Multiple Differences in the Play Fighting of Male and Female Rats. Implications for the Causes and Functions of Play

Juvenile females increase to the same level of play as males if isolated long enough.

“This appears to be because juvenile females increase to the same level of play as males if isolated long enough (Pellis, Field, Smith, & Pellis, 1997).”

At its peak, male rats typically play more than female rats.

“At its peak, males typically play more than females (Olioff and Stewart, 1978; Pellis et al., 1997), making play particularly valuable for studying sex differences in social development.”

Social isolation increases the motivation of rats to play.

“Given that social isolation increases the motivation of rats to play (Panksepp and Beatty 1980; Pellis et al. 1997), this suggests that the males' preference to play with females is robust, not being masked by a general motivation to play.”

The males' preference to play with females is robust and not masked by a general motivation to play.

“Given that social isolation increases the motivation of rats to play (Panksepp and Beatty 1980; Pellis et al. 1997), this suggests that the males' preference to play with females is robust, not being masked by a general motivation to play.”

With the onset of puberty, the frequency of play fighting declines in rats.

“With the onset of puberty, however, the frequency of play fighting declines (Pellis & Pellis, 1990, 1991;Pellis, 2002;Pellis et al., 1997;Thor & Holloway, 1984).”

In male rats, the probability of defense against a playful attack does not change with age.

“With regard to playful defense it has been shown that while the probability of defense against a playful attack does not change with age in males (Pellis & Pellis, 1990;Pellis et al., 1997), the use of complete rotations, in response to playful attack, decreases with the onset of puberty (Pellis & Pellis, 1990, 1991).”

The reversed sex difference is more likely due to differences in the genetic profile of the strain used.

“More likely, this is due to the differences in the genetic profile of the strain used (Pellis et al., 1997;Reinhart et al., 2004).”

The sex difference in the frequency of play (females playing more than males) is contrary to that typically reported in many other studies.

“With regard to the sex difference in the frequency of play, this finding is contrary to that typically reported in many other studies (Pellis, 2002;Pellis et al., 1997;Thor & Holloway, 1986).”

Spontaneous levels of play are much lower than the levels of play exhibited following some period of social isolation.

“It has been shown, however, that spontaneous levels of play are much lower than the levels of play that are exhibited following some period of social isolation (Panksepp, 1981;Pellis et al., 1997), and in the present study, rats were observed following 24 hr of social isolation.”

Social isolation increases the probability that play initiations result in the play partner being pinned, as opposed to adopting other defensive strategies.

“Social isolation also increases the probability that play initiations result in the play partner being pinned, as opposed to adopting other defensive strategies (Pellis et al., 1997).”

Play fighting in rats has been thoroughly studied and characterized.

“Play fighting in rats has been thoroughly studied and characterized (Pellis & McKenna, 1992;Pellis et al., 1997), and is distinguishable from aggressive behavior (Pellis & Pellis, 1987).”

Initiations of social play almost exclusively led to pinning behavior, with very few instances of other playful defensive strategies such as partial rotation and evasion.

“Likely due to the relatively small space available in the home cage and to the high motivation to play induced by isolation, initiations of social play (i.e., attacks to the nape) almost exclusively led to pinning behavior, with very few instances of other playful defensive strategies such as partial rotation and evasion (preliminary experimenter's observations; see also Pellis & Pellis, 1991;Pell”

Hierarchy formation was present and affected the play bouts already at the age of 41-43 days.

“This may support the fact that, although alternations of play roles are characteristic of social play in adolescent rats (Pellis et al., 1997), hierarchy formation was present and affected the play bouts already at the age of 41-43 days.”

This result may relate to hierarchy formation, the onset of which already occurs during adolescence, and might be driven by repetitions of Pinning by the animals which are to become dominant.

“This result may relate to hierarchy formation, the onset of which already occurs during adolescence (Pellis & McKenna, 1992), and might be driven by repetitions of Pinning by the animals which are to become dominant (Pellis & McKenna, 1992;Pellis et al., 1997).”