Dead Mouse

Question
I have 3 mice, 1 male and 2 females and I had to go out of town from Friday morning to Sunday evening. I filled up both the food and water bottle for the mice before leaving, and when I got back earlier this evening I went to check on them to see if they needed more food, cause they can be pigs sometimes, and I noticed that one of the females was dead right outside the nest box. I have had them together since the beginning of Feb with no fighting, so I was wondering why either the male or the other female, also her sister, would kill and then partially eat the one.

Answer
Hi Randy,

I am assuming they are together for breeding purposes?  Mice of both genders can become unpredictable when breeding, especially if they are not from known lines (if you know the past few generations you may be able to work out who was the more likely culprit).  Some females can become aggressive toward others when pregnant, especially if left with the male, as having a male around and constantly bothering the does is usually a very stressful environment.  Males become more territorial when breeding, and although this isn't usually directed toward females it certainly can be.  Aggression during breeding from either gender is very, very uncommon in responsibly bred lines, but for randomly bred mice, it is impossible to guess these sort of behaviors.  It is also possible that she passed on from natural causes (lifespans are only an average - surprises can absolutely occur) and the body was eaten afterward.  Was there, in fact, plenty of food and water remaining when you checked on them?  If resources ran out it is always possible that could have sparked aggression, too.  Always tap the water bottle sipper when you refill it to check for flow, as they can sometimes become stuck.

Since you do not know if the cause of death was from aggression, I would not breed the two remaining mice.  Aggressive temperaments can be genetic, as well as learned in the first few weeks of life, and this isn't a trait you generally want to pass on.  I would separate the two survivors just to be safe, since you don't know if one of them is aggressive or not.

-Tam