Neck muscle twitch properties are associated with constraint on drum speed in woodpeckers, but not drum length

Animal displays are often limited by the properties of the muscles that generate them. Here, using in situ muscle stimulation, we investigated the twitch properties of the longus colli ventralis (LCv), a primary muscle used to protract the head and neck during territorial drumming displays in woodpeckers. Specifically, we tested LCv twitch kinetics and endurance in a manner that simulates drum speed (beats s-1) and length (total beats), two signal features that can evolve independently of each other. We identified a maximum muscle contraction rate that may represent a physiological constraint relevant to drumming speed, but no relevant constraint on the repetition of contractions that might affect drum length. This suggests that twitch properties may differentially affect display components. Broadly, our findings highlight how certain display features may freely diversify independent of others owing to physiological limits, while pointing to the way complex signals can evolve under partial performance constraints.