Abstract
20 female Long-Evans rats were tested for aggression on the day they gave birth. Only 30% of females tested with male intruders appreciably larger than themselves bit their opponents, and none showed offensive behaviors. By contrast, 80% of females tested with smaller males bit the intruders, and all of these used male-like offensive tactics. All attacked intruders exhibited defensive behavior, and all but one emitted ultrasonic vocalizations usually associated with pain. None of the males retaliated against the females. It is suggested that the size of the larger males evokes fear and that fear reduces offensive tendencies.
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