fatdeer mouse?

Question
Hello. We have a deer mouse, found as a tiny orphan around 2 years and 3 months ago. We have kept him (or her?) in a terrarium, with recommended mouse/rat food and water for his at will consumption. He has always been active at night, with normal, slim build and always has seemed very clean, active and healthy. Recently, in the last 2 weeks, he has suddenly changed. It looks like he has added a huge amount of weight suddenly, on both sides of his body. H appears asymmetrical, with the weight slightly more forward on his left side than the weight deposit on his right side. Also, it appears as though there is more deposit on the right side of his back, although the weight is primarily on the sides.
The very sudden onset of this weight, the amount of increase in his size, and the fact that the weight is not symmetrical, is very disturbing!
An additional feature to all this is that his normal schedule seems to have changed somewhat. He is normally very nocturnal, as expected. With this added weight has come a change to this, and he is often moving around during the day, and seems to be eating a lot.
So what gives???!!! What do you think is going on with our foundling, "Peanut"?
Thanks,
Donna

Answer
Hi Donna,

First of all, congratulations on your orphan rescue success!  It's tough to raise those little guys, and two and a quarter years so far is wonderful!  Great job.

This might seem like a silly question, but he is definitely a boy, right?  I ask because along the sides from armpit to hip is a very common place for mammary tumors in female mice, but is almost never seen in males.  Mammary tumors, while not lethal on their own, could grow very quickly and become quite large, giving a fat appearance.  Pregnancy from a sneaky wild mouse who's moved in for the winter could also explain a sudden weight gain around the sides.

If he's definitely a boy, however, there are a few other possible causes.  It could simply be weight gain related to his age, as two years and change is decently old for a wild mouse.  As animals age, their hormones and metabolism change, and you may need to switch to a food a little lower in unnecessary proteins and fats.  An allergic reaction is also possible, if anything recently has changed in his environment or diet.  Another possibility would be internal tumor growth, which sounds scarier than it is.  Tumors in and of themselves are not painful, and might not be lethal unless they put pressure on the body's systems.  Since mice are too small for surgery, tumor growth would not really be anything you could fix, so I would look at other possibilities that CAN be remedied before assuming that tumors are the cause.

Do you have a vet in your area who works with pocket pets like mice and can take a look at him?  If not, could you send a clear photo so I could try and help you figure out what is going on?

-Tam