Parametric Modulation of a Shared Midbrain Circuit Drives Distinct Vocal Modes in a Singing Mouse
The ability of neural circuits to generate multiple outputs is critical for behavioral flexibility. Here, we leverage the rich vocal behavior of the singing mouse (Scotinomys teguina) to investigate the organizational logic of multifunctional motor circuits. We show that two distinct vocal modes--soft, unstructured ultrasonic vocalizations (USVs) for short-range and loud, rhythmic songs for long-range communication--arise not via parallel pathways but through shared brainstem phonatory circuitry involving the caudolateral periaqueductual gray (clPAG). Using a three-parameter linear model of song rhythm, we demonstrate that synaptic silencing of clPAG progressively alters song duration through a single parameter controlling its termination. This parameter also explains sexual dimorphism in songs, identifying clPAG as a key locus for driving natural behavioral variability. Our findings reveal how parametric modulation of a central circuit node can produce distinct behavioral modes, providing a mechanistic basis for rapid behavioral evolution in mammals.