Vocalizations convey sex, seasonal phenotype, and aggression in a seasonal mammal.

Seasonal variation in social behavior is often accompanied by seasonal variation in communication. In mammals, how seasonal environmental cues influence aggressive vocalizations remains underexplored. Photoperiod is the primary cue coordinating seasonal responses in most temperate zone animals, including Siberian hamsters (Phodopus sungorus), a species that undergoes reproductive inhibition and increased aggression in winter. During same-sex aggressive encounters, hamsters emit both broadband calls (BBCs) and ultrasonic vocalizations (USVs) that indicate aggression and the vocalizer's sex, respectively; however, it is not known whether these rodents adjust specific elements of their vocal repertoire to reflect their photoperiod-induced seasonal phenotypes. To address this, we recorded vocalizations emitted during dyadic interactions between male or female pairs of hamsters housed in long or short photoperiods and measured serum testosterone levels. USV emission rate remained stable across photoperiods, but proportional use of USV subtypes varied in novel ways: 'jump' USVs were sensitive to seasonal phenotype, but not the vocalizer's sex, whereas 'plain' USVs were sensitive only to the sex of the vocalizer. BBC emission rate varied with seasonal phenotype; short-day non-reproductive hamsters produced more BBCs and demonstrated increased aggression compared with reproductive hamsters. Testosterone, however, was not related to vocalization rates. Collectively, these findings demonstrate that changes in the vocal repertoire of Siberian hamsters reflect sex, aggression, and seasonal phenotype, suggesting that both BBCs and USVs are important signals used during same-sex social encounters.