On the significance of similarities between ultrasonic vocalizations of infant and adult rats.

The communicatory significance of the 40 kHz vocalization of rat pups and the 22 kHz vocalization of adult rats have been topics of research for over three decades. The 40 kHz vocalization is emitted by pups during cold exposure, whereas the 22 kHz vocalization is emitted by adults following ejaculation, following defeat in aggressive encounters, as well as in other contexts. Recent research suggests that the 40 kHz vocalization is the acoustic by-product of a respiratory mechanism that enhances gas-exchange in the lungs during times of increased oxygen consumption. Furthermore, a reevaluation of research into the physiological basis of the 22 kHz vocalization suggests a similar conclusion. In the present paper, we discuss mechanistic and contextual aspects of ultrasound production. We conclude that these two vocalizations, produced by identical mechanisms and reflecting identical physiological states, are actually the same vocalization, albeit at different frequencies. This alternative view of ultrasound production has implications for our interpretation of the communicatory significance of these vocalizations.