Play fighting of rats in comparative perspective: a schema for neurobehavioral analyses
Introduction
Play is one of those perplexing phenomena that, despite the near universal agreement on recognizing instances of this behavior [48], there is little consensus about the objective criteria that are used to arrive at that agreement (see Appendix A in Ref. [26]). One strategy to resolve this problem is to use an heuristic definition that is modified or discarded as more knowledge about the phenomenon becomes available. Such an approach avoids potentially acrid semantic debates that could delay further research [2]. This is clearly a sensible approach to take, but it can also have some potentially negative consequences. A major problem is that once a behavior is labeled, it tends to be reified [90]. That is, all the qualifiers that should go along with the label are gradually dropped, and we grow to believe that the definition stands for a real phenomenon, rather than being a temporary convenience which may or may not stand up to further scrutiny. Other behavioral categories, such as aggression, have similarly faced this problem. However, aggression, unlike play, can often be linked to fitness enhancing outcomes (e.g., access to a mate or food, protection of a territory or a brood), and these functional outcomes have provided a non-arbitrary framework by which to classify the phenomena generally considered to be “aggressive” [3]. In the absence of such identifiable, biologically relevant outcomes [43], a reified label of “play” can retard, rather than advance, our understanding of the phenomena involved.
From a comparative perspective, as shown by Fagen's [26]extensive survey of the natural history of play, phenomena labeled as play are widespread amongst mammals and birds. Importantly, Fagen notes that the lack of reports of play in some species cannot be taken as evidence of its absence, but rather as the absence of evidence. Where play is reported, it appears that some taxa will exhibit some forms of play, whereas others will not. For example, play with objects is widespread amongst primates and carnivores, but absent or rudimentary amongst rodents 26, 36. Social play, especially play fighting, is the most widespread form of play reported for mammals. Play fighting is also one of the least ambiguous forms of play, whereas in many cases, play with objects or prey may be better explained as misdirected predatory behavior 25, 60, 84. The widespread occurrence of play fighting and the greater confidence in its categorical status may account for the fact that it is the form of play most often studied and reported in the animal literature. Based on the entries in Animal Behavior Abstracts from 1984–1994, 166 report empirical data on play. Of these, 82.5% were either solely devoted to play fighting, or included play fighting as a major component. For better or for worse, play fighting has become the sine qua non in the study of play.
With regard to our knowledge about the neurobehavioral underpinnings of play, almost all that we know has been derived from one species, the laboratory rat 94, 108. Fortunately, the most commonly occurring form of play in rats is play fighting [88], and most neurobiological studies of rats have focused on this form of play 52, 104. Thus, even though a definition of play fighting that contains uncontested objective criteria does not exist, and given that the rat is a practical species for neurobehavioral studies of these phenomena, the conclusion to sidestep further debate on definition and get on with experimental analyses is very tempting [51].
This paper reviews what we know about the play fighting of rats, and contrasts this with information about the play fighting of other mammals. If we are to use the rat as a model for play in mammals, then it is essential that we determine what aspects of this phenomenon are generalizable from rats. For such a purpose, the differences are just as important as the similarities. That is, how good a model is the rat for mammalian play fighting? We will show that a research perspective that treats play fighting as a unitary phenomenon, where the principle dependent variables measured are the frequency and duration of its occurrence, necessarily leads to an impoverished outcome, with limited generalizability. Play fighting can also vary markedly in content, both amongst species 58, 61and within 13, 82. It will be shown that play fighting in rats is a multifaceted phenomenon, with some aspects being useful as a model for other species and others not.
Section snippets
Play fighting
A commonly held definition of play fighting is that it involves the use of the species-typical behavior patterns of agonism, which are used in a non-serious manner. That is, their use does not lead to the functional consequences that are derived from their serious use (e.g., access to a resource). Furthermore, this definition holds that variation amongst species arises from species-specific differences in agonistic repertoires and from species differences in body shape and maneuverability 1, 26
On labeling and classifying play fighting
The body targets competed over during play fighting can be those otherwise contacted during serious conspecific fighting 1, 58, predation 42, 92, and sexual 58, 61and non-sexual [4]amicable behavior. Given the heterogeneity of the origin of the body targets involved, the use of the term “play fighting” seems inappropriate, as it appears to emphasize the linkage to serious fighting. An logical alternative is to label such non-serious competitive social behavior as “rough-and-tumble play”, a term
Acknowledgements
We thank the reviewers for their thoughtful comments, Lori Smith for help with some of the laboratory work, Evelyn Field for help with the graphics and Leanne Wehlage for some of the typing. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada to SMP.
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