daf bit: Bechorot 23-24

The mishna teaches: R. Shimon ben Gamaliel says: if one buys an animal (from a gentile) that is suckling young, he need not fear that the young belongs to another animal. Why does this matter? Because when the animal he bought later gives birth, he needs to know if that offspring is a first-born male (which is subject to special laws). The g'mara on 24 has a long discussion of what is natural or unnatural in animal behavior and concludes that an animal that has never given birth does not suckle, so it must have given birth, and it's natural to assume that the animal it's suckling is its own. (23b-24)

(I think this last part, that the young is the mother's own offspring, is important only if the Jew buys both the mother and the young. In that case he would be subject to a torah prohibition against slaughtering them both on the same day. But if he only buys the mother I don't think this matters.)