When the tune shapes morphology: The origins of vocatives

Many languages use pitch to express pragmatic meaning (henceforth ‘tune’). This requires segmental carriers with rich harmonic structure and high periodic energy, making vowels the optimal carriers of the tune. Tunes can be phonetically impoverished when there is a shortage of vowels, endangering the recovery of their function. This biases sound systems towards the optimisation of tune transmission by processes such as the insertion of vowels. Vocative constructions—used to attract and maintain the addressee’s attention—are often characterised by specific tunes. Many languages additionally mark vocatives morphologically. In this article, we argue that one potential pathway for the emergence of vocative morphemes is the morphological re-analysis of tune-driven phonetic variation that helps to carry pitch patterns. Looking at a corpus of 101 languages, we compare vocatives to structural case markers in terms of their phonological make-up. We find that vocatives are often characterised by additional prosodic modulation (vowel lengthening, stress shift, tone change) and contain substantially fewer consonants, supporting our hypothesis that the acoustic properties of tunes interact with segmental features and can shape the emergence of morphological markers. This fits with the view that the efficient transmission of information is a driving force in the evolution of languages, but also highlights the importance of defining ‘information’ broadly to include pragmatic, social, and affectual components alongside propositional meaning.