Why does your mouse have bruises on its tail?
Bruises on a mouse tail can be caused by an injury, illness, or
may be self-inflicted. Check the cage for any areas where the tail
may get caught or be accessible to other pets, including cage
mates, and small children. Observe the mouse for chewing. Make sure
the mouse is handled gently and never by the tail. If no cause can
be found and the bruising continues, have the mouse checked out by
your vet.