The sound of silence: Sex and individual differences in rat ultrasonic vocalisation

Rats produce a diverse repertoire of ultrasonic vocalisations (USVs) that, despite their inaudibility to the human ear, have been widely studied in behavioural research. Besides alarm calls in the 22-kHz range, research has also focussed on the play-associated 50-kHz range, typically examining overall calling rates in males. However, evidence for sex and individual differences in rat USVs has accumulated in recent years. Building up on this work, we here aimed to further explore sex and individual differences in rats' vocal response during human-induced play behaviour, a context often used to study rat vocalisation. Specifically, we investigated calling rates across different 50-kHz and 22-kHz call subtypes and additionally analysed the sequence of used call subtypes, offering novel insights into the complexity of rat vocalisation. In line with recent work on rats' USV emission during play, male rats vocalised more than females. Moreover, calling rates per call subtype showed temporally stable inter-individual differences. Additionally, individual differences were found regarding call subtype sequence, pointing towards the existence of "vocal fingerprints". Thus, in line with previous knowledge on rat USVs, the present study supports the trend of individual specific research in the field of behavioural biology.