A ewe births its first offspring. That offspring is the firstborn and laws of sanctity (only priests may eat of its flesh) apply to it. But it is not always possible to follow a ewe and see if it has already birthed its first offspring or not. Therefore the sages asked: what is the rule in a case where a pig follows a ewe and suckles from her? Is this the ewe’s first offspring, born in the form of a pig, and so subject to the laws of the firstborn (forbidden for non-priests to eat) or is this a real pig, born to a sow, but since its own mother abandoned him he follows the ewe and has adopted her as his mother? (This question is not answered in the Talmud.)
(Babylonian Talmud, Tractate Bechorot 24b)
The Talmudic text:
Achai asked: if we see a pig cling to and nurse from a ewe, what is the law? What does he ask about? He asks about firstborn obligations, whether the law follows Rabbi Shimon son of Gamliel or whether it follows the sages.
If so, why did he discuss a pig — he should have discussed a ewe nursing a lamb! He asks about first born obligations, [even] according to the sages; he asks about eating the offspring, [even] according to Rabbi Shimon son of Gamliel.
He asks about firstborn obligations, even according to the sages — they say that an animal is merciful, but perhaps this is limited to its own species.
He asks about eating, even according to Rabbi Shimon son of Gamliel. If Rabbi Shimon son of Gamliel says that a mother might be merciful, perhaps this is limited to its own species, or, perhaps it might be merciful even to another species! These questions are unsettled.