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.