Lenition and fortition of /r/ in utterance-final position, an ultrasound tongue imaging study of lingual gesture timing in spontaneous speech

•Linguistic boundary strength conditions syllable rime duration.•Utterance-final position conditions the most delayed /r/ tongue gestures.•Greater gesture delay correlates with weaker-sounding /r/.•Tip-up tongue shape variants permit lenition of /r/ in utterance-final position.•Utterance-final position is a key location for the performance of social variation. The most fundamental division in English dialects is the rhotic/non-rhotic division. The mechanisms of historical /r/-loss sound change are not well understood, but studying a contemporary /r/-loss sound change in a rhotic variety of English can provide new insights. We know that /r/ weakening in contemporary Scottish English is a gesture-timing based phenomenon and that it is socially indexical, but we have no phonetic explanation for the predominance of weak /r/ variants in utterance-final position. Using a socially-stratified conversational ultrasound tongue imaging speech corpus, this study investigates the effects of boundary context, along with other linguistic and social factors such as syllable stress, following-consonant place and social class, on lingual gesture timing in /r/ and strength of rhoticity. Mixed-effects modelling identified that utterance-final context conditions greater anterior lingual gesture delay in /r/ and weaker-sounding /r/s, but only in working-class speech. Middle-class speech shows no anterior lingual gesture delay for /r/ in utterance-final position and /r/ is audibly strengthened in this position. It is unclear whether this divergence is due to variation in underlying tongue shape for /r/ in these social-class communities, or whether utterance-final position provides a key location for the performance of social class using salient variants of /r/.