The concept that discrete actions act as an atomic basis of behavior has pervaded ethology from Lorenz and Tinbergen’s foundational studies to the current computational era. Recent research corroborates these atomic units, showing that laboratory mice choose actions that appear syllabic: i.e. both discontinuous in time and syntactically structured. Yet, little is known about how these syllables vary across phylogeny, where species adapt both behavior and physiology to form ecological niches. Here, we address this question by analyzing the syllabic repertoire and distribution of 18 species and subspecies of rodents spanning 25 million years of evolution. We discovered that these species exhibit traits that are both deeply conserved and other traits that vary on surprisingly fast evolutionary timescales. Consistent across phylogeny, actions are discrete in time, behaviorally stereotyped, and syntactically structured. Moreover, these repertoires of syllables are conserved across each of the species studied. Though the syllables themselves are stable, both the distributions of syllables, and the fine kinematics of those syllables vary from species to species, adapting on rapid timescales. Ongoing physiological work supports the discrete syllabic model of action selection and aims to identify the substrates which drive this variation. These results provide evidence for discrete syllables as a privileged unit not only as a cognitive and physiological substrate for action selection but also as a unit of evolutionary inheritance. Our study lays the groundwork for further research into the genetic mechanisms of these traits, the role of maladaptive genetics in leading to behavioral and psychiatric disease states, and how adaptive genetics and physiology can drive adaptation toward new ecological niches.