ultrasonic vocalizations (usvs) occur at frequencies ranging from approximately 20–100 khz. they are emitted by animals such as bats and rodents, and have been extensively studied in rats and mice. as opposed to sonic vocalizations, ultrasonic vocalizations cannot be detected by the human ear. usvs serve as social signals, and are categorized according to their frequency. different categories of usvs are elicited in response to different situations and varying affective states. the behavioural functions of usvs vary as a rat or mouse pup reaches the juvenile/adult stage of their development. the brain mechanisms behind calling behaviour have also been studied, and some studies have used pharmacological manipulation.