Mary has been gone for a while and they weren't housed together...I had Mary and Moore together and Gerogette and Tyler together. They have been in separate cages now for months also...and Tyler would go after Mary when ever she could (they were aggressive attacks where Tyler went out of her way to get Mary) Tyler is ten months old.This is exactly what I went through with Harper. I got her tested for every disease imaginable and all were negative. My vet said it was hormonal. How old is Tyler? That doesn't mean that this is hormonal for Tyler though. Maybe she misses Mary?