I chcked all over, Tab said to run your hand all over the inside of the cage, which I did, didn't find any sharp edges or anything. So I'm not sure what caused it. There were no signs of fur anywhere, and they were all cuddling when I found them, they hopped away and I noticed blood on the hidey house that they have tipped over and used as a bed. When I looked at the chins that's when I noticed the blood on the shelves. I'm still keeping a watch over him, the blood stopped bleeding on it's own and it appeared to have happened over night. There was no blood when I said good night to them last night, and what freaks me out the most (though not as much as finding blood everywhere) is I didn't hear any chinny cries.