Have you added to the group or changed "partners". How long has your group been together. I have kept thousands of rats over the last five years, and the only time my rats would fight was when their groups were changed. Adding a new female usually causes minor squabbles with the other females as they try to determine hierarchy. Adding a new male in a long term group, can cause the dominant female to become very aggressive toward the new guy. I once had a male rat whose ear was virtually removed by an aggressive girl. Adding an adult male rat into a situation where there is another male will usually result in fights as well, but most people don't keep adult males together unless they've been raised together.
Did they have food available? Sometimes, if food or especially if water is scarce (check your water source...rats love to plug it up), older individuals will die, and the others will feast on the dead body. This is more common when dehydration is an issue than food, as rats learn quick that blood hydrates!
I gave up breeding rats as having an attached garage with breeders tends to make the house stinky...I wish I could still breed them though as I spend almost $800 on frozen feeders a year, which is over 8 times what I used to spend!
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Thanks,
Amy
My Boids