The function of ultrasonic vocalizations during territorial defence by pair-bonded male and female California mice

Acoustic communication is vital to complex social behaviours such as territorial defence. The use of ultrasonic vocalizations, particularly in territorial defence by monogamous species and females, remains understudied. We studied ultrasonic vocalization production and associated aggression in the monogamous, biparental and territorial California mouse, Peromyscus californicus, in which both males and females were found to display similar levels of physical aggression against same-sex intruders. We identified specific ultrasonic vocalization calls that are modulated based on social context: (1) sustained vocalizations, which are long, low-bandwidth calls ranging from 22 to 25 kHz, and (2) barks, which are short, high-intensity calls beginning and ending in the audible range. Despite similarities in physical aggression, sex differences emerged in vocal communication. Only resident males, and not females, produced sustained vocalizations prior to the onset of physical aggression, and were found to shorten the duration of individual sustained vocalization calls over both the course of the pre-encounter phase and from the pre-encounter to encounter phase. In addition, the degree of sustained vocalization shortening in males predicted offensive aggression of the resident. Males exhibited shorter sustained vocalization calls during encounters than females. Barks occurred more frequently during female-female physical aggression than in male-male encounters, and correlated highly with defensive aggression by intruders. Finally, a newly identified highly complex call, sweep phrases, was recorded in a subset of both sexes in the pre- and post-encounter phases. The overall results indicate that ultrasonic vocalizations may play an important role in territorial defence during both territorial advertisement and aggression in a monogamous rodent. Overall, this monogamous species showed sex similarities in physical aggression but sex differences in vocal communication and a more sophisticated function for sustained vocalizations than previously recognized.