Bechoros 2:5-6

Bechoros 2:5

If a ewe gave birth to something that resembled a goat, or if a goat gave birth to something that resembled a sheep, it is exempt from the law of firstborn animals, though if the offspring has characteristics that make its lineage clear, then it is obligated.

Bechoros 2:6

Let’s say that a ewe that never gave birth before gave birth to two males and their heads emerged simultaneously. In such a case, Rabbi Yosi HaGlili says that both belong to the kohein based on Exodus 13:12, “the males shall belong to Hashem.” The Sages say that this scenario is impossible so one animal belongs to the owner and the other belongs to the kohein. Rabbi Tarfon says that the kohein can select the better one, while Rabbi Akiva says that the two parties decide between them. Since the animal retained by the owner might be the firstborn, it must be left to graze until it develops a blemish. When it is slaughtered, it is obligated in the portions given to a kohein, though Rabbi Yosi exempts it. If one of the animals died before the kohein receives one, Rabbi Tarfon says that the two parties divide the surviving animal, though Rabbi Akiva says that the burden of proof lies is on the kohein to prove that the surviving animal is the firstborn. If the mother gave birth to a male and a female, the kohein receives nothing (because the male might not be the firstborn).