Weight loss in mouse

Question
I have two mice approximately one year old.  Over the last month(?)I have noticed that one is gaining weight while the other looks underweight.  No competition.  Thin one still eats, drinks, defecates and is active. Also still has shiny coat but looks very thin.  Can't figure out why or what to do.  Don't want to offer too much fatty food since her companion is fat but starting to worry that the weight loss is unhealthy.

Answer
Dear Nati,

Mice age at very different rates.  The shortest-lived can actually die of old age at about 15 months.  Thus is is possible that your slender mouse is simply aging.  

There are as many reasons (except perhaps social ones) for mice to be fat or skinny as for people.  Genetics plays a large role.  Illness or disease can cause weight gain or loss.  Depression can cause weight loss.  Exercise and food are crucial of course as well, but I assume these factors are the same for both mice.

One unfortunate condition that I have experienced a number of times is a kind of wasting away of a young mouse, as though it were anorexic.  However, this is usually with younger mice.  And it appears to be genetic; there's nothing a vet can really do.

If you want to be sure both are as healthy as they can be, the best thing is to take them to a vet and have them checked out.  However, if both are happy looking, pooping, eating, ambulating, and have glossy coats, my own course of action would be to leave it be.  

Sorry not to be able to give a definitive answer.  As small as they are, mice are complex little creatures.

Best of luck and health to both mice.

squeaks,

Natasha