Cloutier, S., Panksepp, J., Newberry, R. C. 2012. Playful handling by caretakers reduces fear of humans in the laboratory rat. Applied Animal Behaviour Science 140(3-4), 161-171.

Handling of laboratory rats can increase physiological and emotional stress, leading to a fearful relationship with humans. We hypothesized that the affective quality of handling techniques used during routine care influences the animals' fear of humans. We assessed responses of male Sprague-Dawley rats (N = 32) to four handling treatments provided by caretakers from 57 to 74 days of age: (1) Minimal handling once weekly during cage cleaning (this handling was common to all treatments); (2) Exposed to a Passive hand for 2 min daily; (3) Tickled for 2 min daily, mimicking social play; (4) Restrained on back for 2 min daily, intended to mimic unreciprocated pinning by a dominant rat. Behaviour was assessed on days 77-87 during a Human Approach test of attraction to a human hand (HA; 2 min without hand, followed by 3 min with a familiar or unfamiliar passive hand), Elevated Plus-Maze (EPM) and Cat Odour (CAT) tests of anxiety, and weighing and injection procedures (to assess ease of handling). In the HA test, Tickled rats reared (interpreted as non-fearful monitoring behaviour) more often than Restrained rats prior to hand presentation (P < 0.05). In the presence of the hand, Tickled rats reared more often, and made more hand contacts than Minimally-handled rats, and uttered more 50-kHz ultrasonic vocalizations (interpreted as indicating positive affect) than rats from the Passive treatment. Both Tickled and Restrained rats spent more time near the hand than Minimally-handled rats (P < 0.05). Although rats from all treatments directed more behaviour towards a familiar hand, Tickled rats visited an unfamiliar hand more often than Minimally-handled rats (P < 0.05). The handling treatments did not affect ease of handling during weighing, or behaviour in the EPM or CAT tests. During the minute after receiving an intra-peritoneal injection, rats from the Tickled and Passive treatments spent more time rearing close to the experimenter than rats from the Minimally-handled and Restraint treatments (P < 0.05). In Experiment 2, rats (N = 16) were tickled by one handler, and restrained by another, for 2 min daily over 15 days. When preference for the two handlers was compared, rats spent similar time near, and in contact with, both handlers, but nibbled their tickler's hand (interpreted as play solicitation) more often than the hand of their restraint handler (P < 0.05). The results from both experiments indicate that, unexpectedly, the Restraint treatment was not aversive to the rats. Both the Passive and Restraint treatments had some value in habituating rats to humans relative to minimal handling. Nevertheless, tickling was the most effective method for reducing rats' fear of humans and hence improving their emotional welfare.