I just came across this article that I thought everyone would find interesting...

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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.

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