I once had a gerbil who refused to die, and I loved her deeply. When she finally went, she was nearly 6 years old and had a tumor sticking out of her ear. She would scratch and scratch at it, and within a few days it would grow back to more or less the shape it was before. At her age, there wasn't really anything a vet could do. Surprisingly, it wasn't the tumor that killed her. Her liver failed.