Try separating the mice. The mouse being aggressive is probably wanting morespace to itself, or coming on baby making (baby part depends on if it's old or young and if it has lived with males before). Make shure the aggresive one is female. Male are always aggresive with another mouse. the last reason is that it might have gotten a little lonley with out human company. At the place you bought them they probably got much attention, and now not as much.
It depends. Is the male becoming aggressive? If it is then something's wrong. Males are usually protective of the females and pups (depending on breed) so if it is acting aggressive towards the female then it will hurt the puppy's. Other male dogs will be fine around the pregnant female but become aggressive towards the pups. Do your research on the breed to determine what is the best way to protect not only the female and pups but the male because the female will attack the male if he try's to attack the puppy's.
An "aggressive."
If there is another female around she can get domanate of her surroundings. A female pit in heat can be aggressive and even more so if she has has a litter recently.
no they grow aggressive when there is a male in the breeding season
The male is far more aggressive than the female - they are large, and need to assert their dominance over their territory.
No, there is no difference.
yes, all of my hamsters that i have owned, the girls have been more aggressive
I don't think that is true. It's like saying a blonde is dumb, but they aren't. But female budgies are more aggressive than males.
Female
Try to hold her as much as you can and just keep loving her and she won`t be aggressive long.
only when the puppies arrive
It depends on which species you're asking about, and it depends on conditions. For humans, for example, in a random situation the male is statistically more aggressive, but in defense of hearth and home, the female is likely to be more aggressive.