The prejuvenile onset of play fighting in laboratory rats (Rattus norvegicus)

Play fighting in rats is most frequent in the juvenile phase (30-40 days) and then wanes following puberty. During the juvenile phase, the most commonly used defensive tactic to block access to the nape (i.e., the play target) is to rotate around the longitudinal axis to supine. From this position of lying on its back, the defender uses its limbs to hold off the attacking pup. With the onset of puberty males, but not females, switch to more adultlike patterns of defense. The adultlike pattern involves partially rotating around the longitudinal axis of the body, but retaining ground contact with the hindpaws. In this position, the defender is able to rear to a defensive upright posture, or can slam into the partner with its hip. In this study, the manner of onset of play fighting and its components was investigated by daily videotaped observations of six litters of Long Evans hooded rats, starting at 15 or 16 days and ending at 30 days postnatally. The predominant form of social interaction in the midteens was allogrooming, but by 20 days, playful attacks to the nape were the most common form of contact. Allogrooming was most often directed to the head, not the nape. With respect to playful defense, the more adultlike tactics matured first, with all tactics present in the repertoire by 20 days. The fully juvenile pattern of defense did not mature until 25-30 days with both males and females exhibiting the same developmental pattern. These data lead to several conclusions. First, play fighting is a separate category of behavior from the outset, and does not emerge from allogrooming. Second, the adultlike defense patterns do not emerge gradually from the juvenile ones at puberty, but rather, all are present in the repertoire from infancy; and third, both males and females have the same pattern of onset of play fighting. These conclusions are discussed with respect to the possible functions of juvenile play fighting.