Sound production in piranhas is associated with modifications of the spinal locomotor pattern

In piranhas, sounds are produced through the vibration of the swim
bladder wall caused by the contraction of bilateral sonic muscles.
Because they are solely innervated by spinal nerves, these muscles
likely evolved from the locomotor hypaxial musculature. The transition
from a neuromuscular system initially shaped for slow movements
(locomotion) to a system that requires a high contraction rate (sound
production) was accompanied with major peripheral structural
modifications, yet the associated neural adjustments remain to this
date unclear. To close this gap, we investigated the activity of both the
locomotor and the sonic musculature using electromyography. The
comparison between the activation patterns of both systems
highlighted modifications of the neural motor pathway: (1) a
transition from a bilateral alternating pattern to a synchronous
activation pattern, (2) a switch from a slow- to a high-frequency
regime, and (3) an increase in the synchrony of motor neuron
activation. Furthermore, our results demonstrate that sound features
correspond to the activity of the sonic muscles, as both the variation
patterns of periods and amplitudes of sounds highly correspond to
those seen in the sonic muscle electromyograms (EMGsonic).
Assuming that the premotor network for sound production in
piranhas is of spinal origin, our results show that the neural circuit
associated with spinal motor neurons transitioned from the slow
alternating pattern originally used for locomotion to a much faster
simultaneous activation pattern to generate vocal signals.