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If one buys a nursing animal from an idolater and does not know whether it already had its first offspring, he can safely assume that the animal is nursing its own child and is therefore not subject to the law of firstborn anymore.
While there are some animals that lactate before giving birth, in general animals do not nurse a child that is not their own. Also, in a flock one can assume that each animal nurses its own child. A different point of view is that an animal will nurse a child not of its own even before it has given birth.
If one saw a young pig clinging to an ewe and nursing from it, the ewe is exempt from the law of a firstborn, because we can assume that this is not a pig but a mutant lamb. One can't eat it, because perhaps it is a pig after all. This rule seems to combine the two contradictory points of view above: does an animal feed a child that is not its own or it doesn't?
We explore how Slifkin gives us a zoological route into the much deeper question of whether סימני טהרה identify a pre-existing מין or actually constitute halakhic identity.