Bekhorot 2: Beginning with Donkeys

A new tractate, taking the first-born animals as its major focus: including human first-born sons, first male domesticated animals, and first-born donkeys, commemorating the first-borns who were saved in Egypt. We open with the focus on the donkeys. And, of course, with a new mishnah. The mitzvah applies only to donkeys that are owned fully by Jews. The question is, then, what happens if the donkey is jointly owned, and it lists 5 cases. The Gemara addresses why all of those 5 cases were necessary to spell out (instead of learning from one to the other).