TABLE 1.

Definition of play and non-play behaviors as scored in the observation.

Play behaviors	Pinning	One rat lying with the dorsal surface on the floor with the other rat standing over it.
	Wrestling	A group of play-specific behaviors, including wrestling and boxing.
	Chasing	One rat moving in the direction of or pursuing the partner while the partner is moving away.
	Nape Attack	One rat actively nuzzles or pounces on the nape of the other.
	Evasion	The partner evades by running or jumping away.
	Self-pinning	One rat rotates to a supine position without preceding occurrence of an obvious attach by the other.
	Crawl-over	One rat crawls over the other rat.
	Biting	One rat bites and occasionally pulls the tail, ear, leg or fur of the other rat.
Non-play behaviors	Sniffing	One rat sniffs the other rats face, neck or anogenital area.
	Grooming – self	One rat licks paws, and cleans fur of themselves.
	Grooming– other	One partner actively grooms the other (licking, cleaning fur).
	Physical contact	One rat exhibits physical contact with the other without showing one of the other aforementioned behaviors.

While evasion, self-pinning, crawl-over, and biting was observed they did not occur with sufficient frequency and therefore, apart from being added to the total duration of play behaviors, are not analyzed or discussed.