Postweaning guinea pigs housed with their mother and littermates since birth vocalized more and exhibited greater increases in plasma cortisol levels when placed for 1 hr into a novel test cage alone than they did when tested in the identical fashion with the mother present. These responses were apparent beyond 50 days of age, but had waned by 90 days of age. When tested with a familiar sibling cagemate, postweaning guinea pigs emitted fewer vocalizations than when tested alone but exhibited no less of an elevation of plasma cortisol levels. These results were obtained regardless of whether the subjects had been housed with mother and littermates from birth until the time of testing or with mother and littermates until weaning and then just the single sibling cagemate until the time of testing. The present findings closely approximate those seen in preweaning guinea pigs during tests of maternal and sibling separation, and indicate that guinea pigs can continue to exhibit a specific attachment to the mother beyond the time of weaning.

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