Playful defensive responses in adult male rats depend on the status of theunfamiliar opponent

Adult male rats reared as pairmates from weaning were tested in a neutral a rena with both members of another pair (one at a time). The unfamiliar pair s were found to engage in play fighting, although they were more likely to escalate the encounter into serious fighting than were pairs of familiar ra ts. Based on their within-home pair behavior, each pairmate was designated as a dominant or a subordinate. When the test encounters between unfamiliar males were analyzed with regard to whether the pairings consisted of two d ominants, two subordinates, or a mixed pair, the pattern of play fighting w as found to be attenuated. Both dominants and subordinates were more likely to initiate playful encounters, to respond defensively during these encoun ters, and to do so using adult-typical tactics of defense when paired with an unfamiliar rat that was dominant in its home cage. The mechanisms by whi ch the home status of unfamiliar male rats can be identified by another mal e are discussed, particularly with regard to the role that play fighting ma y serve for this function. It is concluded that the data support the hypoth esis that play fighting can be used by adult rats for social testing, which in this case seems to involve ascertaining the opponent's fighting capabil ity. (C) 1999 Wiley-Liss, Inc.