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Long Ultrasonic Calls in Male Rats Following Mating, Defeat and Aversive Stimulation: Frequency Modulation and Bout Structure

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Ultrasonic calls by rats may represent affective expressions and serve communicative functions. The temporal patterning and frequency modulation of the long, 21-30 kHz ultrasonic calls were studied in 7 male rats. Following ejaculation, defeat and aversive stimulation large samples of consecutive long calls were recorded and analyzed off-line using a spectrum analyzer. The analysis was limited to long calls because other types of calls were relatively rare under the circumstances of this study and the emitter could be determined by a typical breathing pattern with reasonable certainty only for long calls. Log survivor analysis of the intervals between calls revealed that rats emit 95 % of long calls in bouts of less than 6 calls. Except for the higher bout rate after defeat, there were no significant differences between tests in call duration, percent time spent vocalizing, bout duration and intervals between bouts. However, there were significant differences between individuals across tests in call duration, number of calls per bout and bout rate. The pattern and capacity to breathe are likely important limiting factors for several temporal characteristics of rat ultrasounds, in particular maximal call duration and range of interval duration within a bout. Post-ejaculatory calls were often strongly modulated in frequency (i.e. pitch) in the medial and terminal segments. These segments of calls emitted after defeat or aversive stimulation were essentially monotonous. We propose that rats emit at least 2 types of long ultrasonic calls-one monotonous, the other modulated-each reflecting a different affective state of the animal. The modulated type may reflect a state of behavioral inhibition such as initiated by ejaculation whereas the monotonous type may reflect a state of intense fear induced by an actual physical threat in situations where escape is blocked. Both types may serve a "desist-contact" function.

Affiliations: 1: Department of Pharmacology, Sylvius Laboratories, State University Leiden, Leiden, The Netherlands; 2: Department of Psychology, Tufts University, Medford, MA 01890, USA


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