Cute or aggressive? When to not pair?

Under what circumstances do you not pair up a male and female based on their behavior? What does female aggression look like?

This pretty cute, but also looks like competition or even dominant behavior by the female. He's in the water bowl. I made her give him a little space and then she went elsewhere in the tub on her own. This is day 2 of them being paired, and I did see them in a more normal configuration earlier (not locking yet though).

https://lh4.googleusercontent.com/-y...310_093727.jpg

It's pretty typical for a female to sit on a male when she's had enough of him -- she uses her bulk to control him and block irritating courtship behavior. Sometimes they do it when they're not receptive at all, not receptive to a certain male, after they've locked and the female does not want to lock with him again . . . really, whenever they decide breeding is over for a given session. (The reverse of when a pair is locked, and most males use the lower half of their bodies to control hers.) Some girls do it this way, some are more subtle. When I see something like this, I give the pair a couple weeks/a month before reintroducing. After that, I kinda play it by ear as to whether or not to try another male or wait for next season.