field mice reaching maturity

Question
Our grand daughter rescued 2 field mice mid September '08.  Eyes not opened yet.  Here it is almost mid February and they are thriving and enjoying the life of Riley in a tank in her room.  She called yesterday they were "fighting".  Since their eyes opened she hasn't been able to lay a hand on them so unsure of their sex.  We were hoping for 2 of the same sex.  She said the fighting was a bit risque so we are wondering if maybe she has a boy and girl.  If this is so wouldn't they have matured before 4 months?  Thank you for the help.

Answer
Dear Joanne,

A pair of males or females trying to establish dominance can show mounting behavior, so there's no obvious answer.  If you have two girls, it will sort itself out moderately quietly within a month or so.  Two boys may fight to the death, and a pair will overrun the place with  mice before you can blink.  However, if the fighting is all in one direction (one chases the other but never or rarely vice versa) and only includes mounting without other fighting behavior, it's more likely to be a pair.  If there is bloodshed (in which case they must be separated), it's probably two boys.  

As male mice mature their testes enlarge, making it look like they have buttocks.    There is no other obvious difference without examining the mice more closely.  

Please let me know if I can be of further assistance.

squeaks n giggles,

Natasha