Male and female syringeal muscles exhibit superfast shortening velocities in Zebra finches

Abstract

Vocalisations play a key role in the communication behaviour of many vertebrates. Vocal production requires extremely precise motor control, which is executed by superfast vocal muscles that can operate at cycle frequencies over 100 Hz and up to 250 Hz. The mechanical performance of these muscles has been quantified with isometric performance and the workloop technique, but due to methodological limitations we lack a key muscle property characterising muscle performance, the force-velocity (FV) relationship. Here we establish a method that allows quantification of the FV relationship in extremely fast muscles, and test if the maximal shortening velocity of zebra finch syringeal muscles is different between males and females. We show that syringeal muscles exhibit extremely high maximal shortening velocities of 46 L0 s-1, far exceeding most other vocal and skeletal muscles, and that isometric properties positively correlate with maximal shortening velocities. While male and female muscles differ in isometric speed measures, maximal shortening velocity surprisingly is not sex-dependent. We also show that cyclical methods to measure force-length properties used in classical laryngeal studies give the same result as conventional stepwise methodologies, suggesting either approach is appropriate. Next to force, instantaneous power also trades for speed, further highlighting these muscles are tuned to operate at high frequencies. We argue that the high thermal dependence of superfast vocal muscle performance may impact vocal behaviour. Summary statementZebra finch syringeal muscle exhibits superfast shortening of 46 L0 s-1. Shortening is not sex-specific but correlates with isometric performance - faster twitches and tetani are associated with faster shortening.