Situational factors, conditions and individual variables which can determine ultrasonic vocalizations in male adult Wistar rats

Abstract

The fact that rats emit different types of ultrasonic vocalizations in a variety of contexts has received increasing experimental attention, since such calls might serve as indices of the animal's subjective state, and/or as social signals in various types of interactions with other rats. Here, we present two experiments in adult male Wistar rats where we tested several different situations and conditions with respect to the occurrence of high-frequency (50-kHz) and low-frequency (22-kHz) calls. These experiments showed that rats emitted high-frequency calls when tested singly in a housing cage, which was situated in a room with no other rats present. Calling did not habituate with repeated testing, and occurred in the animal's own home cage, or a fresh housing cage, and irrespective of whether the animal's motivational status was high or low, that is, irrespective of whether they were food-deprived or fed ad libitum. Furthermore, high- and low-frequency calls were observed when applying a standardized new tickling procedure, which provided evidence for effective types of tickling. Most, but not all, young adult rats still accepted this stimulation as play. Therefore, this procedure might be a useful method to elicit high-frequency calls in adult rats. Overall, substantial evidence for inter-individual variability and intra-individual stability in vocalization was provided both, within and between housing cage and tickle tests. This variability seems to depend at least partly on dispositions or traits, which can be gauged by specific screening tests, like measuring risk-assessment in the elevated plus-maze, since animals with more risk-assessment were more likely to emit high-frequency calls during cage and tickle tests. These findings are discussed with respect to the major hypotheses concerning the functional significance of ultrasonic vocalizations, namely the social/communicatory and the motivational/emotional hypothesis.