mouse getting skinny

Question
I have 2 pet male mice (Miles Davis and Thelonious Monk) that I bought in a pair about 6 weeks ago. One was always smaller than the other, but they always have gotten along very well. I noticed yesterday that the smaller one (Theo) looked even tinier, like he had lost weight and did not look well. He was slow-moving, breathing quickly and got annoyed when Miles was trying to be friendly with him (letting out quiet squeaks), as though he just wanted to be left alone. I cleaned out their aquarium and when I put everything back in, included a large box that takes up about half the cage and put a separate water bottle and food dish in there so that the Theo would not be disturbed. I'm not sure what is wrong with him or if there is anything that I can do to help him. All I can tell is that he looks skinny and weak. Did I do the right thing by keeping them separate? Should I be worried about the something happening to Miles (the healthy one)? If something happens to Theo, should I get a replacement for Miles to play with?

Answer
Dear Amanda,

Miles Davis and Thelonius Monk must be littermates; males don't usually get along.  

Sometimes mice get a sickness where they just waste away.  This is genetic.  When I have had this, siblings have been perfectly healthy, so Miles will probably not get sick too.   

The best you can do for Miles is make him as comfortable as possible.  If he isn't eating, try to tempt him with treats such as crackers or popcorn and even a tiny dab of commercial peanut butter.  

I would try to keep the two mice together.  Mice are social animals and if possible like mousie companionship.  Soft squeaks may not be a problem.  Still, you are the best judge of whether Miles is unhappy living with Theo.  

If Theo passes away, unfortunately it is not a good idea to get a replacement male.  Males will actually fight to the death.  They just want a lot more territory each than can fit in a mouse cage.  Because males must live alone, they need a whole lot of human love.  You would need to take Theo out at least an hour a day for human contact.  If you got another mouse in another cage, the two would communicate between the cages, and that would help.  

squeaks,

Natasha