It is known that long-term post-weaning individual housing significantly reduces emissions of 22-kHz calls in male rats. In this study, we assessed post-weaning successive changes in 22-kHz calls emitted by male rats under two different types of post-weaning housing conditions (individually and socially). In addition, we evaluated the critical point at which a significant reduction in 22-kHz calls could be observed in male rats housed individually after weaning. Significantly fewer 22-kHz calls

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... were emitted by individually housed rats compared to socially housed rats at 16 weeks of age, indicating that 13 weeks after weaning may be a critical point for the reduction of 22-kHz calls caused by post-weaning individual housing. KEY WORDS: early social isolation, indicator of stress, ultrasonic vocalization.