Rat ultrasonic vocalization shows features of a modular behavior.

Small units of production, or modules, can be effective building blocks of more complex motor behaviors. Recording underlying movements of vocal production in awake and spontaneously behaving male Sprague Dawley rats interacting with a female, I tested whether the underlying movements of ultrasonic calls can be described by modules. Movements were quantified by laryngeal muscle EMG activity and subglottal pressure changes. A module was defined by uniformity in both larynx movement and pressure pattern that resulted in a specific spectrographic feature. Modules are produced either singly (single module call type) or in combination with a different module (composite call type). Distinct modules were shown to be linearly (re)combined. Additionally, I found that modules produced during the same expiratory phase can be linked with or without a pause in laryngeal activity, the latter creating the spectrographic appearance of two separate calls. Results suggest that combining discrete modules facilitates generation of higher-order patterns, thereby increasing overall complexity of the vocal repertoire. With additional study, modularity and flexible laryngeal-respiratory coordination may prove to be a basal feature of mammalian vocal motor control.