybrid stress and tone systems have arisen, as the historical penultimate stress accent described for Nahuatl generally has been retained in the tonal dialects. Though such systems are typologically rare, a comparison of the development of other such hybrid systems indicates that they follow a similar historical path. That is, a stress language develops tone through the reinterpretation of a coarticulatory F0 effect as a tonal specification. We suggest that hybrid stress and tone systems may be unstable: Our results indicate that the historical stress accent may be transitioning to tone in subdialects with innovated tone.