Ontogeny of the IBI difference, USVs and immobilization during manual carrying. (A) Ontogeny of the inter-beat interval (IBI) difference between Carrying and Holding during the first two postnatal weeks (n = 96). (B) Comparison of the number of ultrasonic vocalizations (USVs) emitted during the Undisturbed conditions (UD), Holding or Carrying from PND 4 to PND 9 (n = 36). (C) The mean ± SEM immobilization times during manual carrying (black filled circles, at PND 0–20, 35, 56; n ≥ 18 in each PND) are shown. (D) The mean ± SEM immobilization times of two groups of pups are shown. One group of pups was undisturbed except for a cage change once every other week (–). The other group of pups received continuous daily stroking by the experimenter’s hands for 20 s from PND 15 to PND 34 (+). All of the pups were picked up at PND 35 to examine their immobilization response (n = 18 for each condition). *p < 0.05.