The developmental onset of a rudimentary form of play fighting in C57 mice

Play fighting in its most elaborate form involves nonagonistic wrestling between pairmates, where one partner grabs, holds, bites, or otherwise contacts the other. Such play occurs in the absence of the functional consequences associated with serious fighting (e.g., resource acquisition or protection). Typically, the biting, nosing, or grooming contact during play fighting is directed at specific body targets. House mice have been classified as a species that lacks such play, even though play fighting is present in closely related species such as the rat. In this study, six litters of C57 mice were observed daily from the week before weaning until the week after weaning (15-30 days postnatally). Thirty-min videotaped records were collected daily for each litter. Consistent with other studies, over 85% of all play involved locomotor play, and most of the social play involved noncontact locomotion (86%). However, a rudimentary pattern of the "attack and defense" typical of play fighting was found to occur, albeit at a low frequency (2% of all play). Most playful attacks involved snout contact with the partner's rump, but evidence is provided that suggests that this rump contact may be transitory, with the nape area being the primary target for play. Most of the playful attacks elicited playful defense (97%), which in all cases involved the defender evading such contact by leaping or running away, or by dodging laterally away from the attacker. Therefore, there appears to be directed playful attacks in this species, with defense limited to evasion. Defensive tactics leading to wrestling were never observed. That is, play fighting in mice involves only a small subset of what other species, such as rats, exhibit. Nonetheless, the basic components of attack and defense are present in mice.