Development of “Anchoring” in the Play Fighting of Rats: Evidence for an Adaptive Age-Reversal in the Juvenile Phase

Abstract

During play fighting, rats often assume a pinning configuration, where one animal stands over its supine partner. The on-top partner can stand on the ground or on its supine partner with its hind paws. When standing on the ground, the rat is more stable and is better able to respond to its partner’s actions. The frequency of this more stable pattern of standing during pinning (called anchoring) is higher following puberty than during the juvenile phase. Two hypotheses explaining this developmental change in anchoring were tested. The first hypothesis maintains that the lower level of anchoring in juvenile rats reflects an immature sensorimotor capability. The second hypothesis suggests that, since rats are more playful as juveniles, such heightened levels of play may interfere with movements otherwise used to maintain the stable anchored position at this age. Neither hypothesis was supported: infants have similar anchoring levels to postpubertal rats, and juveniles have a relatively low level of anchoring irrespective of how frequently they play fight. Therefore, the lower level of anchoring in the juvenile phase appears to be a developmental peculiarity of this age. These findings support the view that play fighting in the juvenile phase may be organized in a manner to increase the occurrence of the experiences that are developmentally beneficial in this activity.