Why does Art find it difficult to find the right animal for Françoise?

Francoise is Art's wife. She is French and converted to Judaism in preparation for their marriage to please Vladek. She is intelligent, kind, and opinionated, and their relationship is strong. She plays a relatively minor role in the story, serving mostly as a means for Art to discuss his relationship with his father and the Holocaust. She does not have a direct role in a historical sense which makes finding an animal for her difficult.