Domestication and Diversification: A Comparative Analysis of the Play Fighting of the Brown Norway, Sprague-Dawley, and Wistar Laboratory Strains of (Rattus norvegicus) Stephanie M. Himmler University of Lethbridge Klaudia Modlinska and Rafal Stryjek Polish Academy of Sciences Brett T. Himmler University of Lethbridge Wojciech Pisula Polish Academy of Sciences Sergio M. Pellis University of Lethbridge Laboratory strains of rats are a commonly used subject to study play behavior. Recent research has shown that play in one laboratory strain of rat (e.g., Long-Evans hooded) differs in a number of ways from its wild counterparts. These findings suggest that domestication affects some aspects of play behavior. However, there are multiple strains of laboratory rats, which have been domesticated through different lineages all derived from wild rats and it cannot be assumed that all domestic strains are identical in their play. Therefore, the aim of this study was to compare the play behavior of three other strains of laboratory rats (e.g., Wistar, Sprague-Dawley, and Brown Norway). All strains were similar to each other as they all engaged in high frequencies of play, tolerated similar interanimal distances before initiating playful defense and displayed similar acrobatic capacities, suggesting domestication produces some common changes in play and other factors that influence play. However, strains differed significantly from one another in the use of tactics that promote bodily contact during play. Indeed, in this regard, some strains were more similar to wild rats than others, suggesting that some domestication-induced changes are either unique or more prominent in some laboratory strains than others. Such a mosaic pattern of transformation not only offers the possibility of using strain differences to characterize the genetic factors contributing to different facets of play, but also cautions researchers from making rat-general conclusions from studies on any one strain. Keywords: rough-and-tumble play, domestication, rats, strain differences, attack and defense Rough-and-tumble play is one of the most commonly studied forms of play (Pellis & Pellis, 1998) with domesticated laboratory rats being the subjects most often used in experimental studies (e.g., Bolles & Woods, 1964; Meaney & Stewart, 1981; Panksepp, Siviy, & Normansell, 1984; Pellis, 2002; Thor & Holloway, 1984; Vanderschuren, Niesink, & van Ree, 1997). However, domestica- tion affects a variety of traits, including bodily composition, phys- iology, neural mechanisms, and behavior (e.g., Albiach-Serrano, Brauer, Cacchione, Zickert, & Amici, 2012; Castle, 1947; Cop- pinger & Coppinger, 2001; Lockard, 1968; Takahashi & Blanchard, 1982; Pisula, Turlejski, Stryjek, Nałle ˛ cz-Tolak, Gra- biec, & Djavadian, 2012). Many of these domestication-induced changes in psychological characteristics are likely to affect how animals play (Pisula, Gonzalez-Szwacka, & Rojek, 2003). There- fore, it cannot be assumed that the play of domesticated rats represents the situation across all mammals, domesticated and nondomesticated. Indeed, there are differences in the behavioral organization of play in wild and domesticated rats (Himmler, Stryjek, Modlinska, Derksen, Pisula, & Pellis, 2013b). The Himmler et al. (2013b) study compared play behavior in the Long-Evans hood strain of domesticated rats with the Wild War- saw Captive Pisula-Stryjek (WWCPS). The WWCPS strain was derived from five independent colonies of wild rats in Warsaw, Poland, and was handled in a way that minimized human contact Stephanie M. Himmler, Department of Neuroscience, University of Lethbridge, Lethbridge, Alberta, Canada; Klaudia Modlinska, and Rafal Stryjek, Institute of Psychology, Polish Academy of Sciences, Warsaw, Poland; Brett T. Himmler, Department of Neuroscience, University of Lethbridge; Wojciech Pisula, Institute of Psychology, Polish Academy of Sciences; Sergio M. Pellis, Department of Neuroscience, University of Lethbridge. We thank Vivien Pellis for her comments on this article. Wistar and Brown Norway research was funded by the Polish Ministry of Science #UMO-2011/01/B/HS6/06442. The funders had no role in study design, data collection and analysis, decision to publish, or in the preparation of the manuscript. The Natural Sciences and Engineering Research Council of Canada supported the Sprague-Dawley research. Correspondence concerning this article should be addressed to Stephanie M. Himmler, Department of Neuroscience, University of Lethbridge, Leth- bridge, Alberta, Canada. E-mail: s.derksen@uleth.ca This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. Journal of Comparative Psychology © 2014 American Psychological Association 2014, Vol. 128, No. 2, 000 0735-7036/14/$12.00 DOI: 10.1037/a0036104 1