Bechorot 21: Jews and Bechorot

The Gemara discusses three options regarding how we determine the status of a possible bechor when given to a priest by a Jew.   

• Rav states that the first offspring is certain to be a bachor because if the female had already given birth, the owner would brag about that happening
• Shmuel says that we cannot be sure of the animal's status because the owner might not have intended to slaughter it and might have intended to sell it before it gave birth
• Rabbi Yochanan counts on a Jew to understand the halachic consequences of a firstborn versus a first offspring, and since s/he said nothing about the animal being a bechor, we can assume that the mother had already given birth once

Rashi and others continue to discuss the possible reasoning of 'selling points' and 'disadvantages' to selling an animal, including the animal's fertility, its predictability as a good breeder, and the likelihood of exaggeration of points that might lead one to believe that an animal is a bachor.