I don't know who started it -- the first thing I saw was a bit of mutual threatening and a quick scuffle that ended with Hera on her back, squeaking, with a tuft of Thor's fur in her teeth. So that was inconclusive. She ran off and hid in the yellow thing, and he took up a post outside and hissed at her. The way the cage is set up right now, she could go around behind the yellow thing under a short ladder, or she could go down the ramp to the lower half of the cage, but she couldn't get to the ramp up without going past him. Hera spent an hour or so poking around under the ladder and around the front of the yellow thing, getting hissed at. Eventually she got bored, wandered down a level or two, groomed a bit, then strolled upstairs for a snack, ignoring Thor completely. I'm not sure if that counts as a win or a truce or what; I've not seen this blocking-into-a-corner behavior before.
Historically, among the females I've had, dominance goes not to the biggest or strongest rat, but rather to the most persistent rat. This would tend to weigh in Hera's favor. However, Thor is twice her size and since the stroke her grasp on the concept of "down" has been fuzzy, so she's very easy to knock over. (Oddly, she seems to have a pretty good grasp of using the terrain to her advantage, reaching down from above or into a hammock from a nice stable position so she can slap the boys around without much fear of retaliation.)
It's always interesting to watch the rats work out their social structure. Once they've got the hierarchy figured out, they won't test it much, so the only chance I have to see how it's going to work out is to watch while it's still unsettled. My current prediction:
I'm really not sure where Aphrodite's going to fit in, because she's being a bit odd around the boys so she may wind up lower than I expect. Odin seems to have fairly strong feelings about whom he should be allowed to knock over (he was picking on Hera yesterday).
Perhaps I should establish a betting pool.