Young rats play intensely with each other. They chase and flee, roll and box, jump on each other and bounce around the cage. In fact, they use many of the same behavioral sequences we see later in agonistic encounters. So, what is the difference between play fighting and real fighting?

Is play fighting just immature fighting? Or is play fighting a stage unto itself, separate from adult fighting which develops later alongside it?

 At what age do rats play?

According to Panksepp (1981), rats start to play around 18 days of age, playing increases and peaks at 30-36 days, and declines thereafter.

Both male and female juvenile rats play, though males play at higher rates than females.

Does play lead to adult dominance hierarchies?

From weaning to sexual maturity at around age 56-60 days, the rates at which young male rats pin each other to the ground become progressively asymmetrical -- eventually, one animal pinned the other on average 70% of the time, and his pins lasted longer too (Panksepp 1981). At first, it was believed that this young top rat would become the adult alpha rat. In other words, stable adult dominance hierarchies emerged out of this asymmetry in play. However, the picture turns out to be more complicated than this!

Pellis & Pellis (1991) discovered a more complex story: early in development, one rat initiated more playful attacks, and the partner receiving more attacks got pinned the most. This makes sense, because usually the defender gets pinned. However, after sexual maturity, the same pairmate goes on to initiate the majority of playful attacks, but he, the attacker, is the one most frequently pinned! Why the reversal?

The reversal occurs because when attacked, the least frequent attacker becomes progressively more proficient at modifying his defense in order to launch a successful counterattack. Specifically, after weaning, most rats defend themselves by rotating to the belly-up position. With increasing age, rats are less likely to rotate completely, and only rotate the forequarters, leaving the hindquarters standing. From this standing position, they are more able to counterattack (Pellis & Pellis 1987). It is, in fact, the least frequent playful attacker, who learns to launch successful counterattacks, who goes on to become the most dominant adult rat.

This asymmetry is present before sexual maturity, and is highly predictive of which pairmate becomes dominant after sexual maturity: the rat that initiated the least playful attacks before sexual maturity becomes dominant after sexual maturity.

So, although play fighting is not a precursor to serious fighting (see below), the relationship between pairmates during play fighting is very consistent starting from an early age (Pellis & Pellis 1991).

As adults, the subordinate rat initiates most of the playful attacks, which the dominant rat tends to evade (Pellis & Pellis 1992). The subordinate rat responds in a more juvenile manner (belly-up roll) when playfully attacked by the dominant. Playful attack may therefore be an affiliative behavior and not an inhibited form of aggression (Pellis & Pellis 1991). After sexual maturity, the dominant rat launches most of the agonistic attacks. 

Play-fighting targets the nape, serious fighting targets the rump

Play fighting and serious fighting involve different targets. Play fighting involves playful contact of the nape, which, if successfully contacted, is rubbed gently with the snout. Playful defense involves various maneuvers to withdraw the nape from contact (Pellis & Pellis 1987). Serious fighting involves attack and defense of the rump and lower flanks, in which the goal is to inflict a bite on the rump (Blanchard & Blanchard 1977; Pellis & Pellis 1987).

Fig 1. Play fighting in young rats: attack on nape	Fig 2: Sequence of play fighting in young rats, with attempts to attack & defend the nape	Fig 3: Breaking free of nape hold with lateral movement.	Fig 4. Serious fighting sequence in adult rats: sidling attack on flank
			Pellis & Pellis. 1987. ref

The behavior patterns used during play fighting and serious fighting are similar, but the targets of attack are different. Rats continue to play into adulthood at low rates and using adult tactics such as boxing and sidling, but still gently targeting the napes of their opponents (Pellis & Pellis 1987).

Female juveniles play too, but there are qualitative and quantitative differences between females and males. Females play less than males, and respond to a nape attack at a greater distance than males do (i.e. they responded when their partner's nose was further from the nape). This enables females to use different defense postures. If females are given testosterone, their play becomes male-like (Pellis et al. 1994).

Play-fighting is therefore not merely a juvenile form of adult aggression, nor does play merge seamlessly into fighting at sexual maturity. The different targets persist well into adulthood: adult rats continue to play. Nor is play necessarily "practice" for adult fighting, because play fighting differs from adult fighting in tactics and goals. Play fighting and adult fighting therefore appear to be related but separate activities (Pellis & Pellis 1987).

Play fighting in adult rats

Pellis et al. (1993) found that subordinate rats directed more playful contacts (touching or nearly touching a second rat's nape with snout) at the dominant rat than at each other, and maintained a symmetrical play relationship between themselves. When the dominant rat was removed, one subordinate became dominant and the remaining subordinate initiated more playful contacts toward the new dominant. Playful contact may therefore serve a friendship maintenance function, to maintain familiarity and to be tolerated by the dominant rat.

An interesting side note: in groups of three rats, subordinates one of two strategies in their relationship with the dominant rat. Subordinates either avoid the dominant rat or they maintain close proximity with him. "Avoiders" are called omega subordinates, and the "close-in" rats are called beta subordinates (Barnett 1975 p. 125). Avoiders are more severely pummeled when they are encountered (Blanchard & Blanchard 1990), and if free to do so are more likely to emigrate from the colony (Barnett 1975 p. 125). However, if the dominant rat is removed, the omega subordinate is the one most likely to rise to become the new dominant rat (Pellis et al. 1993).

So, the two strategies have different costs and benefits. In the beta strategy, the rat becomes fully submissive toward the dominant, who in turn tolerates his presence more. The subordinate may therefore gain more access to food and females. In the omega strategy, the subordinate maintains an uneasy truce with the dominant. This may be more costly to the subordinate, as he is the target of more aggression from the dominant rat. But it may have the advantage of positioning him so as to take over the dominant's role if it becomes available (Pellis et al. 1993).

What causes a rat to adopt one strategy or the other? Rats vary in temperamental factors that influence aggressiveness and boldness (which may have a genetic component). In colonies of rats that differ in boldness, the boldest rat may become dominant, intermediates may adopt the omega strategy, and the least bold may become betas (Pellis et al 1993).

Summary

Play fighting is different from adult fighting, in that it targets a different area of the body and may involve different tactics. Play fighting involves attack and defense of the nape, while serious fighting involves attack and defense of the rump.

Adult dominance hierarchies can be predicted from juvenile play fighting: the most frequent play-attacker becomes the subordinate rat after sexual maturity. This reversal is due to the least-frequent-attackers' emerging proficiency at launching successful counterattacks.

Play fighting serves a social maintenance function in both juveniles and adults. Subordinates direct more playful contacts at dominant rat than at each other. As adults, the dominant rat tends to evade these encounters with adult defense tactics, while subordinate rats, when playfully attacked, roll over into the juvenile defense position. Initiating such playful attacks may lead the dominant rat to tolerate the subordinate's presence.

Not all subordinates are equal, however. Some initiate many playful contacts with the dominant, and in turn tend to be tolerated by him. Others avoid the dominant and are more frequently attacked by him when encountered. These avoiders are more likely to rise to the dominant position if the dominant rat is removed.