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.