Feigning ferocity

I'm routinely amazed by the intensity of the beatings that Cayley and Nyx inflict upon each other in their play-fights.  I can often hear thumps, smacks, thuds, and other sounds of mutual combat from clear across the house.  They're just horsing around, but it's genuinely rough.

There's a shot above of Cayley stomping Nyx's guts out, and check out the close-up of those snarling teeth!  Holy crap!!

And then when the mutual thrashing is finished, there's always a big slobbering scene of hugs and kissy-kissy:

This behavioral sequence must be innate and universal, because neither one of them have ever observed anything like it in this house!  There's never been any example for either one of them to mimic, so it must come from somewhere on the primal inside.  Maybe this is what happens when kids don't have siblings (?).