Beyond dichotomy: diversity of rat’s ultrasonic vocalizations

Summary Understanding animal emotions is of great interest to researchers in various fields, from neuroscience to animal welfare. Ultrasonic vocalizations (USVs) in rats (Rattus norvegicus) have been established as an ideal model for studying emotional expression because they reflect internal emotional states. Since the discovery of USVs in rats, it has gradually been revealed that two types of adult rats’ USVs with distinct acoustic features (high-short and low-long) are closely linked to positive and negative affect, respectively. Thus, for many years, most studies have grouped USVs into dichotomous categories: 50-kHz (positive) USVs and 22-kHz (negative) USVs, using these numbers as labels rather than measured values. However, the diversity of rats’ USVs may have been overlooked due to this 50- / 22-kHz dichotomous framework, as several recent studies claim to have found new categories in USVs outside the framework. Here, through a systematic survey of descriptions of rats’ USVs in the past literature, we confirmed a non-negligible number of reports on USVs that are outside the dichotomous framework. Our results support the abundance of diverse USVs beyond the dichotomy. This extends the traditional association between acoustic features and emotional states, highlighting the importance of research on diverse vocalizations that might reflect complex affective states beyond the simple distinction between positive and negative.