International Journal of Comparative Psychology

During play fighting, rats attack and defend the nape and during the course of this competitive interaction, they may adopt a configuration in which one animal stands over its supine partner (i.e., pin). Because the pin configuration is typically frequent and relatively easy to identify, it has been widely used as a marker to detect the effects of experimental treatments. In the present study, the frequency of pinning during standardized, 10min trials in three strains of rats, Long Evans hooded (LE), Sprague-Dawley (SD) and wild (WWCPS), was compared. LE and SD had higher rates than WWCPS rats (#/min: 6.5, 5.5, 1.5, respectively). When adjusted for strain differences in the frequency of attacks, SD as well as WWCPS rats had lower rates of pinning compared to LE rats. Both SD and WWCPS rats were less likely to use tactics of defense that promote pinning. Moreover, while the majority of the pins achieved in LE rats arose from the defender actively rolling over onto its back, the majority of pins in WWCPS rats arose because one partner pushed the other onto its back. SD rats were intermediate in this regard. Finally, once they do adopt the pin configuration, SD rats are less likely to remain supine than LE and WWCPS rats. That is, both SD and WWCPS rats have significantly fewer pins than LE rats, but a different combination of factors account for this. These data highlight the need to use a battery of measures for ascertaining the effects of experimental manipulations on play. Some suggested guidelines are provided.