Misgendered Mice

Question
Ok I know it is common for petstores to misgender small pets like rodents and I think that is the issue I have here at the moment. I got two mice about five months ago from a pet store (I know it is better to go through a breeder) and I was led to believe that they were both female which was perfect since I was getting them to keep another female mouse I had company. They got along great and since the other mouse was much older and recently lost her friend I wasn't to surprised when she passed on naturally about a month later.

Well not wanting the two 'females' I have get lonely in their big cage I adopted two friendly and sweet girls, Ivy (little girl that looks like a little black and white cow) and Rose (mostly solid black), that someone had turned over to the pet store. When I brought them home I went through the introduction process and as I was handling my grey 'girl' with black and white flecks in her fur I noticed something odd.. the distance between her anus and where she pees was much further away from each other then on 'her' original cage mate, Oreo. Not only that I saw a distinct part in the fur that looked like where two balls would be located.

Looking back at Oreo I noticed that she was much larger then her counter part and looked like she had saddle bags developing. I realize now I may have been mislead into believing I had two female mice and may have been given a male and female instead! What strikes me as odd is that normally I am pretty good at sexing rodents so how I could possibly miss this is a shock.

Though my question is if I really do have a male and female why didn't they breed sooner? Why wait until five months later? Also is it possible for even the trained eye to mislabel a mouse's gender?

I plan on taking my grey one, Milky Way, to a professional tomorrow to make sure that my suspicions are correct. For now he/she is being housed in a spare cage that isn't as roomy as the other cage but still offers plenty of room for one mouse and has everything they could possibly need for the night.

Answer
Dear Drew,

Don't worry, you didn't lose your touch ;)

Some mice and rats are just weird, sex-wise. Every few months someone posts the underside of a rat or mouse in my rat and mouse group on Facebook and everyone, experts included, argues about its sex. Some boys drop very late. Some girls have a defect which causes them to look like they have growing testes. And some are just confused. There was one rat everyone was arguing about, and its human brought it to the vet and the vet said "male" and tried to neuter.. but there were no testicles inside. There was no vagina; whether there was a uterus or not was not something the rat really wanted anyone to find out, thank you very much.

So you might have a mixture, where this is not 100% male or 100% female, which would explain the absence of mating. Or you might have a boy who just never dropped. In the first case pregnancy is probably not possible. In the second, maybe it is. There are also sterile males. They mate but nothing happens. Other males "miss" and - um- try the pulling-out method of contraception (or pre-ejaculate? I'm not sure). Other males are just uninterested in sex. Either of the last two can change-- a boy can learn how, and a boy can have a hormone surge.

I am a little confused about you writing "why didn't they breed sooner? Why wait until five months later? "-- you didn't mention that the female was pregnant. Is she?

If he is a boy, maybe you could get him fixed so he could stay with your girls and not be lonely.

Best to luck to all of your confused little cross-dressing gender-confused mousie children!

squeaks n giggles,

Natasha