I have two pet male mice (long story, lol), and even though they're both males, they usually get along just fine... but whenever I clean their cage, the younger one attacks the older one pretty violently for a few minutes. Sometimes the attacks are bad enough to leave blood on the other guy! After a few minutes they settle down, and all is well until the next cleaning.

My best guess is that it has something to do with their environment smelling differently - which probably confuses the one mouse, to where he believes there's a "strange male" in his home. Kind of like when you take one cat to the vet, and the other cat hisses at them when you return? Again, the change in smell throws off their familiarity. Anyone know what's happening here, and/or how to stop it? I had an idea of maybe saving a tiny bit of the dirty bedding, and adding that to the new stuff each time. Whaddya think?